Nestling growth and thermoregulatory development in subalpine dusky flycatchers

Auk, The, Jan 2001 by Pereyra, Maria Elena, Morton, Martin L

ABSTRACT.-The Dusky Flycatcher (Empidonax oberholseri), a small, open-nesting Neotropical migrant, is a relatively common summer inhabitant of subalpine environments in the western and southwestern United States. Nestling growth and development of thermoregulation were studied in a population at Tioga Pass (3,000 in) in the eastern Sierra Nevada of California. Despite the selective advantages that might be expected from accelerating rates of growth and thermoregulatory development, in an environment where the season was relatively short and cold temperatures and inclement weather were common, there was little evidence of adaptive modification in either of those parameters; growth rates were comparable to those of other tyrant flycatchers. Feather eruption began around day 5, brushing in dorsal and ventral tracts began two days later, and brushing of the flight feathers around day 9. Asymptotic body masses were attained around day 12, but nestlings did not fledge for another 4 to 5 days. The physiological development of endothermy was closely correlated with increases in body mass and accompanying decreases in surface-area-to-volume ratio and age, but was not significantly correlated with changes in plumage development, when mass and age were statistically controlled. On exposure to ambient temperatures between -3 and 5(deg)C for 10 min (roughly twice as long as the average female bout of inattentiveness), individual nestlings were unable to prevent deep hypothermia until after day 7. Homeothermy, at 80% of adult levels, was not attained until after day 12, although broods of nestlings maintained relatively stable body temperatures as early as day 5. Whereas clutch sizes in this population generally ranged from two to four eggs, the most stable thermal environments were provided by broods of three or four chicks. Although broods of nestlings were generally able to maintain temperatures within 5(deg)C of adult levels, temperature profiles during storms indicated a remarkable capacity for nestlings to tolerate temperatures in the nest as low as 15(deg)C for periods as long as 4 h without apparent ill effect. Received 19 May 1999, accepted 8 August 2000.

IN MANY VERTEBRATES, accelerated growth is positively correlated with vulnerability during the juvenile stage (Williams 1966, Case 1978). In birds whose young develop in relatively exposed nest sites, this is often reflected in early thermoregulatory development and early mobility (Morton et al. 1972, Austin and Ricklefs 1977, Finch 1984). Altricial birds are particularly vulnerable during the nestling period, depending completely upon parental care for warmth, food, and protection from the elements. To escape predators or weather, chicks of open-nesting species must quickly develop in size and degree of neuromuscular development and mature physiological function that is necessary for survival outside the nest. The ability to maintain elevated body temperatures is an important correlate of this, enabling efficient physiological responses and the rapid, coordinated movements needed for locomotion and feeding. Homeothermy tends to occur at a lower percentage of asymptotic body mass than in cavity nesters of similar adult body size (Visser 1998). Emerging adult capabilities in those areas do not come without costs, however. The energy invested in activities such as thermoregulation is unavailable for growth and tissue development, as indicated by the slower growth rates that occur in tissues with a high degree of mature function (Ricklefs 1979, Visser and Ricklefs 1993, Hohtola and Visser 1998, Ricklefs et al. 1998).

The geographical trends for rapid growth in temperate-zone birds, as compared with tropical and subtropical species (Nice 1954, Lack 1968, Ricklefs 1968, Klaasen and Drent 1991) and in species breeding in areas where they are relatively exposed to predators or weather, form the basis of the view advocated first by Lack (1948, 1954, 1968) and others since (Ricklefs 1968), that environmental factors are the driving selective force behind those patterns. Further, those ecological hypotheses postulate that growth rates are shaped by environmental pressures such as predation, weather, and food supply, largely through their effects on nestling mortality (O'Connor 1977, 1978; Ricklefs 1983).

Tests of those ideas are often complicated, however, by the taxonomic biases that accompany geographical comparisons (see, for example, Ricklefs et al. 1998). Regional comparisons between growth rates in temperate and tropical zones can be biased by the zoogeographical dominance of some families in certain regions (Vuilleumier 1975). Shared attributes within families often extend to types of nest sites, foraging modes. and anatomical specializations for feeding and locomotion (see, for example, Traylor and Fitzpatrick 1982, for the Tyrannidae) further obscuring effects of environmental factors on development. Perhaps in part due to those factors, attempts to test the validity of the ecological hypotheses have sometimes yielded equivocal results. In some studies, there is evidence to support the idea that rates of mortality and development are related (Bosque and Bosque 1995, Ricklefs et al. 1998); in others, fledging age but not growth rates appear to be related to nestling mortality rates (Maher 1964; Ricklefs 1969a, b; King and Hubbard 1981); and in still others, there is no support at all (Ricklefs 1969a, c, 1976, 1982). As a result, alternative hypotheses have been developed to explain interspecific variability in growth. Of those, one set revolves around constraints on development imposed by energetics of parental effort (Ricklefs 1968, Drent and Daan 1980), and another focuses on potential effects of anatomical and physiological constraints upon development (Ricklefs 1969c, 1983, 1984; Ricklefs et al. 1998).