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Format:
Print
Author:
Dombrowski, Peter G.
Dept./Program:
Wildlife and Fisheries Biology Program
Year:
2008
Degree:
M.S.
Abstract:
The normal diet of many birds, including most insect- and seed-eating birds, contains insufficient calcium when nestling skeletons are mineralizing. During these times, parents must seek out calcium-rich prey or other calcium-rich objects and feed them to nestlings. Such sources of calcium are rare in naturally base-poor environments that have been depleted of calcium by acid precipitation and other causes. In some cases, calcium is scarce enough to limit reproduction of birds, but the prevalence of such limitation is unknown. As an insectivorous bird that does not normally forage on the ground, and whose range includes calcium-depleted forest, the Black-throated Blue Warbler may be at risk for calcium limitation. I carried out a calcium supplementation in a calcium-depleted forest in central Vermont to test the hypothesis that nestlings in a population of Blackthroated Blue Warblers are limited by dietary calcium availability. There was no effect of the supplementation on nestling mass, tarsus length, wing measurements, or beak measurements.
The main calcium source for these warblers is snails, but during dry weather snails retreat to lower layers of leaf litter where they are unavailable to surface foraging Blackthroated Blue Warblers. Data on temporal variation in the availability of snails at the same site showed that snail availability is influenced by time since a rain event and by weather variables relating to evaporation. Weather variables were not useful for predicting nestling fecal calcium concentration (an indicator of snail consumption), and fecal calcium was not useful as predictor of nestling growth parameters. Additional fecal samples from nestlings at Hubbard Brook, a similarly calcium-depleted forest in New Hampshire, confirmed the lack of a relationship between weather, fecal calcium, and nestling growth. Snail availability on the surface of the forest floor is apparently not a limiting factor for Black-throated Blue Warblers and does not prevent them from providing sufficient calcium to nestlings.