title: Comparing food limitation among three stages of nesting: supplementation experiments with the burrowing owl.
abstract: Food availability is an important limiting factor for avian reproduction. In altricial birds, food limitation is assumed to be more severe during the nestling stage than during laying or incubation, but this has yet to be adequately tested. Using food-supplementation experiments over a 5-year period, we determined the degree and timing of food limitation for burrowing owls (Athene cunicularia) breeding in Canada. Burrowing owls are an endangered species and food limitation during the nestling stage could influence reproductive performance of this species at the northern extent of their range. Supplemented pairs fledged on average 47% more owlets than unfed pairs, except during a year when natural food was not limiting (i.e., a prey irruption year). The difference in fledgling production resulted from high nestling mortality in unfed broods, with 96% of all nestling deaths being attributed to food shortage. Supplemental feeding during the nestling period also increased fledgling structural size. Pairs fed from the start of laying produced the same number of hatchlings as pairs that received no supplemental food before hatch. Furthermore, pairs supplemented from egg laying to fledging and pairs supplemented during the nestling period alone had the same patterns of nestling survival, equal numbers of fledglings, and similar fledgling mass and structural size. Our results provide empirical support for the hypothesis that the nestling period is the most food-limited phase of the breeding cycle. The experimental design we introduce here could be used with other altricial species to examine how the timing of food limitation differs among birds with a variety of life-history strategies. For burrowing owls, and other species with similar life histories, long-term, large-scale, and appropriately timed habitat management increasing prey abundance or availability is critical for conservation. Our results provide empirical support for the hypothesis that the nestling period is the most food-limited phase of the breeding cycle. For burrowing owls, and other species with similar life histories, long-term, large-scale, and appropriately timed habitat management increasing prey abundance or availability is critical for conservation.
authors.affiliation: Department of Biological Sciences, University of Alberta T6G 2E9, Edmonton, Canada ; Canadian Wildlife Service Room 200, 4999-98th Ave., T6B 2X3, Edmonton, Canada.
authors.forename: Troy I
authors.initials: TI
authors.lastname: Wellicome
authors.affiliation: Department of Biology, University of Regina S4S 0A2, Regina, Canada.
authors.forename: L
authors.initials: L
authors.lastname: Danielle Todd
authors.affiliation: Department of Biology, University of Regina S4S 0A2, Regina, Canada ; Royal Saskatchewan Museum 2340 Albert St., S4P 2V7, Regina, Canada.
authors.forename: Ray G
authors.initials: RG
authors.lastname: Poulin
authors.affiliation: Canadian Wildlife Service Room 200, 4999-98th Ave., T6B 2X3, Edmonton, Canada.
authors.forename: Geoffrey L
authors.initials: GL
authors.lastname: Holroyd
authors.affiliation: Department of Biological Sciences, University of Alberta T6G 2E9, Edmonton, Canada.
authors.forename: Ryan J
authors.initials: RJ
authors.lastname: Fisher
keywords: Brood reduction; fledgling production; fledgling size; grassland; nestling survival; raptor; timing food limitation