Human population growth has resulted in more frequent interactions between humans and wildlife, making it increasingly important to understand how anthropogenic disturbance affects animal populations. A number of recent studies on birds have shown that individuals experiencing high levels of disturbance are frequently more aggressive than conspecifics living in less disturbed areas. Our study asked whether heterospecific aggression varied in Western Bluebirds (Sialia mexicana) breeding in artificial nest boxes over a gradient of human disturbance in the Okanagan Valley of British Columbia, Canada. Unlike most previous studies, which have investigated effects of high disturbance (urbanization), our study focused on sites with lower rates of human disturbance, including ranch lands, vineyards, and recreational trails. Using decoys and playback, we measured the response of bluebird pairs to a simulated territory intrusion by one of two heterospecifics: a competing cavity nester, the House Wren (Troglodytes aedon), and a non-competing species, the American Goldfinch (Spinus tristus). We asked whether this response varied with the level of human disturbance experienced by the pair, and also whether aggression was correlated with female flight initiation distance (FID; a measure of boldness), local density of boxes in a territory, and presence of neighboring cavity-nesters. Overall, aggression was significantly higher towards the competitor stimulus (wren) than the non-competitor (goldfinch). Aggression towards the wren model did not change with disturbance level; however, aggression towards the goldfinch model increased significantly in more disturbed areas. Female FID did not vary across disturbance categories, but was correlated with aggression towards the wren, where individuals that responded more aggressively had marginally larger FIDs (i.e., behaved less boldly). Female FID was not correlated with aggression towards the goldfinch. Birds nesting in territories with more boxes tended to be more aggressive towards the wren, and birds nesting in territories with one or more box-nesting neighbors were significantly more aggressive towards the wren than birds without neighbors. Neither of these factors was correlated with aggression towards the goldfinch. Thus, the predicted association between disturbance and aggression was detected only in response to the non-competitor species (goldfinch), whereas aggression towards the competitor species (wren) varied with territory quality and number of competitors. Our study shows that variation in low levels of anthropogenic disturbance may be associated with behavioral effects in bird populations, and serves as a reminder that different factors may influence the expression of aggression in different contexts.

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