Song measure averages of allopatric and sympatric Black-capped and Mountain Chickadee individuals

Character displacement theory predicts that closely related co-occurring
species should diverge in relevant traits to reduce costly interspecific
interactions such as competition or hybridization. While many studies
document character shifts in sympatry, few provide corresponding evidence
that these shifts are driven by the costs of co-occurrence. Black-capped
(Poecile atricapillus) and Mountain Chickadees (P. gambeli) are closely
related, ecologically similar, and broadly distributed songbirds with both
allopatric and sympatric populations. In sympatry, both species appear to
suffer the costs of their co-occurrence: 1) both species are in worse body
condition compared to allopatry and 2) hybridization sometimes yields
sterile offspring. Here, we explored character displacement in the songs
of Black-capped and Mountain Chickadees by characterizing variation in
male songs from sympatric and allopatric populations. We found that
Mountain Chickadees sing differently in sympatry versus allopatry.
Specifically, they produced more notes per song, were more likely to
include an extra introductory note, and produced a smaller glissando in
their first notes compared to all other populations. Combined with
previous research on social dominance and maladaptive hybridization
between Black-capped and Mountain Chickadees, we posit that differences in
sympatric Mountain Chickadee songs are population-wide shifts to reduce
aggression from dominant Black-capped Chickadees and/or prevent
maladaptive hybridization.