Molt in Relation to Migration

Fig. 1.Composition by age and sex as found in one or more series of each of eight species of birds included in the Topeka sample. Each separate series is represented by a single histogram, the histograms for a species being grouped with the earliest series on the left. Each histogram expresses the numbers of adults (left-hand column) and immatures (right-hand column) in terms of percentage of the whole series. Thus the two bars of each couplet add up to 100 per cent. The hatched portion of each bar represents males, the clear portion females.

It would be difficult to imagine a better way of resolving the problems and uncertainties just reviewed than by the detailed analysis of large samples of migratory birds killed at random at various points and times. An analysis of the sample of birds accidentally killed at Topeka is presented here as an initial step in this direction. Although the conclusions based on this sample are tentative and may in time be altered, the data themselves are definite. If this general type of analysis is repeated again and againâ€”applied to samples taken on many dates and in many localitiesâ€”a mass of hitherto unparalleled evidence for the study of migration will emerge.

Differential Migration OF Sex- and Age-classes as shown by the Topeka Sample.â€”Smaller samples have not been treated. Species affording samples seemingly large enough to justify at least preliminary analysis were: Catbird, Red-eyed Vireo, Mourning Warbler, Dickcissel, Nashville Warbler, Orange-crowned Warbler, Yellow-throat, and Lincoln Sparrow (Fig. 1). For all of these except the Catbird and Dickcissel, at least two samples from a week or more apart were available for comparison in an effort to detect trends in migration. Fig. 1 shows the actual ratios of sex- and age-classes observed in samples of the species listed above. Each of the last four species provided two separate samples, of sufficient size to warrant an attempt at measuring the statistical significance of the observed changes in adult-immature ratios (Table 2).

Table 2.â€”Statistics of the Ratios of Adults to Immatures in Four Species

SpeciesDates of samplesTotal numberNumber and percentage of adults[1]Difference (in %)[2]P[3]Nashville WarblerOct. 1 (93)Oct. 5-7 (63)15645 (.484)26 (.413).071.36Orange-crowned WarblerSept. 25-Oct. 1 (19)Oct. 5-9 (25)443 (.158)11 (.440).282.05Yellow-throatOct. 1 (115)Oct. 5-8 (44)15962 (.540)20 (.455).085.34Lincoln SparrowOct. 1-3 (44)Oct. 6-10 (27)7127 (.614)8 (.296).318.01

[1]Percentage of immatures equals 1.000 minus percentage of adults.

[2]Standard error of the difference between ratios was computed by the formulaÏƒe=âˆšPeQe(1+1),N1N2where Peequals percentage of adults and Qeequals percentage of immatures in the entire sample.

[2]Standard error of the difference between ratios was computed by the formula

Ïƒe=âˆšPeQe(1+1),N1N2

where Peequals percentage of adults and Qeequals percentage of immatures in the entire sample.

[3]Probability of error;i. e., a P of .01 means there is one chance in 100 that the difference observed does not represent an actual difference in nature.

Upon the application of statistical methods it soon became evident that, unless changes in ratio between two samples are marked, large samples would be required in order to reach conclusions of high statistical significance in a single study of the present type. In this case (see Table 2), the Lincoln Sparrow and Orange-crowned Warbler, though represented by only moderate-sized series, show marked changes in age composition over the period studied, and the statistical treatment indicates a high degree of probability that these changes are real. Assurance that the lesser changes observed in the Nashville Warbler and Yellow-throat are real, on the other hand, is much less, even though the samples are larger. Few if any of the samples here discussed are as large as might be desired. Therefore, conclusions based upon them (see below) are to be regarded as tentative. Many other, future, samples will perhaps also be insufficient in size in themselves. There are, however, statistical advantages to repetition which will serve to make the repeated analysis even of small samples significant and valuable.

Certain of the samples not treated statistically show ratios that can be seen by inspection to be probably significant. For example the almost complete absence of adults from the three samples of Red-eyed Vireos (Fig. 1E) cannot be disregarded in view of the size of the whole sample of the species. The same applies to the high percentage of adult females and the near absence of adult males in the sample of the Dickcissel (Fig. 1F). The continuity in direction of changes observed in the three samples of the Mourning Warbler (Fig. 1G) and Red-eyed Vireo is likewise probably significant, even though some of the samples compared are small. It seems to us that the application of statistical methods to these species should await the accumulation of more material. For anyone desiring to treat them statistically now, the data are inherent in this paper.

We have not computed the standard errors of the ratios of sexes within age groups (except experimentally in a few cases). This can easily be done, however, and the significance of a given ratio determined, on the assumption (perhaps sometimes dubiously justifiable) that the sex-ratio in the species concerned is one:one. Obviously there is no point in computation of the standard errors of adult-immature ratios in single samples (such as that of the Dickcissel) until the actual ratio prevailing in the species in nature at the season in question is known for comparison with the observed ratio. Our formal statistical treatment, therefore, has been limitedto an examination of the significance of thechangesbetween adult-immature ratios in samples of the same species taken a number of days apart.

The samples suggest several patterns of differential migration of sex- and age-classes. Indeed, the important consideration brought outâ€”in our opinion not hitherto sufficiently emphasized in literatureâ€”seems to be that in generalizing about adults and immatures, one must be careful to take sexes into account, and conversely, in generalizing about males and females, one must consider also age. In other words, there are really four classes to be considered. This poses additional problems in analysis and introduces the need for still larger samples in order to reach significant conclusions. To illustrate: an adult-immature ratio of 40:20 (N = 60) may be satisfactorily significant, while within the 40 adults a ratio of 25 males:15 females may not be. Were the original sample 80:40 (N = 120) with male adults 50 and female adults 30, it is obvious that the significance of the latter ratio would be greater. The same applies in reverse if the greater emphasis is placed on sex and the lesser on age. Because of the moderate size of the samples this problem has been felt in the present study in respect to sex ratios within age groups, many of which must at present be regarded as of tentative significance.

In short, what the earlier ornithologists regarded as a simple problem is in reality a complex one. There are only two patterns in what may be called the Brewster-GÃ¤tke argument: adults first or immatures first (with of course the further possibility of both at the same time). Both patterns occur, as is now known, at least to some extent. But actual patterns, as suggested by our samples, are more complex when all classes are considered. It will readily be seen that, if adult males, immature males, adult females, and immature females be regarded as units, each with certain migratory characteristics, the combinations of these units in various orders of migratory precedence are potentially numerous. In fact, of course, they do not behave strictly as units (or perhaps very rarely so), but our data strongly indicate that the tendency exists in many cases. This may be stated another way. The present samples may be reduced to two basic patterns, fitting the classic early American (adults first) and early European (immatures first) theories. But, either such simple arrangement is compounded in some, perhaps in truth in all, instances by differential migration of the sexeswithineach age class. This proposition can also be stated backwards:the samples show differential times of migration of the sexes, compounded by differential times of migration of the age groups within each sex. The order in which these matters are approached depends on what one is trying to find out. Influenced by the literature, in which most emphasis has been placed on age, we have approached the problem from that standpoint. The data and figures here given, however, can be juggled if one wishes to place first emphasis on the order of sexes in migration.

Bearing in mind what has just been said, particularly in respect to sizes of samples necessary for significance, let us consider the patterns of migration suggested by the Topeka sample. These are as follows:

(1)An early migration largely composed of adults, giving way later on to a preponderance of immatures.Regardless of variations among them, samples showing this basic pattern are in line with the opinions of Brewster (1886) and his followers. This pattern is here shown by the Lincoln Sparrow, Yellow-throat, Nashville Warbler, Catbird (one sample only), and Red-eyed Vireo (Fig. 1, A, B, C, D, E). The evidence of these and all other samples would admittedly be more conclusive if the samples were further apart in time or, better still, were there more of them. There is evidence that differences in migration of the sexes, within age classes, influence this pattern, sharply in some instances. In the later samples of Lincoln Sparrow, Yellow-throat, and Red-eyed Vireo (Fig. 1, A, B, E) there are relatively fewer males, both adult and immature, than in the earlier samples and this may be true also of the Catbird, judging from the single sample. The Red-eyed Vireo (Fig. 1, E) is characterized by small number, or absence of, males in each sample but the samples are not significantly different, and can be regarded as one. Although the samples of the Dickcissel and Mourning Warbler (Fig. 1, F, G) show a somewhat different over-all pattern and are discussed further on, they also contain few adult males. Since these samples are from a period that is near the end of the migration of Red-eyed Vireos, Mourning Warblers, and Dickcissels, it may be assumed tentatively that the adult males have already migrated. Meinertzhagen (1930:56) postulated that in many species there is an earlier or more rapid migration of adults, particularly males, and the data for the above species in our sample tend to support his assumption. But our data suggest in addition that in some speciesimmature malesmigrate earlier, or more rapidly, than doimmature females, just as adult males precede adultfemales in some instances. Within this general pattern (adults first) another variation is shown by the Nashville Warbler (Fig. 1, C) in which the later sample of adults is heavily weighted towards males, even though an increasing over-all proportion of immatures is evidenced. In this case, and contrary to Meinertzhagen's suggestion, it would seem that adult females have preceded or outstripped adult males in migration.

(2)An early preponderance of immatures, followed by a preponderance of adults.The several species of birds at Topeka that display this pattern conform with the conclusions of GÃ¤tke and other early Old World ornithologists that in most species immatures precede adults in migration. In the present sample two variations of this pattern occur.

(a) In the Dickcissel (Fig. 1, F) and the Mourning Warbler (Fig. 1, G), immatures decrease more markedly than adults (visible in samples of Mourning Warbler; inferred in Dickcissel), leaving the adults in the majority in the closing phase of migration. The distinctive and interesting feature in each of these two species is the ascendancy in numbers of adultsdespitethe almost complete disappearance of adult males. The relative increase of adults is here caused by a retarded migration of adult females, which linger conspicuously behind all other classes. Something of this nature was suggested, in theory, by Dixon (1892:70) who thought that adult females are delayed by "maternal duties." It was hinted at also by Dwight (1900:127) who thought that in some species females molted later than males as a result of prolongation of parental responsibilities. As mentioned already, there is need for caution in interpreting the present samples because the Dickcissel is represented only by one sample and two of the three samples of Mourning Warblers are small. In the case of the Mourning Warbler, the samples may be regarded as one, nearly lacking in adult males. The progressive increase of adult females, however, may be significant; at least there are enough of these to make division of the birds into three samples enlightening. There is, of course, some chance that the majority of adult males have not yet migrated, or are migrating by a different route. This seems unlikely in both cases. October 1 is late in the migration of the Dickcissel and it seems that large-scale migration would not occur much later, and in the case of the Mourning Warbler adult males are rare in all three samples, extending over a considerable period and reaching late into the probable migration period of the species. It is interesting to conjecture just when and where adult male Mourning Warblers do migrate in autumn. Brewster (1886:16) wrote: "This species arrives at Cambridge [Massachusetts] about September 12, and during the remainder of the month is ... abundant.... The adults, however, are so very uncommon that I have never known them [to] represent more than five per cent of the total number of individuals. They do not seem to be more numerous in the earlier flights than towards the close of the month, and I am very sure that they cannot be found in this locality before the young begin to appear." While the present samples show an abundance of adultfemalesof this species (could Brewster have failed to recognize these as adults?) the whereabouts of the adult males remains a mystery.

(b) Another variation is displayed by the Orange-crowned Warbler (Fig. 1, H). Here also there is an increase of adults towards the end of migration, but this increase is marked by a growing percentage not of females but of males. Locally this species is a late migrant compared with most others of the Parulidae. Thus the first sample, composed of birds taken September 25-October 1, may be regarded as fairly early in the fall migration. Immature birds compose 84.2 per cent of this sample, there being no adult males at all. By October 5-9 the picture has changed markedly, the sample being composed of 44 per cent adults (82 per cent of which, in turn, are males) and 56 per cent immatures. In view of this trend one can not help suspecting that a still later sample would show a majority of adults, perhaps nearly all males. This of course does not necessarily follow; the migration of immatures could simply be more protracted, and could have commenced earlier, than that of adults.

Little imagination is required to see how enlightening it might be could we analyze thoroughly the patterns of all migrating species. When the detailed facts are available, it seems likely that general trends will emerge which may be of great significance to the study of migration in general. A final point which must eventually be clarified is determination of the extent of variability in the pattern of each species from year to year and locality to locality.

Once patterns of precedence in migration of different classes are established, search into the life-histories of the species concerned may help to explain the peculiarities discovered. In the present case, for instance, we find a possible clue to the reason for the high proportion of adult females of the Dickcissel late in migration, asshown by our sample. Gross (1921:14-15) presented evidence that adult female Dickcissels molt considerably later than their mates, and we have independent evidence that individuals of this species are at times almost flightless when molting the remiges!

General comment.â€”The exact relationship between molt and migration seems not to have been definitely established. The subject has received cursory attention in the literature and conflicting opinions have been expressed. Dwight (1900:126-128) believed that molt is completed or nearly completed before migration in nearly all passerine species that occur in New York save for certain swallows and flycatchers. Molt has since been found to precede migration of at least one of the flycatchers (Empidonax virescens) considered by Dwight to be an exception to this rule (Mengel, 1952). In Great Britain the subject of molt in migration was considered in some detail by Rintoul and Baxter (1914) and Ticehurst (1916), who arrived at conclusions similar to Dwight's. These workers also found certain swallows to be exceptions to the rule.

The above authors and others have shown that, at least among passerines, some body molt is frequently found in migrating individuals but that molt of tail feathers is much less often found and molt of remiges almost nonexistent. Baxter and Rintoul noted only four cases of molting wing feathers among hundreds of migrants. Among the diverse non-passerine orders the picture seems to be more complicated, as might be expected. We do not, however, comprehend the reasoning which led Meinertzhagen (1930:56) to summarize: "... on the whole it can be said that though birds seldom migrate when flight feathers are in quill, moult in general does not influence migration." This seems to us an obviousnon sequitur. Meinertzhagen (loc. cit.) went on to say: "Males and females of one species are believed to moult simultaneously [see, however, Dwight, 1900:127], and there is no doubt that in some cases the two sexes migrate at slightly different times, and occasionally prefer different winter quarters. Birds of the year never moult their quills previous to their first autumn migration [Consultation of Dwight, 1900, who gives many examples of this, would have spared Meinertzhagen this error.], and yet they frequently follow adults on passage and sometimes precede them. There are no grounds for believing that adults have moulted their quills before birds of the year are prepared to migrate [but there are, inmany cases;cf.Dwight, 1900:127], in the case where adults precede the latter. Neither is there any evidence to show that adults have not moulted their quills till after their offspring are ready for passage, in the case where they follow their offspring. It does not, therefore, appear that moult is an important factor."

Comments interpolated above show our feeling that this summary is inadequate and misleading. To us it seems that the extreme rarity in migration of birds with remiges in molt is strong evidence that moltdoesinfluence at least the time of migration. It is immaterial whether this molt occurs before or after migration, although in the majority of cases it seems to take place before. Much more needs to be known of the migration pattern and molt of each species before generalizations can safely be made.

Analysis of samples of migrants can show only the presence and nature or the absence of molt in birds actually migrating. In the present instance shortage of time and manpower for preserving some and processing all of the sample resulted in incomplete data being kept on molt. We include this section to emphasize uncertainties still prevalent and to stimulate further work.

Molt in the Topeka sample.â€”Our limited findings coincide with those of Rintoul and Baxter (1914). Body molt was noted in a number of individuals and species. When present, this molt almost invariably was in its final stages. One immature male Rose-breasted Grosbeak (October 1) was in heavy body molt. It is perhaps worthy of mention here that this grosbeak evidently migrates at times in extensive molt. An adult male (RMM 1102) taken by Mengel near Henderson, Kentucky, on September 9, 1949, was molting plumage of body, wings, and tail, no feather of the last being longer than one half inch. This remarkable specimen had only five primaries on one side and four on the other fully functional. The outermost on the left and two outermost on the right were from the previous plumage, not yet dropped; the three innermost of each wing were new and full-length.

In the present sample molt of remiges was noted in one specimen, an adult female Indigo Bunting (October 1) with outer primaries sheathed and with molt in progress in the body plumage. The one (immature) Yellow-breasted Chat in the sample (October 1) had all of its tail feathers nearly full-length but in quill, possibly as a result of accident, and two feathers were being replaced also in the tail of an immature Clay-colored Sparrow (October 6), which was also in body molt and had some, juvenal feathers on the belly and flanks.

Body molt near completion was further noted as follows: immature male Yellow-throated Vireo (October 1), adult male Blue-headed Vireo (October 1), immature female Leconte Sparrow (October 23), several Lincoln Sparrows (various dates).

Linear measurements.â€”Taxonomists long have recognized in many species that males differ in size from females. Less attention, until recently, has been paid to the relative sizes of adult and immature birds. Many taxonomists, however, seem to have had an uneasy suspicion that immature birds are "untrustworthy" in comparison with adults, and immatures have often been excluded from samples when recognizable. Since, however, there are still relatively few reliably aged specimens in collections, for the most part only those immature birds immediately recognizable as such by obvious plumage differences (which are often present only in juvenal plumage) have been excluded from series. The majority of birds in first winter plumage so closely resemble adults that the two ages have been included in series for measurement. In most passerines these younger birds still bear the juvenal feathers in wing and tail and are, in size of these important parts, quite as "untrustworthy" as birds still in juvenal body plumage. Even if a complete postjuvenal molt occurs we still should not assume that first winter feathers are as long as adult winter feathers without first determining that this is so. Although aware of this problem, systematists until recently seemingly have been more or less content to disregard it, or forced to do so for practical reasons. Miller (1941:179) had little choice but to hope that size differences between adult and immature juncos were unimportant. Behle (1942:217) wrote of Horned Larks,Eremophila alpestris: "... the plumages of first-year birds and adults seem indistinguishable, though I have never quite satisfied myself that there are no differences in lengths of rectrices and remiges." He added, with logic confusing to us: "Since it is a difficult problem to determine the ages of horned larks that have passed the postjuvenal molt, this similarity of plumages is fortunate for the systematist."

In recent years, some workers have analyzed size differences between adults and immatures. Sibley (1950:115) showed that adult Red-eyed Towhees (Pipilo erythrophthalmus) had notably longer wings and tails than immatures, and the same was demonstrated in Red Crossbills (Loxia curvirostra) by Tordoff (1952). In workwith jays (Aphelocoma), Pitelka (1951:199) found that: "... in comparisons of dimensions of sex and age groups within a given sample, although magnitude of difference varies from one character to another, most of the averages are successively smaller for first-year males and adult and first-year females." He listed exceptions and concluded: "Segregation [of sex and age classes] has proved to be of extreme significance in an interpretation of individual and geographic variation."

Much along these lines can be learned by examination of large random samples such as that afforded by the Topeka accident. Although only a few species in this sample were measured, the results secured seem to show further the need for segregation of age classes in taxonomic work with some species.

Figure 2 shows the variation in the lengths of wing and tail in the Nashville Warbler. It is evident from the figure that in both of these characters the four sex- and age-classes differ significantly. An accurate idea of geographic variation in this species could not be obtained without separating these classes in comparisons. Age classes in spring and summer, long after the skull is completely ossified, can be segregated only if differences in plumage can be found. In the Nashville Warbler, such differences occur in fall (see annotated list) but these probably are obliterated by the partial prenuptial molt. These facts emphasize the importance, for taxonomic studies, of aged specimens collected in late summer or early fall on their breeding ground and in fresh winter plumage. Figure 3 shows the variation in length of wing in the Lincoln Sparrow in which age seems to be of much less importance than in the Nashville Warbler. Males and females of the Lincoln Sparrow differ significantly in wing-length, but adults and immatures are of nearly the same size. It would seemingly not be necessary to separate age classes in studies of geographic variation in the Lincoln Sparrow. Size data for some other species are given in the annotated list.

Weights.â€”Little seems to have been done to determine the correlation of weights with sex- and age-classes. Weight may be the best single index of over-all size and is especially useful to students of the physiology of migration. Weight, however, is subject to the considerable variable imposed by fat condition. In large and comparable series, this variable is probably removed insofar as comparability of means is concerned, but the high variability of weight in most cases naturally results in more overlap (less separability) between populations with means close together than is found with most linear measurements.

Fig. 2.Statistics of variation in length of wing and tail in the Nashville Warbler. The solid lines represent the observed ranges in millimeters. The stippled boxes represent two standard errors (Ïƒm) to each side of the means (vertical lines). The open boxes represent one standard deviation (Ïƒ) to each side of the means.

Fig. 3.Statistics of variation in length of wing in the Lincoln Sparrow.

Figures 4-6 show variation in weight in the samples of Nashville Warbler, Mourning Warbler, Yellow-throat, Dickcissel, and Lincoln Sparrow. Each figure is essentially self-explanatory. It will be seen that in the Nashville Warbler and Lincoln Sparrow, weight is roughly proportional to wing-length (shown in figs. 2 and 3), giving about equally good separation of sex- and age-classes in the latter and poorer separation in the former. Data for these and other species indicate a generally greater weight of males than of females, but less difference according to age. In some other species, for example the Yellow-throat, immatures seem to be a little heavier on the average than adults. It is unfortunate that wing-lengths are not at present available for these.

Fig. 4.Statistics of variation in weight in the Nashville Warbler and Mourning Warbler.

Fig. 5.Statistics of variation in weight in the Yellow-throat and Dickcissel.

Fig. 6.Statistics of variation in weight in the Lincoln Sparrow.

These comments on weight suggest an additional factor which may play a part in rate of migration and which some day may be profitably studied. Suppose that in some species adults and immatures are nearly the same in weight but that immatures have shorter wings. In such a species the immatures are relatively shorter-winged for their weight than adults. In aerodynamic terms,they would have a higher "wing-loading." (Wing-loading is the result obtained by dividing area of effective wing by total weight; it is here assumed that in a single species wing area is directly proportional to wing length.) This being the case, immatures with higher "wing-loading" would require more energy (derived from fat) to fly the same distance as adults, or with the same amount of fat they would fly a shorter distance. Thus they might tend to be outstripped in migration by adults starting at the same time. The reverse, of course, would also be theoretically true, if adults possessed a higher wing-loading than immatures. Physical factors such as these rather than the differential "virility" postulated by Meinertzhagen (1930:56) might account for the arrival of certain classes of some species on the wintering grounds in advance of others. There are, of course, many other factors which must be taken into account before the effect, if any, of the wing-loading factor can be evaluated. Data for illuminating calculations will become available, however, with the accumulation of abundant information on weights, measurements, and migration patterns.

Tanner (ms., and letter, April 21, 1955) recently devoted considerable ingenuity to computing by actuarial methods the longevity of the Oven-bird, using the adult-immature ratio in samples killed at a ceilometer at Knoxville, Tennessee. Tanner's computations were based on the simple assumption that

S (survival rate) =Number of adults in population (or sample)Total size of population (or sample).

Further application of such techniques may prove desirable and rewarding. It would seem at present, however, to be a risky procedure, as it has been abundantly shown (see above) that adults and immatures often do not migrate at the same times and rates, and the ratios of adults to immatures in samples of migrants are likely to be far from representative of the true proportions in the populations concerned. It should be added that Tanner is perfectly aware of this objection.

Thorough processing of large samples of birds killed accidentally is time-consuming. We were fortunate in having considerable assistance; even so, all desirable data could not be obtained from the 1090 birds of the present sample. As aids to others conducting studies of this kind we should mention a few points which may be of assistance.

Birds should be picked up as soon as possible after death, certainly by the end of the day after the accident and preferably much sooner. They should be weighed as soon as possible after collection (weights decrease rapidly, even under refrigeration), and the weights (in grams, to one tenth of a gram) written on tags attached to a leg of each specimen. The sample should then be sorted by species or groups of species of approximately equal size (to avoid crushing of smaller birds by larger ones), placed in boxes, paper bags, or better, air-tight containers clearly marked with date, locality, and other necessary particulars, and relegated to a deep-freeze compartment. For all but the smallest samples, such freezing units are indispensable to complete study. Once frozen, the birds may be selected for study at leisure, but time is still important, as, even when frozen, gonads may eventually deteriorate, and birdseventually become desiccated which is a disadvantage if skins are to be made.

In the cases of large kills, or limited manpower, or both, it may be impossible to process all birds, however desirable this might be. If possible, however, all should be collected, identified, the numbers and species recorded, and rarities saved. Further, partial analysis, or more properly, complete analysis of a partial sample, can be made. Analyses which for any reason require randomness of sample pose a special problem. We think that in very large kills the best way to solve this problem is probably to make one or more transects across the area where dead birds are found. These transects should cross both the areas of greatest and least density (to allow for fast and slow flying species). Their width may be adjusted to give the desired number of birds, that is, the number that can be adequately processed. Another alternative would be to decide to study certain abundant species and pick up all of these. There are other possibilities, but in any event the method of sampling should be thoroughly described wherever all birds have not been processed.

The foregoing paper discusses accidents in which large numbers of night-migrating birds are killed. A brief historical review of ornithological interest in such occurrences is given, and the types of data provided by the accidents are listed and discussed. In particular, recent accidents occurring in early October, 1954, through much of eastern United States are cited, and detailed analysis is presented of a sample of 1090 birds killed one mile west of Topeka, Shawnee County, Kansas, between September 25 and October 23, 1954.

At Topeka during the period mentioned, 1090 birds representing 61 species were collected and were processed at the University of Kansas. For all specimens, weight, sex, age, and fat condition were recorded, and certain species were measured as well. Some notes on molt were taken. A total of 193 birds was preserved as study skins, and 49 as skeletons. Comments on weight, size, sex, age, subspecific identity, and status in Kansas are presented in an annotated list.

Randomness of this and other similar samples is discussed. A theoretical computation is given for several nights of the numbersof migrants passing through a plane one mile in width, from 450 to 950 feet above ground level, and oriented to face the arriving migrants. The computed totals give some idea of the tremendous volume of nocturnal migration under some conditions. Potentialities of further study of such events are discussed and a comparison is made with lunar observations.

Differential migration of sex- and age-groups as shown by the larger samples taken at Topeka (Catbird, Red-eyed Vireo, Nashville Warbler, Yellow-throat, Mourning Warbler, Dickcissel, Lincoln Sparrow) is discussed and the history of theories on this subject reviewed. It is shown that age and sex must both be taken into account in studies of differential migration. Several patterns of migration are displayed by the species analyzed, adults migrating earlier than immatures in some instances, young earlier than adults in others, but all seemingly being complicated to varying degrees by differential migration of sexes within age groups. It is suggested that explanations of these patterns may be sought in the life histories of the species involved.

Molt in relation to migration is discussed briefly, and it is held that there is an important relationship between molt and time of migration. Specimens noted to be in molt are listed.

Size differences, in wing length, tail length, and weight are discussed in relation to sex and age, and variation in one or more of these characters is analyzed as found in the Topeka samples of Nashville Warbler, Mourning Warbler, Yellow-throat, Dickcissel, and Lincoln Sparrow. It seems that in some instances significant size differences prevail between adults and immatures and that these age classes should be separated in taxonomic work with species where differences in size are known to exist. When the facts are not known they should be determined, and the large samples collected in accidents to nocturnal migrants present excellent opportunities for ascertaining the facts.

Other uses of material obtained in large migration accidents are discussed, such as computations of longevity and the problems of processing large, accidentally-killed samples. Care should be taken to select samples as nearly random as possible when all birds cannot be processed.

Repeated and thorough analysis of accidental kills should provide a mass of valuable data bearing on many questions and problems which have thus far been little studied.

Allen, J. A.

1880. Destruction of birds by light-houses. Bull. Nuttall Orn. Club, 5(3):131-138, July.1896. GÃ¤tke's 'Heligoland.' Auk, 13(2):137-153, April.1901. Barrington's 'The Migration of Birds at Irish Light Stations.' [Review.] Auk, 18(2):205-206, April.1909. An American's views of bird migration. Brit. Birds, 3(1):12-19, June 1.

1880. Destruction of birds by light-houses. Bull. Nuttall Orn. Club, 5(3):131-138, July.

1896. GÃ¤tke's 'Heligoland.' Auk, 13(2):137-153, April.

1901. Barrington's 'The Migration of Birds at Irish Light Stations.' [Review.] Auk, 18(2):205-206, April.

1909. An American's views of bird migration. Brit. Birds, 3(1):12-19, June 1.

Barrington, R. M.

1900. The migration of birds as observed at Irish lighthouses and lightships [etc.]. London, R. H. Porter; Dublin, Edward Ponsonby. Pp. XXV, 285, 667. (Not seen; citation from Mullens and Swann, Bibliogr. Brit. Orn., 1917, p. 43 and Zimmer, Cat. Ayer Coll., I, p. 40, 1926; see also reviews, Ibis, 1900:677-679, Auk, 1901:205-206; some sources list pagination as XXV + 667, possibly in error.)

Behle, W. H.

1942. Distribution and variation of the Horned Larks (Otocoris alpestris) of western North America. Univ. California Publ. ZoÃ¶l., 46(3):205-316, May 20.1950. Clines in the Yellow-throats of western North America. Condor, 52(5):193-219, September-October (September 25).

1942. Distribution and variation of the Horned Larks (Otocoris alpestris) of western North America. Univ. California Publ. ZoÃ¶l., 46(3):205-316, May 20.

1950. Clines in the Yellow-throats of western North America. Condor, 52(5):193-219, September-October (September 25).

Brewster, W.

1886. Bird migration. Mem. Nuttall Orn. Club, No. I, pp. 1-22, Cambridge, March.

Carson, L. B.

1954a. [Destruction of birds at a television tower at Topeka, Kansas.] Topeka Aud. News, 9(1):pp. = 1-2, July, August, September [published October]. [Unsigned, unpaged, and untitled article by L. B. Carson, ed.]1954b. [Further destruction of birds at a television tower at Topeka, Kansas.] Topeka Aud. News, 9(2):pp. = 5-7, October, November, December [published December]. [Unsigned, unpaged, and untitled article by L. B. Carson, ed.]1954c. New records for fall migrants in eastern Kansas. Kansas Orn. Soc. Bull., 5(4):27-29, December.

1954b. [Further destruction of birds at a television tower at Topeka, Kansas.] Topeka Aud. News, 9(2):pp. = 5-7, October, November, December [published December]. [Unsigned, unpaged, and untitled article by L. B. Carson, ed.]

1954c. New records for fall migrants in eastern Kansas. Kansas Orn. Soc. Bull., 5(4):27-29, December.

Dixon, C.

1892. The migration of birds. London, Chapman and Hall. Pp. XVI + 300.

Dobben, W. W. H. van, andM. F. Bruyns

1939. Zug nach Alter und Geschlecht an niederlÃ¤ndischen LeuchttÃ¼rmen. Ardea, 28:61-79, December (not seen; see Auk, 1940:271).

Drost, R.

1935. Ueber das ZahlenverhÃ¤ltnis von Alter und Geschlecht auf dem Herbst-und FrÃ¼hjahrszuge. Vogelzug, 6(4):177-182, October.

Dwight, J., Jr.

1900. The sequence of plumages and moults of the passerine birds of New York. Annals New York Acad. Sci., 13(1):73-360, October 19.

Ganier, A. F.

1949. The late summer Dickcissel departure. Migrant, 20(3):52-53, September.

GÃ¤tke, H.

1895. Heligoland as an ornithological observatory. Translated by Rudolph Rosenstock. Edinburgh, David Douglas. Pp. X + II + 599 + 11 II. (advt.).

Gross, A. O.

1921. The Dickcissel (Spiza americana) of the Illinois prairies. Auk, 38(1):1-26, January 18 (first of two parts).

Gurney, J. H.

1923. Bird migration as observed on the east coast of England. Ibis, 11th ser., 5(4):573-603, October 3.

Howell, J. C.,Laskey, A. R., andJ. T. Tanner

1954. Bird mortality at airport ceilometers. Wilson Bull., 66(3):207-215, September [published October 29].

Howell, J. C., andJ. T. Tanner

1951. An accident to migrating birds at the Knoxville airport. Migrant, 22(4):61-62, December.

Laskey, A. R.

1951. Another disaster to migrating birds at the Nashville airport. Migrant, 22(4):57-60, December.

Long, W. S.

1934. Notes from eastern Kansas. Auk, 51(2):255, April 4.1940. Check-list of Kansas birds. Trans. Kansas Acad. Sci., 43:433-456.

1934. Notes from eastern Kansas. Auk, 51(2):255, April 4.

1940. Check-list of Kansas birds. Trans. Kansas Acad. Sci., 43:433-456.

Lovell, H. B.

1952. Catastrophe to birds at a Louisville airport. Kentucky Warbler, 28(1):5-6, February.

Lowery, G. H., Jr.

1951. A quantitative study of the nocturnal migration of birds. Univ. Kansas Publ., Mus. Nat. Hist., 3(2):361-472, June 29.

McCabe, T. T.

1943. An aspect of collectors' technique. Auk, 60(4):550-558, October 7.

McCabe, T. T., andA. H. Miller

1933. Geographic variation in the Northern Water-thrushes. Condor, 35(5):192-197, September-October (September 15).

Meinertzhagen, R.

1930. Nicoll's birds of Egypt. Vol. I (of 2). London, Hugh Rees Ltd. Pp. XVI + 348.

Mengel, R. M.

1952. Certain molts and plumages of Acadian and Yellow-bellied flycatchers. Auk, 69(3):273-283, July 7.

Miller, A. H.

1941. Speciation in the avian genus Junco. Univ. California Publ. ZoÃ¶l., 44(3):173-434, May 24.

Miller, A. H., andT. T. McCabe

1935. Racial differentiation in Passerella (Melospiza) lincolnii. Condor, 37(3):144-160, May-June (May 15).

PalmÃ©n, J. A.

1876. Ueber die Zugstrassen der VÃ¶gel. Leipzig, Wilhelm Engelmann. Pp. VI + 292 + I.

Peters, J. L., andL. Griscom

1938. Geographical variation in the Savannah Sparrow. Bull. Mus. Comp. ZoÃ¶l., 80(13):445-478, January.

Pitelka, F. A.

1946. Age in relation to migration in the Blue Jay. Auk, 63(1):82-84, January 25.1951. Speciation and ecologic distribution in American jays of the genus Aphelocoma. Univ. California Publ. ZoÃ¶l., 50(3):195-464, July 20.

1946. Age in relation to migration in the Blue Jay. Auk, 63(1):82-84, January 25.

1951. Speciation and ecologic distribution in American jays of the genus Aphelocoma. Univ. California Publ. ZoÃ¶l., 50(3):195-464, July 20.

Rintoul, L. J., andE. V. Baxter

1914. Notes on some passerine birds found migrating in moult. Scottish Naturalist, no. 35, pp. 245-252, November.

Rowan, W.

1926. Notes on Alberta waders included on the British list. Part II. Brit. Birds, 20(2):34-42, June 1.

Ruth, E. L.

1952. The Bay-breasted Warbler in Kansas. Kansas Orn. Soc. Bull., 3(3):18-19.

Sibley, C. G.

1950. Species formation in the Red-eyed Towhees of Mexico. Univ. California Publ. ZoÃ¶l., 50(2):109-194, November 24.

Spofford, W. R.

1949. Mortality of birds at the ceilometer of the Nashville airport. Wilson Bull., 61(2):86-90, June.

Stone, W.

1906. Some light on night migration. Auk, 23(3):249-252, July.

Storer, R. W.

1951. Variation in the Painted Bunting (Passerina ciris), with special reference to wintering populations. Occas. Papers Mus. Zool., Univ. Michigan, no. 532, pp. 1-12, June 29.

Thomson, A. L.

1926. Problems of bird-migration. Boston and New York, Houghton Mifflin Company. Pp. XV + I + 350.1936. Recent progress in the study of bird-migration: a review of the literature, 1926-35. Ibis, 13th ser., 6(3):472-530, July 1.

1926. Problems of bird-migration. Boston and New York, Houghton Mifflin Company. Pp. XV + I + 350.

1936. Recent progress in the study of bird-migration: a review of the literature, 1926-35. Ibis, 13th ser., 6(3):472-530, July 1.

Ticehurst, C. B.

1916. Notes on migrants and moult, with special reference to the moults of some of our summer visitants. Scottish Naturalist, no. 50, pp. 29-38, February.

Tordoff, H. B.

1952. Notes on plumages, molts, and age variation of the Red Crossbill. Condor, 54(4):200-203, July-August.

Wiegold, H.

1926. Masse, Gewichte und Zug nach Alter und Geschlecht bei HelgolÃ¤nder ZugvÃ¶geln. Wissenschaftliche Meeresuntersuchungen, Abt. Helgoland, Neue Folge, 15ter Band, Heft 3, No. 17, Lipsius & Tischer, Kiel und Leipzig, pp. 1-73.

Wolfson, A.

1954. Weight and fat deposition in relation to spring migration in transient White-throated Sparrows. Auk, 71(4):413-434.

Transmitted June 30, 1955.

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