by Katherine Monroe (CLAS ’18, Biology), May 2017
A fascinating aspect of climate change that has received relatively little attention is the impact of changing environments on species interaction. Climate change is causing many species – across all phyla – to relocate so that they may remain within their temperature tolerance ranges. A rare well-studied model of changing species interaction is the black-capped and Carolina chickadee hybrid zone that stretches from east to west from New Jersey to Kansas. Though Carolina chickadees (CACH) live south of this border and black-capped chickadees (BCCH) live farther north, the hybrid zone has been moving farther north over the generations. This raises some questions: why is the hybrid zone moving north, and how does this occur?
Stephanie Wright-Nelson addressed such questions in her 2016 dissertation. BCCH and CACH have distinct songs, despite their physiological and ecological similarities. Song is known to be a key component in intraspecific songbird interactions such as mate choice and territory defense. Moreover, some hybrid-zone chickadees can sing both CACH and BCCH songs. It thus seems likely that song influences hybridization.
In Wright-Nelson’s first of three chapters, she sought to determine if the extent of geographic song variation differs between these two species, as an indicator of ecological and evolutionary similarity. She measured song qualities – number of notes, song duration, note patterns – of recordings taken from hundreds of BCCH and CACH individuals, at dozens of locations across the species’ respective ranges. Her results revealed that each species has 2 distinct song categories, but that CACH males more frequently sing from both. This, she hypothesized, may allow CACHs to convey different messages: one song for intrasexual interactions such as territory defense, and one for intersexual interactions such as mate attraction and pair bond maintenance. CACH also had particularly strong song similarity in a north-south direction, which is interesting in relation to the northward moving hybrid zone. It is also interesting to note that the BCCH song that is most similar to CACH songs generally exist at the edges of the BCCH ranges; however, these songs do not occur near hybrid zones. Selective pressures against hybridization may push to maintain song types distinct to BCCHs, as hybrid fitness is often lower than pure species fitness. These data suggest that geographic variation can provide information on song function.
Wright-Nelson also found that CACHs have more overall song variability. She explored this further in chapter 2 by comparing song-learning styles. Much like how human infants are predisposed to learn language, birds have “neural templates” that focus their learning and enable them to produce their species’ sounds. Broad templates foster wider song repertoires, while narrow ones prevent birds from learning heterospecific signals – which may have a fitness cost. Wright studies these templates in young, lab-raised, genetically pure BCCHs and CACHs. She measured the frequency of food begging calls in response to playback recordings of adult CACHs, BCCHs, and house wrens (heterospecific control). Testing was done before and after 10 days of exposure to these recordings. Wright found that young BCCHs increased their begging response to BCCH calls, whereas CACHs responded to any chickadee song – and responded more overall. These patterns were retained as the chickadees grew up: all but one BCCH sang over 50% BCCH songs, while the CACH’s repertoires consisted of 0-100% BCCH-like songs. Wright thus concluded that CACHs have a more flexible song-learning template. Though sister species, these two species differ significantly in their preference for learning heterospecific songs. It is thus likely that genetic factors may contribute to the difference in repertoire size in wild chickadees.
Having established that the songs of these sister species vary geographically and genetically, Wright-Nelson studied the interplay of these differences in relation to the moving hybrid zone. She compared recordings of male chickadees at two locations in eastern Pennsylvania (Hawk Mountain Sanctuary and Nolde Forest) between 1999 and 2015. As Nolde is about 30 miles south of Hawk Mountain, it was hybridized first. Initially, CACH songs were rare at Nolde, and nonexistent at Hawk Mountain. By 2009, the Nolde population began including hybrid CACH-like songs. This trend was mirrored at Hawk Mountain just two years later. These data are especially compelling in relation to genetic patterns. CACHs were first detected at Hawk Mountain in 2001, but CACH-like songs did not appear for another 10 years. Currently, Nolde forest is composed entirely CACHs, but BCCH-like songs remain as a cultural artifact of hybridization. Wright suggests that the interaction between chickadee genetics and culture is complex. Though chickadee genes and songs may be moving north, their asynchrony indicates that they are not necessarily coupled.
As any good scientific study, Wright-Nelson’s dissertation inspires as many questions as it answers. Hawk Mountain is currently within the hybrid zone, making it an ideal place to study song patterns. This summer, I intend to determine the extent to which male chickadees’ songs match their genotype. If they do not always match, then my results might give insight into how CACHs seem to be outcompeting their BCCH counterparts. For instance, males might use variant songs strategically, to deter aggression from other males or to attract females into extra-pair copulations. Since chickadees form pair bonds during the winter when relatively little singing occurs, it is likely that song does not affect mate choice. However, it very well may influence extra-pair copulations in the spring. Another member of the Curry Lab, Emily Burton, will study paternity to determine what females of each species look for in extra-pair copulations. As climate change continues to cause species to modify their ranges, studies on hybridization such as those done in the Curry Lab will become increasingly relevant.
Wright Nelson, S.G. (2016). Song learning, song variation, and cultural change in two hybridizing sister songbird species, Black-capped (Poecile atricapillus) and Carolina (P. carolinensis) chickadees. PhD dissertation. The Ohio State University, Columbus, Ohio