Thermal environment of the nest during development of tree swallow (Tachycineta bicolor) chicks

Auk, The, Jul 2002 by Chaplin, Susan B, Cervenka, Mora L, Mickelson, Alison C

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

RESUMEN.-Registramos continuamente la temperatura de la copa del nido (T^sub n^) y del aire (T^sub a^) inmediatamente exterior a la caja de nidificaci6n durante el periodo de desarrollo de polluelos de Tachycineta bicolor en el centro-este de Minnesota. Manipulamos el tamaho de la nidada (tres o seis poIluelos por nido) para estudiar el efecto del ndmero de polluelos en el nido sobre el ambiente termico de este. Desde el dia 1 al dia 4, T^sub n^ fluctuo en forma paralela con T^sub a^, pero fue aproximadamente 2-7 deg C mayor; la T^sub n^ nocturna se mantuvo a 24 deg C mientras que la diaria a 26 deg C. Desde el dfa 6 hasta el dfa 12, T^sub n^ fue relativamente constante a traves del dia, manteni6ndose a 32 deg C durante el dfa y a 29 deg C durante la noche.

Mediante un analisis de regresion de T^sub n^ versus T^sub a^ determinamos que la independencia entre T^sub n^ y T^sub a^ fue incrementando con el tiempo. La T^sub n^ nocturna de las nidadas grandes (seis polluelos) fue -2 deg C mas alta que la de las nidadas pequenas (tres polluelos), y al decimo dia la T^sub n^ de las nidadas grandes presento una mayor independencia de la T^sub a^ que la de las nidadas pequenas. La presencia de homeotermia en el decimo dia de los nidos con seis polluelos se correlacion6 con la masa corporal maxima de los polluelos y con la finalizaci6n del aislamiento por plumas. Con 1012 dias de edad, T^sub n^ mostro una mayor dependencia de la T^sub a^ en las nidadas pequenas (tres polluelos) que en las nidadas grandes (seis polluelos). Proponemos que el desarrollo de la capacidad termoregulatoria podria haber procedido mas lentamente en polluelos de nidadas mas pequenas.

Tree Swallows (Tachycineta bicolor) begin breeding in April in Minnesota and the first young hatch approximately 1 June, when air temperatures at night may range from 5-15 deg C. On cool days and during inclement weather, average daytime air temperatures at that latitude may only be 15-20 deg C, posing significant risk of cooling to newly hatched chicks without an attendant brooding parent. Perhaps because of the risk of hypothermia associated with early breeding at northern latitudes, Tree Swallows line their cavity nests of dry grass with feathers (Robertson et al. 1992). The importance of that feather insulation was documented by Lombardo et al. (1995), who found that Tree Swallow chicks from nests with artificially reduced feather insulation had lower growth rates and were smaller and less mature at day 12 (approximately two-thirds of the nestling period).

Few measurements of nest thermal environment during posthatch chick development have been reported. Calder (1971) recorded temperatures of artificial eggs inserted into two Calliope Hummingbird (Stellula calliope) nests by continuous recording during incubation, brooding of young chicks, and late postnatal development. However, most studies have focused on nest or egg temperatures during laying and incubation periods (Haftorn 1978, Cooper and Afton 1981, Zann and Rossetto 1991, Smith and Montgomerie 1992, Flint and Maccluskie 1995, Wilson and Verbeek 1995). This study reports results of continuous recording of nest cup and air temperature (at the surface of the nest box) throughout Tree Swallow nestling development and as a function of clutch size. On the basis of the premise that nest temperature is critical to nestling development, several questions have been investigated with these data. (1) How is nest temperature influenced by the ambient environment (air temperature, variation in air temperature, or time of day)? (2) Does the number of chicks in the nest affect the thermal environment of the nest during development? (3) How is nest temperature influenced by growth and maturation of chicks-that is, does the pattern of nest temperature change daily during the nestling period, specifically with changes in nestling biomass and development of endothermy?