Interspecific interactions between mountain and black-capped chickadees within a contact zone.

Hybridization occurs at least occasionally in more than 9% of bird species. Within the Paridae (chickadees and titmice), hybridization is documented both among the Old World species and North American species. Europe has a larger number of sympatric tit species than does North America, but appears to have less hybridization. These overlapping species may have developed isolating mechanisms to decrease hybridization and as a result maintain distinct species. In North America, little research has focused on potential hybridization between the two closely related species that occasionally occur in sympatry and anecdotal evidence suggests they occasionally hybridize: black-capped (Poecile atricapillus) and mountain (P. gambelli) chickadees. My PhD research focuses on the interspecific relationships and potential hybridization between black-capped chickadees and mountain chickadees at the John Prince Research Forest (JPRF) in northern British Columbia. I focused my work on behavioural mechanisms that may reinforce species isolation and genetic analysis to determine the amount of intermixing that is occurring between species. Through nuclear and mitochondrial DNA analyses, I found evidence of hybridization. All but one of these hybrids were phenotypically mountain chickadees that had black-capped DNA in their genome. Further, all hybrids that were found amongst nestlings occurred in mountain chickadee nests showing genetic patterns indicating extra-pair copulations between female mountain chickadees and males black-capped chickadees. As dominance status is known to affect mate choice in black-capped chickadees, interspecific social hierarchies have the potential to interfere with interspecific interactions and create the potential for hybridization. My field observations and aviary experiments showed that black-capped chickadees are dominant over mountain chickadees in almost all circumstances. In aviary trails comparing responsiveness of either species to flock-rallying chick-a-dee calls, I found that mountain chick