Irruptive fall migrations are linked to elevated breeding abundance but are not associated with body condition or stopover duration in Northern Saw-whet Owls

Bird species that undertake irruptive migrations are good candidates for assessing density-dependent effects on stopover ecology because the number of birds using stopover sites varies considerably from year to year. Using morphometric data from a 9-year banding program and radiotelemetry (n = 25 females), we found that increases in the annual density of Northern Saw-whet Owls (NSWO; Aegolius acadicus) encountered at a fall stopover site in Nova Scotia, Canada, were not linked to female body condition or minimum stopover duration. Rather, most NSWO spent no more than 1 or 2 full days at the stopover site following radio-tagging and during their return visits to the site. Body condition indices were highest for NSWO captured near the end of the migration monitoring season, possibly reflecting birds that had recently established wintering ranges and were elevating energy stores. We used breeding abundance indices derived from Birds Canada's Atlantic Nocturnal Owl Survey to help test the hypothesis that irruptive fall migrations in NSWO were driven by elevated breeding productivity and dispersal of immature birds (breeding success hypothesis). Indeed, irruptive fall migrations were characterized by elevated densities of immatures, but not adults, and mean breeding abundance indices for the Maritime provinces during years with irruptive migrations were higher than those for non-irruptive years. We hypothesize that prey abundance during years with irruptive migrations was sufficient to enable high breeding densities and minimize effects of elevated fall densities of NSWO on rates of mass gain and stopover duration. center dot Elevated migrant densities can affect a bird's rate of mass gain, and individuals in poor condition may need to extend duration of their stopovers to acquire energy to resume migration.center dot Using a banding dataset and radiotelemetry, we assessed whether densities of Northern Saw-whet Owls (Aegolius acadicus; NSWO) encountered at a fall stopover site in Nova Scotia, Canada, were associated with owl body condition and stopover duration.center dot Annual densities of NSWO detected at the stopover site varied across the 9-year study period; however, female body condition and stopover duration were not linked to owl densities.center dot Irruptive migrations consisted of elevated densities of immature, but not adult, NSWO, and mean breeding abundance indices for years with irruptive migrations were higher than those for non-irruptive years, consistent with the hypothesis that irruptive movements can be driven by high breeding productivity in this species.center dot We hypothesize that prey availability during years with irruptive fall migrations was sufficient to enable elevated breeding abundance of NSWO and reduce effects of increased fall densities on rates of mass gain and stopover duration. Les especes d'oiseaux qui entreprennent des migrations irruptives sont de bons candidats pour evaluer les effets dependant de la densite sur l'ecologie de la halte migratoire car le nombre d'oiseaux utilisant les sites de halte migratoire varie considerablement d'une annee a l'autre. En utilisant les donnees morphometriques provenant d'un programme de baguage s'etendant sur 9 ans et la radiotelemetrie (n = 25 femelles), nous avons trouve que les hausses de densite annuelle d'Aegolius acadicus (NSWO) rencontrees sur un site de halte migratoire automnale en Nouvelle-ecosse, au Canada, n'etaient pas liees a la condition physique des femelles ni a une duree minimale de halte migratoire. Au contraire, la plupart des NSWO n'ont pas passe plus d'un ou deux jours complets au site de halte migratoire apres avoir ete munis d'un emetteur et lors de leurs visites de retour sur le site. Les indices de condition physique etaient plus eleves pour les NSWO captures vers la fin de la saison de suivi de la migration, lesquels pourraient etre des oiseaux ayant recemment etabli leur aire d'hivernage et qui etaient en train d'accumuler des reserves d'energie. Nous avons utilise des indices de l'abondance de reproducteurs derives de l'Inventaire des Strigides Nocturnes de l'Atlantique d'Oiseaux Canada pour verifier l'hypothese que les migrations automnales irruptives chez NSWO etaient motivees par une productivite reproductive elevee et la dispersion d'oiseaux immatures (hypothese du succes de la reproduction). En effet, les migrations automnales irruptives etaient caracterisees par des densites elevees d'immatures, mais pas d'adultes, et les valeurs moyennes des indices de l'abondance de reproducteurs pour les provinces maritimes pendant les annees de migrations irruptives etaient plus elevees que celles des annees sans irruption. Nous emettons l'hypothese que l'abondance des proies durant les annees avec des migrations irruptives etait suffisante pour permettre des densites de reproduction elevees et limiter les effets des densites automnales elevees de NSWO sur les taux d'accroissement de la masse et la duree de la halte migratoire.