We continuously recorded temperatures of the nest cup (Tn) and air (Ta) just outside the nest box throughout development of Tree Swallow (Tachycineta bicolor) chicks in east-central Minnesota. Brood size was manipulated (three or six chicks per nest) to study the effect of number of chicks in the nest on its thermal environment. From day 0 to day 4, Tn paralleled Ta, approximately 2–7°C higher, with nocturnal Tn maintained at 24°C and daytime Tn maintained at 26°C. From day 6 to day 12, Tn was relatively constant throughout the day, maintained at 32°C (day) and 29°C (night). The Tn became increasingly independent of Ta, as determined by regression analysis of Tn versus Ta with age. Nocturnal Tn of larger clutches (six chicks) was ∼2°C warmer than smaller clutches (three chicks), and Tn of larger clutches exhibited greater independence from Ta at day 10 than in smaller clutches. The occurrence of nest homeothermy at day 10 in six-chick nests correlates with near-maximal body mass of chicks and completion of feather insulation. Small (three-chick) clutches showed greater dependence of Tn on Ta at 10–12 days of age than large (six-chick) clutches; we propose that development of thermoregulatory capacity may have proceeded more slowly in chicks from those nests.
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Vol. 119 • No. 3