Competition between a migrant and resident passerine during the non-breeding season

Migrant birds spend half their lives coexisting with residents during the non-breeding season and their populations may be limited by environmental conditions during this time, yet the degree to which interspecific interactions affect both migrants and residents is still not well known. I studied the ecologically similar migrant White-eyed Vireo (WEVI: Vireo griseus) and the resident Thick-billed Vireo (TBVI: V. crassirostris), sister taxa that only coexist during the non-breeding season. The intensity of resource competition is expected to depend on resource availability, which can vary spatially and temporally. I used a spatial habitat contrast (forest vs scrub) and a temporal seasonal contrast (fall vs winter) to assess whether variation in resource abundance affected competitive interactions. In order to test for adaptive responses to interspecific competition, I also compared locations allopatric and sympatric with the heterospecific vireo. In chapter 1, I found that TBVI had higher aggression in sympatry than allopatry and were dominant to WEVI yet territories between the species still overlapped. In chapter 2, I showed that there were more food resources in forest than scrub habitats, and that food resources declined from fall to winter. Using stable isotopes as indicators of diet, I also found that the two species had diets that were more similar to each other in sympatry than in allopatry, opposite to predictions under ecological character displacement. In chapter 3, I measured corticosterone as an indicator of stress and documented higher levels of stress in male TBVI sympatric with WEVI compared to allopatric TBVI; body condition was also lower in sympatric than allopatric TBVI. These results indicate that there is little niche partitioning between species, and this has negative physiological consequences at least for the resident species