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Saturday, August 29, 2009

Goldfinch Parents Winter in the South While Kids Remain Behind

Goldfinches are the only finch species that go through a complete color change molt two times a year. In the fall they take off their bright yellow, black, and white coat and switch to a muted olive color.

In October the Goldfinches separate into two groups based on age. Studies show that the birds hatched this year will generally stay in Michigan for the winter but their parents will go further south to winter. One thought is that the first year finches didn’t have to go through a molt and have more energy to survive a winter.

Just the opposite was found with Blue Jays. Studies show that the first year Blue Jays go further south to winter in a more plentiful habitat. While their crafty parents, perhaps knowing several survival tactics, stay the whole year.


Julie Craves said...

Hmm. This may apply to areas further north than Michigan, at least southern Michigan. Of 1200+ goldfinches banded in Dearborn Nov through mid-Feb, about 40% were adults. Given that young should theoretically outnumber adults by 2 to 4 times as winter sets in (based on young birds entering the population, our findings indicate the number of young birds here is about as expected, perhaps even a little low.

We actually started looking at goldfinches because of the number of recaptured birds we had between all seasons, when published studies indicated that northern populations migrated. It appears this must not be true across the board. This is work we hope to publish in the future.

Wild Birds Unlimited Mid-Michigan said...

Thank you for your comment. I took my information from a study called "Age and sex differences in winter distribution of American. Goldfinches in eastern North America" http://www.jstor.org/pss/3676804
The following is an abstract:
Banding data for 1975-1985 were used to determine the latitudinal distribution of sex and age (males only) classes of the American Goldfinch during winter in eastern North America. Males wintered north of females, and among males, immatures wintered farther north than adults. Analysis of recovery data suggested that age and sex differences in winter distribution resulted from differences in distance migrated from the breeding grounds. The results are discussed in light of existing hypotheses proposed to explain differential migration. Geographic differences in the winter distribution of the sexes are qualitatively consistent with predictions of both the body size and social dominance hypotheses, but neither these, nor the arrival time hypothesis, can explain why male goldfinches make longer migrations from the breeding grounds as adults. We propose that young goldfinches may be physiologically or behaviorally incapable of making long migrations during their first autumn.

The Blue Jay info is from a 1992 article in by Bill Hilton Jr in WildBird called "Boisterous blue jays": The following is an quote from the article that can be found at http://www.hiltonpond.org/ArticleJayBlueMain.html:"Although my supporting data are rather sparse, I suspect many young Blue Jays from the northernmost part of their range in Canada and New England migrate southward, while older birds more adept at finding food and surviving cold weather are likely to overwinter in the same region where they were bred."

Julie Craves said...

Yes! Those goldfinch data are part of what prompted us to look at our age ratios, and seasonal patterns of recaptures. Either there are further regional differences not captured in these studies, and/or (given much of the data is from 20-35 years ago) something climate-related may be occurring. There is always something new to learn, even with common species.

Blue Jay data we don't have as much of, but I suspect Bill's theory is true in at least part of their range.