Ecological Relations and Social Interactions of Philadelphia and Red-Eyed Vireos

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Abstract
The ecological and social interactions of the interspecifically aggressive red-eyed (V. olivaceus) and Philadelphia (V. philadelphicus) vireos were studied during 4 breeding seasons in the Hubbard Brook Experimental Forest, New Hampshire [USA]. The 2 spp. here have extensively overlapping territories, but occupy different vegetational strata during most of the breeding season. The aggressiveness of the Philadelphia vireo towards the red-eyed vireo varies within a breeding season, being most frequent and intense during the periods of greatest vertical overlap. The aggressiveness of red-eyed vireos towards Philadelphia vireos varies between years, being most frequent and intense in years of high red-eyed vireo population density. In 1 yr no aggression was apparent between the 2 vireos. During some aggressive encounters the smaller, subordinate Philadelphia vireo used distraction displays when red-eyed vireos came close to their young. Resource competition may be the underlying cause of the aggression between these 2 ecologically similar species, largely because the most intense and frequent interspecific interactions occurred during periods of maximum overlap in foraging microhabitat. A comparison of these findings with those in another study illustrates the importance of forest height and vegetation structure in facilitating their coexistence.