No abstract

Artificial nests are commonly used to investigate relative rates of nest predation in birds, but several methodological considerations need to be addressed before results from natural and artificial nests can be compared. Using field and laboratory experiments, we examined responses of predators to visual and olfactory cues that were associated with wicker nests and their contents. Avian predators did not discriminate between wicker nests dipped in mud and those covered by a camouflage fabric, whereas mammalian predators showed a weak tendency to depredate camouflaged nests. Nests containing plasticine eggs were depredated more often than nests containing only quail eggs and finch eggs, although no response to number of plasticine eggs in nests was found. The higher predation of nests with plasticine eggs may have resulted because small mammals, relying on olfactory cues, comprised a large portion of the predator assemblage. Field results were supported in tests where captive deer mice (Peromyscus maniculatus) were attracted to assortments of egg types that included plasticine. Time required by captive deer mice to penetrate quail eggs and finch eggs versus plasticine eggs varied as a function of egg size and shell thickness and strength. Overall, domestic finch eggs provided a better alternative to quail eggs because they were small enough to allow detection of predation events by small mammals and did not have an unnatural odor like plasticine. Potential problems with nest concealment, egg visibility, egg odors, and other factors must be resolved to enhance the design and reliability of artificial nest experiments.

No abstract

Vegetation characteristics are commonly invoked to explain differential nesting success, but few studies have identified how habitat attributes may be associated with specific predator groups responsible for nest losses. We measured vegetation characteristics at artificial songbird nests deployed on and above the ground in mixedwood forest in west-central Alberta, to discriminate characteristics of successful nests from those of nests destroyed by mice and voles, squirrels, and birds. Successful nests, and those depredated by mice and voles, tended to be on the ground and were well concealed by dense shrubs. Squirrels and birds usually raided aboveground nests at sites with few shrubs and high tree densities. These results suggest that nest visibility is a major factor influencing risk of predation, but the relative importance of concealment varies according to the types of predators and their behavior. The characteristics of nests from which eggs were removed, leaving no egg remains to identify predators, closely resembled those of nests visited by squirrels and birds. Our findings have implications for how individual songbird species cope with selection pressures imposed by nest predators associated with vegetation characteristics of birds' nest patches and, ultimately, how predators may shape the structure of avian communities.Résumé : Les caractéristiques de la végétation sont souvent évoquées pour expliquer les fluctuations du succès de la nidification, mais peu d'études ont tenté de déterminer comment les attributs d'un habitat peuvent être associés à des groupes spécifiques de prédateurs responsables des pertes au nid. Nous avons mesuré les caractéistiques de la végétation près de nids artificiels de passereaux installés sur le sol et au-dessus du sol dans une forêt mixte du centreouest de l'Alberta, afin de déterminer lesquelles de ces caractéistiques sont reliées aux nids réussis par opposition aux nids détruits par les souris et campagnols, les écureuils et les oiseaux. Les nids réussis et ceux pillés par les souris et campagnols sont des nids au sol, bien camouflés par des buissons denses. Les écureuils et les oiseaux pillent généralement les nids au-dessus du sol à des sites où il y a peu de végétation et où les arbres sont denses. Ces résultats semblent indiquer que la visibilité du nid est un facteur très important de risque de prédation, mais l'importance relative du camouflage varie selon le type de prédateur et selon son comportement. Les caractéistiques des nids pillés où il n'est resté aucun débris permettant d'identifier le prédateur ressemblaient fortement à celles des nids pillés par les écureuils et les oiseaux. Nos résultats soulèvent le problème de l'adaptation des passereaux aux pressions de sélection qui sont créées par les prédateurs de nids et qui sont associées aux caractéristiques de la végétation dans l'entourage des nids et, ultimement, nous amènent à nous demander comment les prédateurs peuvent influencer la structure des communautés d'oiseaux.[Traduit par la Rédaction] Rangen et...

No abstract