WEEKLY TOTAL OF ROBINS REPORTEDWEEKLY TOTAL SNOWFALL (INCHES)WEEKLY AVERAGE OF DAILY MINIMUM TEMP. (F.)
WEEKLY TOTAL OF ROBINS REPORTED
WEEKLY TOTAL SNOWFALL (INCHES)
WEEKLY AVERAGE OF DAILY MINIMUM TEMP. (F.)
From thefirst graphit is readily apparent that few Robins were noted in the province prior to the middle of January. After that time the number seen increased rapidly, reaching its maximum about February 1, and decreasing a little more gradually until about February 20, after which date few Robins were seen. On account of the scarcity of observers, before mentioned, this line does not show the total number of Robins which were present about the inhabited parts of Nova Scotia in any week, nor can its relation to such total numbers be readily determined. It does serve, however, as a moderately correct indicator of the relative abundance of the Robins about the inhabited parts of the province in one week as compared with another.
Thegraph indicating the weekly snowfallappears as a line of abrupt changes and sharp angles, showing that the variation in the snowfall from week to week was very marked. Somewhat contrary to expectation, no relation between this line and the Robin graph appears to be traceable. It is possible that, if the average depth of snow on the ground in each week could be depicted graphically, the line thus formed would show more direct relation to the weekly abundance of Robins, but, unfortunately, no data from which such a graph could be prepared are available.
Thetemperature graphappears to correspond very well with the slopes of the Robin graph, especially in the part of the winter prior to February 20. A period of low temperature in the week ending January 5 is found to correspond with a noticeable increase in the number of Robins reported, while higher temperature during the week ending January 12 accompanies a decrease in the number of Robins seen. From January 12 to February 2 increasingly lower average temperatures are contemporaneous with an increasing abundance of Robins observed, and the extremes of both graphs are reached in the same week. In the week ending February 9 both lines fall slightly lower, and in the next week there is a very considerable decline in both. From that time on the relationship appears less close, for a reason hereinafter stated. Such a close correspondence between the two lines as has been pointed out, however, seems most unlikely to be wholly fortuitous, and would appear to indicate that temperature is a greater factor than had been supposed in causing these birds to seek the neighborhood of man.
The question as to why these Robins were so commonly observed in Nova Scotia last winter is one which at present does not seem to be capable of definite answer, for too many of the possible contributory causes are unknown. Some efforts toward a solution of the problem are, however, here submitted.
In the first place, it would appear fair to presume that these Robins were not, as was popularly supposed, misguided arrivals from the south at an unusually early date. It seems probable that they had remained in Nova Scotia, or in regions still further north, from the time of the fall migration until the time when they were seen here. The fact that few were seen between December 1 and the middle of January is explainable by the supposition that during that time they were living in the deep woods, miles from any human being except an occasional Indian or a gang of lumbermen, and that they were then more widely scattered. In the woods at that time large quantities of juniper berries and mountain ash berries would be available for their food supply.
Whether more Robins than usual remained in Nova Scotia in this way last fall seems an open question. Mr. R. W. Tufts, of Wolfville, N. S., in a letter dated February 13, 1918, which was published in the Halifax “Morning Chronicle” of February 15, gives it as his opinion that there was no unusual number of Robins in the province last winter. He attributes the great number of Robins seen in the province at that season solely to the fact that the snowfall was heavier than usual, which, he says, covered the juniper bushes which supplied the Robins with most of their usual winter food, and so forced them to seek sustenance in the inhabited areas of the province, where they were more easily observed. In opposition to this theory it should be noted that the snowfall of last winter, though heavy, was not of a record-breaking character, while I am informed by Mr. Harry Piers, Curator of the Nova Scotia Provincial Museum, and a veteran Nova Scotian ornithologist, that the abundance of Robin observations during the winter of 1917-18 is, so far as is shown by his records or memory, absolutely without parallel. I have experienced some difficulty in obtaining records of snowfall for years other than the more recent ones, but the monthly snowfalls at Halifax for the winter of 1904-05, for instance, compare with those of the winter of 1917-18 as follows.
Although the totals for the two winters are practically alike, yet it will be observed that by February 1, 1918, after a snowfall of 48.5 inches in December, 1917, and January, 1918, Robins were observed as fairly common throughout Nova Scotia, whereas a snowfall of 72.2 inches in December, 1904, and January, 1905, appears to have caused no unusual observations of Robins in the province, nor is there record, so far as I can discover, of any larger number of these birds than usual being seen here at any time that winter. These facts would seem to tend to show either that in the winter of 1917-18 an unusual number of Robins did remain in this part of Canada, or that their appearance in the settled parts of the country was due to other causes than the heavy snowfall, or that both of these hypotheses are true.
It has been suggested to me by Prof. E. C. Allen, of Truro, N. S., that many of the Robins seen in Nova Scotia this winter may have spent the first part of the winter outside of this province, in the neighboring, wilder regions to the northward. In proposing this theory he says, “Granting that scattered Robins do remain [in winter] in regions north of Nova Scotia (a fact concerning which I have no evidence), would not the continued cold weather tend to drive them south, and, owing to the contour of the coast, might they not hesitate to cross the water south of us in winter, and therefore be more or less congested here?... It might be argued that Robins would not hesitate to cross the Atlantic strip of water south of us, as many thousands do cross in the fall. On the other hand, might it not be possible that in winter the migratory instinct might not be sufficiently strong to carry them straight out to sea over rough water?” There is need of data from New Brunswick, Prince Edward Island, and Newfoundland concerning winter Robins to throw additional light on this interesting theory.
If the number of Robins which remained here last winter was greater than usual, the cause of this condition is wholly problematical. I have not had such opportunities as I desire for observing the abundance ofjuniper berries and mountain ash berries in the wilder parts of Nova Scotia last fall or this spring, but no unusual abundance or scarcity of Robin food has been revealed by such observations as I have been able to make. It may be that the migratory instinct failed last fall in a greater number of Robins than usual, and thus more of them were influenced to remain here, or it may be that subtle meteorological forces caused a change in the migration of some of these birds.
It has already been noted that low temperatures seem to have accompanied the appearance of the Robins. In what way the temperature may have caused the Robins to seek the inhabited districts I cannot say, unless it might be by temporarily congealing the surface of swampy and springy areas, which ordinarily remain open in winter weather, and from which the Robins may have obtained food when the rest of the country was covered with snow. Further investigation appears to be much needed here. While considering temperature, it is worthy of note that the past winter was exceptional for one other thing besides the unusual numbers of Robins seen—that is, for its long, unbroken periods of low temperature. A direct relation between these two phenomena may be suggested. In other parts of northern North America this low temperature seems to have caused an unusual scarcity of winter birds, but that was not the case here.
After February 25, although the weather remained severe, there appear to have been no observations of Robins in the province until the arrival of the first spring migrants, noted at Halifax on March 26. This may be due to the birds’ having finally left us for a more congenial climate, but I am strongly inclined to believe that it was caused by the destruction of practically all the Robins in the province, their last available supplies of food having been exhausted. This would account for the disagreement between the Robin graph and the temperature graph after February 20. Although only two dead Robins, other than those killed by an owl, were reported, yet scarcely more than this would be expected, since most of the birds would probably die in out-of-the-way places, and would soon be covered by snow or devoured by animals.
It is hoped that the facts and suggestions here presented may throw some light on the subject of winter Robins and perhaps help to pointout some new lines of inquiry, so that before long additional observations and investigations may make the full truth of the matter clear. The observations of the winter of 1917-18 were unusual, but it is often by a study of the unusual that the usual is understood.