Pomeroy, A. C. 2006. Tradeoffs between food abundance and predation danger in spatial usage of a stopover site by western sandpipers, Calidris mauri . Á/ Oikos 112: 629 Á/637.Foragers use a variety of anti-predator behaviours to increase their safety from predators. While foraging, animals should alter usage within or between sites to balance the benefits of feeding with the costs of predation. I tested how the distribution of food abundance and predation danger interacts to explain spatial usage (i.e. distance from shore) by migratory western sandpipers (Calidris mauri ) at Boundary Bay, British Columbia, Canada, during northward and southward migrations. At Boundary Bay there are opposing spatial gradients in the distribution of food abundance and safety from predators. Predation danger for sandpipers is high near the shoreline where there is approach cover for falcons and decreases with distance from shore. Food abundance for sandpipers declines as distance from the shoreline increases. Food and danger attributes at Boundary Bay also differ temporally, such that food abundance is higher during southward migration, and predation danger is higher during northward migration. The spatial usage by western sandpipers balances the tradeoff between the opposing spatial gradients in food and safety. For both migratory periods spatial usage of the mudflat by sandpipers is highest at distances from the shoreline where food abundance and predation danger are intermediate. During the northward migration sandpiper usage is highest between 150 and 500 m from the shoreline, and during the southward migration sandpiper usage is highest between 100 and 600 m from the shoreline. Despite temporal differences in food and danger attributes, spatial usage of the site by sandpipers does not differ between migratory periods. Understanding how the distribution of food abundance and predation danger interact to affect the within site usage by shorebirds has important implications for assessments of site quality.
ABSTRACT. In recent years, the eastern foothills of the Rocky Mountains in northeastern British Columbia have received interest as a site of industrial wind energy development but, simultaneously, have been the subject of concern about wind development coinciding with a known migratory corridor of Golden Eagles (Aquila chrysaetos). We tracked and quantified eagle flights that crossed or followed ridgelines slated for one such wind development. We found that hourly passage rates during fall migration peaked at midday and increased by 17% with each 1 km/h increase in wind speed and by 11% with each 1°C increase in temperature. The propensity to cross the ridge tops where turbines would be situated differed between age classes, with juvenile eagles almost twice as likely to traverse the ridge-top area as adults or subadults. During fall migration, Golden Eagles were more likely to cross ridges at turbine heights (risk zone, < 150 m above ground) under headwinds or tailwinds, but this likelihood decreased with increasing temperature. Conversely, during spring migration, eagles were more likely to move within the ridge-top area under eastern crosswinds. Identifying Golden Eagle flight routes and altitudes with respect to major weather systems and local topography in the Rockies may help identify scenarios in which the potential for collisions is greatest at this and other installations.RÉSUMÉ. Récemment, les contreforts des Rocheuses dans le nord-est de la Colombie-Britannique ont attiré l'attention comme site potentiel d'un développement éolien industriel, tout en faisant simultanément l'objet de préoccupations puisque ce projet coïnciderait avec un corridor de migration connu d'Aigles royaux (Aquila chrysaetos). Nous avons suivi et quantifié les vols d'aigles qui ont traversé ou longé les lignes de crêtes visées par un projet éolien de ce genre. Nous avons constaté que le taux de passage horaire durant la migration automnale atteignait un maximum à midi et augmentait de 17 % pour chaque km/h d'augmentation de la vitesse du vent, et de 11 % pour chaque °C d'augmentation de la température. La propension à traverser les sommets des crêtes où seraient installées les éoliennes différait selon les classes d'âge, les jeunes aigles de l'année ayant deux fois plus de chance de le faire que les adultes ou les jeunes plus âgés. Durant la migration automnale, les aigles traversaient davantage les crêtes à hauteur d'éoliennes (zone de risque, < 150 m au-dessus du niveau du sol) sous un vent de face ou arrière, mais cette tendance diminuait avec l'augmentation de la température. En revanche, durant la migration printanière, les aigles étaient plus susceptibles de survoler la région des sommets sous un vent latéral de l'est. La détermination des trajectoires et des altitudes de vol des Aigles royaux, selon les systèmes météorologiques prédominants et la topographie locale des Rocheuses, peut contribuer à identifier les scénarios dans lesquels les risques de collision sont les plus élevés, que ce soit pour ce projet éolien ou d'au...
ABSTRACT. To migrate successfully, birds need to store adequate fat reserves to fuel each leg of the journey. Migrants acquire their fuel reserves at stopover sites; this often entails exposure to predators. Therefore, the safety attributes of sites may be as important as the feeding opportunities. Furthermore, site choice might depend on fuel load, with lean birds more willing to accept danger to obtain good feeding. Here, we evaluate the factors underlying stopover-site usage by migrant Western Sandpipers (Calidris mauri) on a landscape scale. We measured the food and danger attributes of 17 potential stopover sites in the Strait of Georgia and Puget Sound region. We used logistic regression models to test whether food, safety, or both were best able to predict usage of these sites by Western Sandpipers. Eight of the 17 sites were used by sandpipers on migration. Generally, sites that were high in food and safety were used, whereas sites that were low in food and safety were not. However, dangerous sites were used if there was ample food abundance, and sites with low food abundance were used if they were safe. The model including both food and safety best-predicted site usage by sandpipers. Furthermore, lean sandpipers used the most dangerous sites, whereas heavier birds (which do not need to risk feeding in dangerous locations) used safer sites. This study demonstrates that both food and danger attributes are considered by migrant birds when selecting stopover sites, thus both these attributes should be considered to prioritize and manage stopover sites for conservation.RÉSUMÉ. Afin de compléter leur migration, les oiseaux ont besoin d'emmagasiner les réserves énergétiques nécessaires pour franchir chaque étape de leur parcours. Les migrateurs obtiennent leurs réserves énergétiques sur les haltes migratoires, où sont souvent présents des prédateurs. Ainsi, les caractéristiques relatives à la sécurité d'un site peuvent être tout aussi importantes que les possibilités de s'y alimenter. De plus, le choix du site peut dépendre des réserves énergétiques de l'oiseau, un oiseau maigre étant plus disposé à accepter le danger afin d'obtenir une bonne alimentation. Dans la présente étude, nous avons évalué les facteurs sur lesquels repose l'utilisation des haltes migratoires par des Bécasseaux d'Alaska (Calidris mauri) migrateurs, à l'échelle du paysage. Nous avons mesuré les caractéristiques relatives à l'alimentation et à la sécurité de 17 haltes migratoires potentielles dans la région des détroits de Georgie et de Puget. Nous avons utilisé des modèles de régression logistique afin de tester si l'alimentation, la sécurité ou les deux, étaient les variables qui prédisaient le mieux l'utilisation de ces haltes par les Bécasseaux d'Alaska. Huit des 17 sites ont été utilisés par les bécasseaux en migration. En général, les sites sécuritaires et ayant un nombre élevé de proies étaient utilisés, tandis que ceux qui étaient non sécuritaires et dont le nombre de proies était faible ne l'étaient pas. Toutefois, les sites non ...
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