Summary
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The purpose of this study was to determine whether the yellow warbler (Dendroica petechia) derives any benefits from nesting in close proximity to its own and other species in reducing predator pressure or the incidence of brood parasitism by the brown-headed cowbird (Molothrus ater).
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Yellow warblers nesting synchronously with their own and with neighbouring species resulted in a proportionate reduction in the number of nests preyed upon, suggesting a ‘swamping effect’ of the local predators. A significantly lower incidence of predation occurred at yellow warbler nests which were inside a gray catbird (Dumetella carolinensis) response range. This was thought to be the result of yellow warblers taking advantage of the catbird's nest-guarding behaviour and aggressive responses to predators.
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Yellow warblers nesting synchronously with other yellow warblers were subject to a proportionately lower incidence of brood parasitism than asynchronous nests, suggesting a ‘swamping effect’ on the cowbird. A significantly lower incidence of brood parasitism occurred at yellow warbler nests which were in a red-winged blackbird (Agelaius phoeniceus) response range. The yellow warbler was thought to be taking advantage of the aggressive response of red-winged blackbirds to cowbirds.
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Overall, yellow warblers which nested within the response range of red-winged blackbirds or gray catbirds had significantly higher nest success than other yellow warblers. This suggests that selection for defence against predators and cowbirds may lead to multi-species aggregations.
Methodology
The study was conducted in two large study areas of 10 ha and 15 ha and five small study areas each between 1-2 ha near Queen's University Biological Station, Chaffey~ Lock, Ontario. All of the areas were wet lowlands along streams. The predominate vegetation was Spirea latifolia, Myrica gale, Alnus rugosa, sedges, and grasses. Most of the areas included a marsh of Typha latifolia. Nests records were kept for all bird species that nested within the study areas. The study areas were completely searched for new nests every 2 or 3 days, from early May to mid-July in 1975-1977. Nests that had been damaged, and/or eggs or young that were missing before their expected fledging time, were assumed to have been preyed upon. Those nests in which the contents remained intact, but were no longer tended by the parents, were considered deserted. At the end of the nesting season the locations of all nests were mapped by using a tape measure and compass. Nesting density was determined by measuring the distance to neighbouring nests from a yellow warbler nest. Using playback recordings, it was found that red-winged blackbirds would participate in group responses which were initiated up to 57 m from their own nests. This distance also often coincided with the responding bird's territorial boundary. If yellow warblers were to derive any benefit from red-winged blackbirds' or catbirds' responses to predators or cowbirds it would be necessary to nest within this range. For this reason, nests that fell within a radius of 57 m of a yellow warbler nest were counted to give a measure of the number of nests per hectare at a given yellow warbler nest. Only those nests were counted that were active at the same time as was the yellow warbler nest. Nesting success, calculated as the number of young fledged per egg laid, was determined for each yellow warbler nest.