Undergraduate Honors Thesis


Character displacement of mountain chickadee song where they co-occur with black-capped chickadees Public Deposited

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    Mechanisms for species recognition are important when closely related species overlap in their ranges because hybridization (i.e., the interbreeding between two species) can be costly. Hybridization is maladaptive when it results in wasted reproductive effort, inviable offspring, or offspring with reduced fitness. When hybridization is costly, the characters used for species recognition may diverge where species co-occur. In passerine birds, song is an important mechanism of species recognition. The black-capped chickadee (Poecile atricapillus) and the mountain chickadee (Poecile gambeli) are two closely related species with both geographically isolated (i.e., allopatric) and geographically overlapping (i.e., sympatric) populations; in the latter, hybridization has been observed on numerous occasions. In this study, we compared songs of allopatric and sympatric populations of black-capped and mountain chickadees. We tested whether song divergence is greater in sympatry compared to allopatry given the potential selective pressures for species to avoid hybridization. As such, we predicted that songs of sympatric black-capped and mountain chickadee populations would be more divergent from each other than those of allopatric populations. We found that sympatric mountain chickadees produced more notes per song and were more likely to include an extra introductory note compared to allopatric mountain chickadees. Our findings support our predictions that character displacement should exist in sympatry, potentially so that chickadees avoid maladaptive hybridization. 

Date Awarded
  • 2023-04-11
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Last Modified
  • 2023-04-19
  • Boulder
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