We continuously recorded temperatures of the nest cup (T_n) and air (T_a) 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, T_n paralleled T_a, approximately 2–7°C higher, with nocturnal T_n maintained at 24°C and daytime T_n maintained at 26°C. From day 6 to day 12, T_n was relatively constant throughout the day, maintained at 32°C (day) and 29°C (night). The T_n became increasingly independent of T_a, as determined by regression analysis of T_n versus T_a with age. Nocturnal T_n of larger clutches (six chicks) was ∼2°C warmer than smaller clutches (three chicks), and T_n of larger clutches exhibited greater independence from T_a 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 T_n on T_a 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.