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Hunting success of wintering Swainson's hawks: environmental effects on timing and choice of foraging method

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Abstract:

We examined the predatory behavior of Swainson's hawks (Buteo swainsoni Bonaparte, 1838) wintering in the Argentine pampas. Aerial and ground foraging were the main hunting methods employed by hawks in this region. The overall hunting success of hawks preying on insects was 50% and age-related differences in hunting success were not significant. The Swainson's hawks, however, hunted more successfully in the air (65% of prey capture attempts) than on the ground (42%). Aerial hunting while soaring was the most successful hunting method based on the number of prey captured per energy unit. Based on the analysis of prey consumed by hawks during the study period, grasshopper species with poor flight capabilities were available in the air as a consequence of the vertical air motion. With regards to daily activity patterns, the time that a hawk spent using each hunting method was not proportional to the cost ratio associated with each method. Hawks foraged in the air only during midday hours when weather conditions permitted the formation of thermals. Thus, the use of soaring flights and the availability of prey in the air were constrained by the physical environment, and hawks could only exploit airborne food sources during limited periods of the day.

Nous avons étudié le comportement prédateur des buses de Swainson (Buteo swainsoni Bonaparte, 1838) qui hivernent dans les pampas d'Argentine. La chasse dans les airs et au sol sont les principales méthodes de capture utilisées par les buses dans cette région. Le succès global de la chasse des buses qui se nourrissent d'insectes est de 50 % et les différences de succès de la chasse observées en fonction de l'âge ne sont pas significatives. Cependant, les buses de Swainson chassent avec plus de succès dans les airs (65 % des tentatives de capture de proies) qu'au sol (42 %). D'après le nombre de proies capturées par unité énergétique, c'est la chasse dans les airs durant le vol plané qui est la méthode la plus efficace. L'analyse des proies consommées par les buses durant la période d'étude montre que les orthop tères à faible pouvoir de vol sont disponibles dans l'air à cause des mouvements verticaux de l'atmosphère. Les patrons d'activité journalière révèlent que le temps que les buses passent à utiliser chacune des méthodes de chasse n'est pas proportionnel au rapport des coûts associés à chaque méthode. Les buses chassent dans les airs seulement au milieu de la journée lorsque les conditions climatiques permettent la formations de courants thermiques ascendants. Ainsi, l'utilisation du vol plané et la disponibilité des proies dans les airs sont restreintes par le milieu physique; les buses ne peuvent donc utiliser les ressources alimentaires atmosphériques que durant de courtes périodes de la journée.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: October 1, 2005

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  • Published since 1929, this monthly journal reports on primary research contributed by respected international scientists in the broad field of zoology, including behaviour, biochemistry and physiology, developmental biology, ecology, genetics, morphology and ultrastructure, parasitology and pathology, and systematics and evolution. It also invites experts to submit review articles on topics of current interest.
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