title: Seasonal patterns in reproductive success of temperate-breeding birds: Experimental tests of the date and quality hypotheses.
abstract: For organisms in seasonal environments, individuals that breed earlier in the season regularly attain higher fitness than their late-breeding counterparts. Two primary hypotheses have been proposed to explain these patterns: The quality hypothesis contends that early breeders are of better phenotypic quality or breed on higher quality territories, whereas the date hypothesis predicts that seasonally declining reproductive success is a response to a seasonal deterioration in environmental quality. In birds, food availability is thought to drive deteriorating environmental conditions, but few experimental studies have demonstrated its importance while also controlling for parental quality. We tested predictions of the date hypothesis in tree swallows (Tachycineta bicolor) over two breeding seasons and in two locations within their breeding range in Canada. Nests were paired by clutch initiation date to control for parental quality, and we delayed the hatching date of one nest within each pair. Subsequently, brood sizes were manipulated to mimic changes in per capita food abundance, and we examined the effects of manipulations, as well as indices of environmental and parental quality, on nestling quality, fledging success, and return rates. Reduced reproductive success of late-breeding individuals was causally related to a seasonal decline in environmental quality. Declining insect biomass and enlarged brood sizes resulted in nestlings that were lighter, in poorer body condition, structurally smaller, had shorter and slower growing flight feathers and were less likely to survive to fledge. Our results provide evidence for the importance of food resources in mediating seasonal declines in offspring quality and survival.
authors.affiliation: Department of Biology University of Saskatchewan Saskatoon SK Canada; Present address: Government of Manitoba Wildlife and Fisheries Branch Winnipeg MB Canada.
authors.forename: Vanessa B
authors.identifier: 0000-0002-3524-6444
authors.initials: VB
authors.lastname: Harriman
authors.affiliation: Ecosystem Science and Management University of Northern British Columbia Prince George BC Canada.
authors.forename: Russell D
authors.initials: RD
authors.lastname: Dawson
authors.affiliation: Department of Biology University of Saskatchewan Saskatoon SK Canada; Present address: Institute for Wetland and Waterfowl Research Ducks Unlimited Canada Stonewall MB Canada.
authors.forename: Lauren E
authors.initials: LE
authors.lastname: Bortolotti
authors.affiliation: Department of Biology University of Saskatchewan Saskatoon SK Canada; Environment and Climate Change Canada Prairie and Northern Wildlife Research Centre Saskatoon SK Canada.
authors.forename: Robert G
authors.initials: RG
authors.lastname: Clark
keywords: Tachycineta bicolor; food supply; hatching date; offspring quality; parental quality; tree swallow