bird migration, Veery (Catharus fuscescens), Swainson’s thrush (Catharus ustulatus), Gray-cheeked thrush (Catharus minimus)
Migration is an energetically demanding and high-risk phase of the annual cycle of intercontinental migrant songbirds. Many accumulate large fat deposits to fuel their semiannual journey. However, the amount of migratory fat stored by birds may be subject to a trade-off. Fat deposition powers the birds’ migration and reduces the risk of starvation, but it increases the cost of carrying large fat loads and the risk of predation. I examined the relationship between the body size and fat deposition of Veery (Catharus fuscescens), Swainson’s thrush (Catharus ustulatus), and Gray-cheeked thrush (Catharus minimus) passing through northeastern Illinois during spring migration. My results contrasted strongly with the results of a previous study (Yong and Moore 1994), which found a negative relationship between the wing length and fat stores both within and among species of North American Catharus thrushes. Within species, I found a statistically significant positive relationship between the wing size and the energy stores of Swainson’s thrushes. Between species, I also found evidence of a positive relationship between wing length and fat deposition, with Gray-cheeked thrushes the longest-winged and fattest, and Veeries the shortest-winged and leanest. It is evident that the populations of Gray-cheeked thrushes and Veeries in my sample were morphologically different from the populations sampled by Yong and Moore (1994). These differences may be accounted for by geographic variations. Wing length is not negatively correlated with fat deposition in Catharus thrushes as Yong and Moore (1994) suggested. It is therefore still unclear what factors govern the deposition of fat by migratory Catharus thrushes. While our data provide some evidence that body size may be an important factor, with larger birds storing more fat, other factors such as migration distance and/or phylogenetic constraints may be important as well.
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