L. Danielle Todd

Post-fledging survival of burrowing owls in Saskatchewan

Extensive research into the decline of the endangered burrowing owl (Athene cunicularia) has focused on breeding biology, but has largely ignored the post-fledging period. To determine whether post-fledging survival was an important factor in the decline we used radiotelemetry to monitor post-fledging juveniles in a migratory population of burrowing owls in southern Saskatchewan, Canada, during 1997-2000. As part of a concurrent study, 92% of the owl pairs nested in artificial nest boxes, and 64% received supplemental food from hatch until fledging. Survival of juvenile owls during the 3-month, post-fledging period was significantly higher in 1997 (1.00, n = 12) than 1998-2000 (0.55 [95% CI: 0.44 to 0.70], n = 64; P = 0.02). Unusually high survival during 1997 may have been related to the unusually high abundance of voles (Microtus spp.) that year. From 1998 to 2000, 27 of 64 radiomarked owls died. Predators caused the majority of post-fledging mortality (15 of 27), with juveniles at greatest risk the first 14 days after fledging. Other sources of mortality were related to anthropogenic factors (5 of 27) and starvation (5 of 27). Following each of 3 years with moderate juvenile post-fledging survival, the breeding population of burrowing owls declined 11-48% and contained correspondingly few yearling recruits (x = 2.1%). However, following the year of high post-fledging survival (1997), the breeding population increased by 32%, and yearling recruitment was substantially higher (8.3%). These results suggest that post-fledging survival has a considerable influence on population size in the subsequen breeding season, and that management strategies for this species of concern should include this life stage.

SEX AND NEST STAGE DIFFERENCES IN THE CIRCADIAN FORAGING BEHAVIORS OF NESTING BURROWING OWLS

Abstract ABSTRACT We used infrared cameras to accurately record the circadian provisioning behaviors of nesting Burrowing Owls (Athene cunicularia) in southern Saskatchewan, Canada. We monitored 19 nests over three years and recorded 4675 prey deliveries. We found a sex-based difference in foraging behavior: males hunted vertebrates during crepuscular periods, and females hunted insects during diurnal periods. Males delivered between 82% and 96% of all vertebrate prey depending on the stage of the nest. Males delivered at least 90% of all insects during early nest stages, after which females delivered an average of 76% of the insects. The rate of vertebrate deliveries increased from 1.3 to 7.7 per 24 hr as the summer progressed, and the number of insect deliveries increased from less than 1 to 18.6 per 24 hr. Vertebrates comprised 98%–99% of prey biomass delivered until females began delivering insects, but even then vertebrates still comprised 94% of prey biomass. Insects were consistently delivered at the highest rate during the day and vertebrates were consistently delivered at the highest rates during the dusk and dawn periods.

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.

Diet of Common Nighthawks (Chordeiles minor: Caprimulgidae) Relative to Prey Abundance

Abstract Optimal foraging theory predicts that when prey density is low, predators should employ a generalist feeding strategy and take prey in proportion to their abundance. The purpose of this study was to compare the diet of common nighthawks (Chordeiles minor: Caprimulgidae) with a measure of prey abundance. Relative to the proportion of insects available, nighthawks consumed Coleoptera and Hymenoptera more than expected while Diptera were avoided. Only Trichoptera and Lepidoptera were consumed in proportion to their abundance. A qualitative comparison of our results with data on nighthawk diets from the Okanagan Valley, British Columbia, suggests that the birds in the two locations have different diets but similar preferences relative to the abundance of different insect orders.

Apparent survival of adult Burrowing Owls that breed in Canada is influenced by weather during migration and on their wintering grounds

ABSTRACT Understanding factors that influence the survival of endangered migratory species is critical for making informed management decisions, yet this understanding relies on long-term recapture datasets for species that are, by definition, rare. Using 3 geographically widespread (Saskatchewan, Alberta, and Manitoba, Canada) and long-term (6–15 yr) mark–recapture datasets, we quantified spatial and temporal variation in apparent annual survival and recapture probabilities of Burrowing Owl (Athene cunicularia), an endangered species that breeds in Canada. We then examined how large-scale weather patterns during migration (storms) and on the wintering and breeding grounds (precipitation), in addition to prey irruptions on the breeding grounds, influenced apparent survival of Burrowing Owls. Female Burrowing Owls had lower apparent survival than males in all 3 study areas. Storms during fall migration and above-average precipitation on the wintering grounds were associated with reduced apparent survival of Burrowing Owls in the longest-running study area, Saskatchewan; in Alberta and Manitoba, there were few correlations between apparent survival of Burrowing Owls and weather or prey irruptions. Increases in stochastic events such as storms during migration or precipitation on the wintering grounds could have adverse consequences on the already small Burrowing Owl population in Canada. Local management actions that focus solely on improving adult apparent survival within Canada are likely insufficient for mitigating susceptibility of adults to inclement weather or other factors outside the breeding season, underscoring the need for management of this species across multiple jurisdictions within North America.