Behavioral Ecology and Population Status of Wood Thrush and Ovenbird in Great Smoky Mountains National Park.

Abstract

Population declines of Neotropical migratory landbirds are attributed primarily to habitat fragmentation, higher rates of predation, and brood parasitism. These findings have stimulated many studies of avian reproductive success and comparisons of the source-sink dynamics of avian populations in fragmented and contiguous forests. Limited demographic data often impose a number of simplifying assumptions on source-sink models of forest passerines, such as assumptions about the number of possible breeding attempts, adult and juvenile survival rates, and pairing success.

In 1999-2001, I studied the relationships between food availability, predation risk, reproductive success, demography, and parental behavior of Ovenbird (Seiurus aurocapillus) and Wood Thrush (Hylocichla mustelina) populations in Great Smoky Mountains National Park. I monitored 178 Wood Thrush and 110 Ovenbird nests, ascertained the pairing status of 326 Ovenbird males, marked and identified the age of 30 reproducing Ovenbird females, and sampled parental behavior of the focal species at 50 food-supplemented nests and 62 control nests during 283 four-hour observational sessions conducted at three times of day and three standardized nestling ages.

For Ovenbirds, I estimated pairing success at 60%, daily nest survival rate at 0.95, annual survival of adult females at 0.63, of juvenile females at 0.32, annual fecundity at 0.96 female offspring per breeding female, and a finite rate of population increase (lambda) of 0.94. However, such lambda-estimate is erroneous, because Ovenbird populations in the park do not appear to be rapidly declining sinks. Neither do they appear to be fast growing sources, so the most likely scenario is a population at equilibrium, or a moderate population sink. In either event, my findings suggest that this large unfragmented tract of presumed high quality forested habitat does not appear to function as a significant population source.

I developed a population viability model for the Ovenbird with varying rates of pairing success, renesting, and double brooding. Model simulations yielded lambdas close to 1 only at high rates of pairing success and renesting after nest failure, and a double brooding rate of 0.33. I propose that at the southern limits of Ovenbird distribution, double brooding may occur at higher rates, than previously thought, and may compensate for its low annual fecundity.

I developed a conceptual model linking parental care of Wood Thrushes and Ovenbirds to their reproductive success and food availability. My major findings were similar for both species. Daily nest survival rates were significantly higher in food-supplemented (treatment), than in control nests. The nestling period of food-supplemented nests was shorter than of control nests, which reduced the exposure of treatment nests to predation. Treatment nests showed much higher productivity, than control nests. Nestlings at treatment nests were heavier prior to fledging, despite the fact that feeding rates at treatment and control nests were similar. Parental attendance was significantly higher at food-supplemented nests than at control nests.

I conclude that parental behavior, mediated by food availability, has adaptive significance in Wood Thrushes and Ovenbirds because it improves their reproductive success when food is abundant.

Food supplementation is rarely applied to ground-foraging insectivorous passerines because of the practical difficulties. I provided mealworms at feeding stations made of plastic transparencies covered with a thin layer of green moss. Only 16% of breeding pairs of Wood Thrush and Ovenbird failed to use supplemental food. Only minor amounts of mealworms were taken by non-target consumers. I conclude that my method is effective for the focal species, and its applicability to other ground-foraging insectivorous passerines should be tested in the field.