Shifting Hybrid Zones: Behavior, Climate, or Both?

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by Alyssa Kist (CLAS ’18, Biology), May 2017

Because we study hybridization in chickadees, the Curry Lab is always searching for current research that relates to our own program. In two of their latest papers, Billerman et al. address an important question regarding shifts in avian hybridization: What is driving these shifts? Billerman and colleagues focused on two concepts that may be contributing to these moving zones between two species of sapsucker (Red-naped and Red-breasted): interspecific interactions and climate change.

To consider how interspecific interactions between the sapsucker species contribute to hybrid zone shifts, Billerman and Carling (2016a) studied aggression toward a simulated territory intruder. The males of both species are intensely territorial and will express aggressive behavior by attacking and chasing intruders; thus, aggression was a strong factor to measure. Red-naped and Red-breasted sapsuckers hybridize along a contact zone from northeastern California to southern British Columbia, and the researchers gathered samples for their research from this region. Billerman et al. tested three predictions: (1) Red-breasted Sapsuckers will be more aggressive than Red-naped Sapsuckers; (2) Red-breasted Sapsuckers will respond equally to conspecific (between the same species) and heterospecific stimuli, while Red-naped Sapsuckers will respond more strongly to conspecific stimuli; (3) differences in aggressive responses between sympatric populations will be stronger than between allopatric populations.

Billerman and Carling used playback of a sapsucker vocalizations, combined with taxidermic mounts (stuffed sapsuckers), to quantify aggressive behavior in regions of sympatry and allopatry among conspecific and heterospecific sapsuckers. The taxidermic mounts mimicked territorial intruders of both species and the playback recordings of each species were combined because each species’ drums and calls are very similar and are both used in response to territorial threats. The researchers kept the mounts in place for 15-minute periods and played the playback recordings  during each period’s entirety. The researchers estimated the closest distance the focal sapsucker approached the mount, and frequency of movements. They also measured the number of times a sapsucker vocalized or drummed and whether the sapsuckers swooped at or physically attacked the mount. The researchers used the phenotype of the birds to categorize which birds responded aggressively to the mounts and playbacks (Red-breasted Sapsuckers have fully red breasts and no black feathering; Red-naped Sapsuckers have fully black breasts, small red nape patches, and black feathers).

Overall, the data did not support two of the hypotheses. The Red-breasted Sapsuckers were not more aggressive than Red-naped Sapsuckers; on the other hand, Red-naped Sapsuckers responded more aggressively acoustically and at a quicker rate to the territorial threats (the mounts and playbacks) than the Red-breasted Sapsuckers. These data contradicts the currently understood effects of interspecific interactions on the movement of hybrid zones. Differences in aggressive responses were not strongest in sympatric areas but equivalent in both sympatric and allopatric areas. Furthermore, Red-naped Sapsuckers responded more aggressively to conspecific mounts. This may be because of mate competition. Billerman et al. also suggest that acoustic aggression (vocal responses to a supposed threat) may not be an effective way to measure physical aggression and may represent differences in communication among the species instead. Despite the fact that the findings disproved the hypotheses, Billerman et al. emphasize the importance of understanding why hybrid zones moved and how species are affected by this movement.

Billerman et al. (2016b) focused more on how climate change contributes to reproductive isolation between related taxa. Climate change can affect hybrid zones by shifting the contact zone to areas where the climate favors one species over another, or by allowing new hybrids to form due to contact expansion. Billerman et al. again studied Red-naped and Red-breasted sapsuckers . Historical shifts in the hybrid phenotypes have been collected and museum-preserved and the researchers estimated current shifts in order to quantify the hybrid zone movement due to climate change. Billerman et al. hypothesized that the sapsucker hybrid zone has moved because of climate changes. Their objective was to define the climate niche of parental and hybrid sapsuckers in order to answer if climate changes have led to shifting areas of contact and hybridization. Therefore, Billerman et al. predicted that climate change has led to the expansion of Red-breasted Sapsuckers and the retraction of Red-naped Sapsuckers, which contributes to the predicted downsizing of the hybrid zone. After conducting their analysis, the researchers found that Red-breasted Sapsuckers prefer areas with shorter, warmer winters and more precipitation, while Red-naped Sapsuckers prefer areas with longer, colder winters and less precipitation. Therefore, the more different the climate becomes, the less likely these sapsuckers are to hybridize, leading to the hybrid zone shift and the predicted genetic introgression, or gene movement from one species to another.

Both articles by Billerman et al. highlight how animal behavior and environmental changes potentially impact hybrid zones. Dr. Curry’s lab similarly is focusing on how hybrid zones for Black-capped Chickadees and Carolina Chickadees in southeastern Pennsylvania are shifting due to interspecies interactions and changing climates. The work of Billerman et al. gives us insight about how to approach studying these factors. One member of our lab, Katherine Monroe, is now investigating  the effect of chickadee songs on hybrid zone dynamics. With the Black-capped Chickadees and Carolina Chickadees, Katherine will determine the extent to which the songs of male chickadees match their genotypes. If these two factors reveal no correlation, Katherine plans to analyze if female chickadees mate with male chickadees based on their genotype or song. This study will measure the effects of vocalizations on the movement of the chickadee hybrid zones and will continue to pursue the answer to this important question why hybridization zone shifts occur.

Literature Cited

Billerman, S. M., & Carling, M. D. (2016a). Differences in aggressive responses do not contribute to shifts in a sapsucker hybrid zone. Auk 134: 202–214

Billerman, S. M., Murphy, M. A., & Carling, M. D. (2016b). Changing climate mediates sapsucker (Aves: Sphyrapicus) hybrid zone movement. Ecology and Evolution 6:7976–7990