Nest Success and Age-Specific Mortality in Traill's Flycatchers

THIS paper presents nest success, factors affecting nest success, agespecific mortality from egg-laying until fledging, and a crude estimate of the mean number of fledglings produced per pair in Traill's Flycatchers (Empidonax t. trailIll).I studied a total of 106 nests in five summers from 1963-1967 at Toledo, Ohio, 1963-65; Fremont, Nebraska, 1966; and Wooster, Ohio, 1967.I visited nests at least once daily and tried to find them before construction or egg-laying was completed.The habitat in Toledo was old fields and ditch banks, and was the same area in which I studied American Goldfinches (Spinus tristis) (Holcomb, 1969a).I measured nest heights to the nearest 0.5 inch from the ground to the top rim of the nest.Unhatched eggs were opened within 2 days after the other nestlings hatched to determine at what stage embryonic development ended.RESULTS AND DISCUSSION Nest success.--Ifound 91 nests in which the number of eggs laid and numbers of young fledged could be determined exactly.The 91 nests had 272 eggs from which 99 young fledged for a total success of 36.4 percent.Of 272 eggs, 149 or 54.8 percent hatched.Factors involved in reduction of nest success were: 6 infertile eggs, 3 embryos interrupted in development, 96 eggs removed by predators, 41 nestlings removed by predators, 8 eggs and 1 nestling destroyed by rain and windstorms, 7 nestlings that starved when abandoned by adults (at least one adult of two pairs probably died or was caught by a predator), 4 eggs not hatching because of Brown-headed Cowbird (Molothrus ater) egg, 3 eggs lost through weak nest construction, and 4 eggs abandoned because of human interference.