Biology ETDs

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Hybridization between black-capped and mountain chickadees was examined in 238 km of riparian woodland along the Rio Grande in central New Mexico. Chickadee densities and distributions were assessed and an attempt made to locate all breeding pairs of chickadees in the 25 km where hybridization was most common. Members of each pair were identified, vegetational aspects of nest sites and territories measured, and breeding success of the different pair combinations monitored when possible.

The hybrid zone probably dates back to the early 1970's, when black-capped chickadees first bred in this portion of the Rio Grande valley. Here, both species are marginal, occur in low densities, and are interspecifically territorial. Black-capped chickadees inhabited areas with taller, denser canopy than mountain chickadees; otherwise few differences were noted in territory structure or nest sites. Mixed-species pairs or pairs involving hybrids accounted for 50% of all pairs in the hybrid zone in 1983, decreasing to 33% in 1984. Mate choice did not differ from random expectation in 1983, but was assortative in 1984 and when data from both years were combined. Breeding success was poor for conspecific pairs in both years of the study. Mixed-species pairs had lower breeding success than conspecific pairs, and only one pair involving hybrids was known to have produced fledglings. Sightings in 1981-1982 of more than 20 hybrids suggests that mixed-species pairs may have been more successful in the recent past.

Hybridization probably occurred due both to recency of contact and to low population densities of both species. The decrease in hybridization during the study suggests that the hybrid zone is ephemeral and that reproductive isolating mechanisms are being reinforced. Hybrid zone stability may be maintained by supplementation of the local breeding population by individuals originating outside the hybrid zone.

Project Sponsors

Funding for 1981-1982 was provided by U.S. Army Corps of Engineers contract DACW47-81-C-0015 to R. D. Ohmart at Arizona State University, and for 1983-1984 by the Graduate Research Allocations Committee and Student Research Allocations Committee at the University of New Mexico.



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Department Name

UNM Biology Department

First Committee Member (Chair)

J. David Ligon

Second Committee Member

James Smith Findley

Third Committee Member

William R. Rice

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