Troy I. Wellicome

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.

Does Doramectin Use on Cattle Indirectly Affect the Endangered Burrowing Owl

Abstract Doramectin is one of several endectocide compounds widely used to treat nematode and arthropod pests affecting cattle. Insecticidal residues in dung of endectocide-treated cattle can reduce numbers of dung-breeding insects. Concerns have been raised that use of endectocides may adversely affect birds that rely on dung-breeding insects as food. However, these concerns have not been specifically addressed in previous studies. We performed two studies to collectively assess whether doramectin adversely affects burrowing owls (Athene cunicularia Molina), which are listed as “Endangered” in Canada. In the first study, insect emergence was monitored from dung of cattle treated with a recommended topical dose of doramectin. Experiments replicated in each of 3 yr showed residues reduce the number of insects developing in dung of cattle treated up to 16 wk previously. In the second study, we identified prey items from regurgitated pellets collected at 206 burrowing owl nests in southern Alberta and Saskatchewan, Canada. A total of 50 213 prey items were identified, of which 90% were invertebrates. Beetles (Coleoptera) comprised 54% of the total prey items, followed next in abundance by grasshoppers (Acrididae, 20%) and crickets (Gryllidae, 16%). Of the beetles, 1 381 specimens were identified as breeding in dung (mainly species of Aphodius, Canthon, Onthophagus). The dung beetles comprised an estimated 2.8% of the total prey items or 0.1% of total prey biomass. Results of the first study validate initial concerns that doramectin use can reduce numbers of insects breeding in dung of treated cattle. Results of the second study show reliance of burrowing owls on dung beetles is sufficiently low that use of doramectin on cattle is unlikely to appreciably affect the food supply of co-occurring burrowing owls.

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.

Off-road sampling reveals a different grassland bird community than roadside sampling: implications for survey design and estimates to guide conservation

Grassland bird species continue to decline steeply across North America. Road-based surveys such as the North American Breeding Bird Survey (BBS) are often used to estimate trends and population sizes and to build species distribution models for grassland birds, although roadside survey counts may introduce bias in estimates because of differences in habitats along roadsides and in off-road surveys. We tested for differences in land cover composition and in the avian community on 21 roadside-based survey routes and in an equal number of adjacent off-road walking routes in the grasslands of southern Alberta, Canada. Off-road routes (n = 225 point counts) had more native grassland and short shrubs and less fallow land and road area than the roadside routes (n = 225 point counts). Consequently, 17 of the 39 bird species differed between the two route types in frequency of occurrence and relative abundance, measured using an indicator species analysis. Six species, including five obligate grassland species, were more prevalent at off-road sites; they included four species listed under the Canadian federal Species At Risk Act or listed by the Committee on the Status of Endangered Wildlife in Canada: Spragues Pipit (Anthus spragueii), Bairds Sparrow (Ammodramus bairdii), the Chestnut-collared Longspur (Calcarius ornatus), and McCowns Longspur (Rhynchophanes mccownii). The six species were as much as four times more abundant on off-road sites. Species more prevalent along roadside routes included common species and those typical of farmland and other human-modified habitats, e.g., the European Starling (Sturnus vulgaris), the Black-billed Magpie (Pica hudsonia), and the House Sparrow (Passer domesticus). Differences in avian community composition between roadside and off-road surveys suggest that the use of BBS data when generating population estimates or distribution models may overestimate certain common species and underestimate others of conservation concern. Our results highlight the need to develop appropriate corrections for bias in estimates derived from roadside sampling, and the need to design surveys that sample bird communities across a more representative cross-section of the landscape, both near and far from roads.

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.

Influence of vegetation on the nocturnal foraging behaviors and vertebrate prey capture by endangered Burrowing Owls

Restrictions in technology have limited past habitat selection studies for many species to the home-range level, as a finerscale understanding was often not possible. Consequently, these studies may not identify the true mechanism driving habitat selection patterns, which may influence how such results are applied in conservation. We used GPS dataloggers with digital video recorders to identify foraging modes and locations in which endangered Burrowing Owls (Athene cunicularia) captured prey. We measured the coarse and fine-scale characteristics of vegetation at locations in which owls searched for, versus where they caught, vertebrate prey. Most prey items were caught using hover-hunting. Burrowing Owls searched for, and caught, vertebrate prey in all cover types, but were more likely to kill prey in areas with sparse and less dense vegetative cover. Management strategies designed to increase Burrowing Owl foraging success in the Canadian prairies should try to ensure a mosaic of vegetation heights across cover types. Influence de la vegetation sur la recherche nocturne de nourriture et la capture de proies vertebrees par la Cheveche des terriers, espece en voie de disparition RESUME. Pour de nombreuses especes, les etudes anterieures sur la selection de l’habitat ont ete limitees a l’echelle du domaine vital en raison des restrictions technologiques de l’epoque, et la comprehension a une echelle plus fine n’etait souvent pas possible. Par consequent, ces etudes ne cernent sans doute pas le mecanisme veritable derriere les tendances dans la selection de l’habitat, influencant peut-etre du coup la facon avec laquelle ces resultats sont transposes en actions de conservation. Nous avons utilise des geolocalisateurs (GPS) et des cameras numeriques afin de determiner les techniques qu’adoptaient la Cheveche des terriers (Athene cunicularia), une espece en voie de disparition, pour capturer ses proies et d’identifier les sites de capture. Nous avons mesure les caracteristiques fines et grossieres de la vegetation aux sites ayant servi a la recherche pour les comparer a celles des sites dans lesquels les proies vertebrees ont ete capturees. La majorite des proies ont ete attrapees lors de vols stationnaires. Les cheveches ont cherche et attrape leurs proies vertebrees dans tous les types de couvert vegetal, mais etaient plus susceptibles d’en tuer aux endroits ou la vegetation etait clairsemee et moins dense. Les strategies d’amenagement destinees a augmenter le succes d’alimentation de la Cheveche des terriers dans les Prairies canadiennes devraient viser la presence d’une mosaique de vegetation de hauteurs differentes dans les divers types de couvert vegetal.

Landscape-scale features affecting small mammal assemblages on the northern Great Plains of North America

Abstract Little is known about the macrohabitat associations of rodents and shrews in prairie landscapes because of the logistic constraints of conventional trapping. We used the remains of 60,972 small mammals in owl pellets to assess factors affecting small mammal composition across 4.3 million hectares of the northern Great Plains of North America. Cropland with clay soils was dominated by deer mice (Peromyscus maniculatus), whereas areas with higher proportions of native grassland and moderately sandy soils supported communities with more sagebrush voles (Lemmiscus curtatus). Areas with clay soils and higher annual precipitation were associated with higher proportions of house mice (Mus musculus), meadow voles (Microtus pennsylvanicus), and shrews (Blarina brevicauda and Sorex species), whereas drier areas with sandier soils and lower annual precipitation were dominated by olive-backed pocket mice (Perognathus fasciatus) and northern grasshopper mice (Onychomys leucogaster). Contrary to extrapolations of previous smaller-scale efforts, soil texture was the primary landscape feature driving small mammal composition in our study, whereas agricultural cropland significantly altered the composition of these assemblages. These associations demonstrate the importance of considering macrohabitats encompassing entire populations.

Pre-Migratory Movements by Juvenile Burrowing Owls in a Patchy Landscape

Dispersal is a fundamental aspect of population dynamics, and can have direct implications on processes such as the colonization of habitat patches. Pre-migratory movements, landscape fragmentation, and body condition have all been hypothesized as key factors influencing dispersal in birds, but little direct evidence exists to support these ideas. We used radio-telemetry and supplementary feeding to test if body condition or landscape pattern influenced pre-migratory movements of juvenile Burrowing Owls (Athene cunicularia) in a fragmented landscape. We categorized grassland patches as either large (≥95 ha) or small and isolated ( ≤58 ha and ≥1.5 km to next nearest grassland patch), and young owls were either provided supplemental food as nestlings or not. Owlets receiving supplemental food and residing in large grassland patches moved a greater maximum distance from their nest than similarly fed owlets residing in small patches (large = 1605 ± 443 m; small = 373 ± 148 m). In contrast, non-supplemented owlets from large and small patches did not differ in their maximum distance moved from the nest (large = 745 ± 307 m; small 555 ± 286 m). Only two of 32 individuals from small patches moved >800 m, whereas ten of 23 owlets from large patches moved >800 m. In addition, owlets from large patches continued to move farther and farther from their nest before migration, whereas owlets in small, isolated patches ultimately moved <400 m from their nests before migration, despite having initiated dispersal in a manner similar to owlets from large patches. Our results, along with evidence we synthesize from previous studies, suggest that juvenile Burrowing Owls in small patches are unwilling or unable to cross the cropland matrix of a fragmented landscape.

Ferruginous Hawks Buteo regalis alter parental behaviours in response to approaching storms

AbstractHeavy and frequent rain, low temperatures, and strong winds may decrease adult foraging time, cause thermoregulatory stress on nestlings, and lead to nest damage or destruction, all of which can negatively affect breeding success. However, certain parental behaviours can mitigate these potentially negative effects of inclement weather. We examined how parents could mitigate these negative weather effects by adjusting three behaviours—nest attendance, prey deliveries, and nest maintenance—before, during, and after storms at 11 nests of the at-risk Ferruginous Hawk (Buteo regalis) in Canada. Female adults spent an increasing amount of time on the nest as wind speed increased. Similar numbers of prey were delivered before and after storms, suggesting that Ferruginous Hawks do not compensate for lost foraging time. They appeared to demonstrate an ability to detect approaching storms, possibly by responding to falling barometric pressure cues, and may have mitigated the risk of nest damage by increasing their nest maintenance behaviours. Our study is among the first to observe storm preparation behaviour, and indicates that some raptorial birds have the ability to alter nesting behaviour in response to approaching inclement weather.ZusammenfassungKönigsbussarde (Buteo regalis) verändern das elterliche Verhalten bei heraufziehenden Stürmen Starke und andauernde Regenfälle, niedrige Temperaturen und starke Winde können die Zeit der Nahrungssuche von Adulten verringern. Dies verursacht thermoregulatorischen Stress bei den Nestlingen und kann zu Schaden oder Vernichtung der Brut führen, was insgesamt den Bruterfolg negativ beeinflussen kann. Dennoch können bestimmte elterliche Verhaltensweisen diese potentiell negativen Schlecht-Wetter-Effekte abmildern. Wir untersuchten, wie Eltern diese negativen Wettereinflüsse durch die Anpassung von drei Verhaltensweisen abschwächen konnten: Anwesenheit am Nest, Beuteübergaben und Brutpflege vor, während und nach Stürmen an 11 Nestern des gefährdeten Königsbussard (Buteo regalis) in Kanada. Adulte Weibchen verbrachten mehr Zeit auf dem Nest mit zunehmender Windgeschwindigkeit und stärker werdenden Regenfällen. Vor und nach den Stürmen wurden ähnliche Anzahlen an Beuteobjekten eingetragen, was darauf hindeutet, dass Königsbussarde verlorene Zeiten zur Nahrungssuche nicht kompensieren. Königsbussarde scheinen die Fähigkeit zu haben, heraufziehende Stürme zu erkennen, in dem sie auf fallenden Luftdruck reagieren und das Risiko eines Brutverlustes durch zunehmende Brutpflege minimieren. Unsere Studie gehört zu den ersten, die sturmvorbereitendes Verhalten beobachten konnte und zeigt, dass Greifvögel in der Lage sind, ihr Brutverhalten als Antwort auf raue Witterungsbedingungen anzupassen.

Using vertebrate prey capture locations to identify cover type selection patterns of nocturnally foraging Burrowing Owls

Studies of habitat selection often measure an animals use of space via radiotelemetry or GPS-based technologies. Such data tend to be analyzed using a resource selection function, despite the fact that the actual resources acquired are typically not recorded. Without explicit proof of resource use, conclusions from RSF models are based on assumptions regarding an animals behavior and the resources gained. Conservation initiatives are often based on space-use models, and could be detrimental to the target species if these assumptions are incorrect. We used GPS dataloggers and digital video recorders to determine precise locations where nocturnally foraging Burrowing Owls acquired food resources (vertebrate prey). We compared land cover type selection patterns using a presence-only resource selection function (RSF) to a model that incorporated prey capture locations (CRSF). We also compared net prey returns in each cover type to better measure reward relative to foraging effort. The RSF method did not reflect prey capture patterns and cover-type rankings from this model were quite different from models that used only locations where prey was known to have been obtained. Burrowing Owls successfully foraged across all cover types; however, return vs. effort models indicate that different cover types were of higher quality than those identified using resource selection functions. Conclusions about the type of resources acquired should not be made from RSF-style models without evidence that the actual resource of interest was acquired. Conservation efforts based on RSF models alone may be ineffective or detrimental to the target species if the limiting resource and where it is acquired are not properly identified.