McGILL BIRD OBSERVATORY
Notes: Finally we had a truly wintry week, cold enough to keep us from putting up the nets this week. The only noteworthy birds of the week were a flock of 25 Common Redpolls and a group of 95 Bohemian Waxwings, both seen on Thursday.
Instead of banding, three of us took advantage of the break to pay a visit to the Canadian Museum of Nature collections in Gatineau, where curator Michel Gosselin kindly provided us the opportunity to examine and measure hundreds of specimens. While some degree of human error is always possible, museum specimens are generally accurately aged and sexed (by examination of internal organs), thus providing a good reference for exploring age and sex differences.
For this visit we focused on three of the top 10 most frequently banded birds at MBO - species that have relatively subtle differences between ages and/or sexes, and for which we would like to develop more accurate and reliable methods of identification.
Male and female Black-capped Chickadees have (virtually?) identical plumage, and sex is generally assessed only during the breeding season. However, it has been suggested by that both wing chord and tail length are shorter in females. Our measurements agreed with this, but whether the differences are enough to be significant, let alone be useful in the field, remains to be tested. While it has also been proposed that the width and shape of the bib varies by age and sex, this was not apparent among the museum specimens (though admittedly they weren't in a particularly natural position) and also has been difficult to assess in field trials to date, as the appearance varies considerably depending on the posture of the bird.
Male and female American Goldfinches are readily distinguished by plumage during the breeding season. Even in winter, they can separated quite well by the colour of their wings. However, we have noticed that all ages and sexes tend to have more extensive white patches on the tail than is documented in Pyle (1997), so we have been documenting this and other tail characteristics at MBO since fall 2005, and took this opportunity to collect baseline data at the museum for comparison with our field measurements. Among the interesting preliminary results are that 100% of individuals with an abrupt transition between the pale spot and the rest of the tail were after-hatch-year birds, and that tail colour closely matches wing colour, and is thus also a strong predictor of sex, with 97% of individuals in the sample with black tails being males, and 100% with pale brown tails being females.
Finally, we examined a sample of Magnolia Warblers, as there is no accepted way of distinguishing male and female hatch-year birds. Our measurements included wing chord, the length of the white spot on each of r2 and r3 (the second and third feathers from the centre of the tail), and a qualitative assessment of the edging of the uppertail coverts. Again, preliminary results show that females are smaller, and consistently have less white on the tail, but there appears to be a fair amount of variation. The colour of the uppertail covert edging showed a fairly predictable pattern, and we plan to test how reliable a predictor it is, in combination with the other measurements.
Reference: Pyle, P. 1997. Identification Guide to North American Birds, Part 1. Slate Creek Press, Bolinas CA.
© 2002- The Migration Research Foundation Inc.