Feeding guilds, diets and foraging behavior of insectivorous passerines in a riparian habitat in Manitoba

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Pohajdak, Gloria C.
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Although riparian habitats are important avian breeding areas, few detailed studies of the foraging ecology of birds in such habitats have been reported. I quantified the diet and feeding behavior of 13 species of insectivorous passerines on two study sites in a riparian habitat near Delta Manitoba, during the 1982-1985 breeding seasons. The Manitoba study area was a typical riparian habitat. Vegetation was dominated by a few tree species, dense populations of birds inhabited the area and arthropods were often abundant. Three distinct foraging guilds were identified by discriminant function analysis followed by cluster analysis. One group of birds primarily gleaned for insects in the canopy, another hawked and hovered to obtain prey and the third gleaned and probed to obtain insects below the canopy. Differential use of distinct feeding sites was not important in describing guilds. The foraging behavior of individual species varied more between years than between the two study sites within a year. Two species, Warbling Vireos (Vireo gilvus) and Gray Catbirds (Dumatella carolinensis) belonged to different foraging guilds in different years. Weather conditions, particularly wind and temperature, primarily affected the height at which birds foraged and their use of different feeding maneuvers. There was a high overlap in diet among the bird species because they all frequently fed on the frequently abundant adult midges. Birds in different behavior-defined foraging guilds often had diets as similar to each other as to members of their own foraging guild. When food resources were not abundant, diet overlap among species decreased and diet breadth increased. Although the breadths of feeding behaviors used by individual species generally remained constant despite differing prey availabilities, overlap in behavior among the bird species decreased as anthropod abundance decreased. These findings are consistent with the hypothesis that interspecific competition for food occurred among the riparian birds when food resources were relatively low. Densities of the Manitoba riparian birds may be limited by both food resources and the availability of nesting sites.