Tufted Titmice, Black-crested Titmice, and their Hybrids Occupy Different Habitats within their Hybrid Zone
Habitat can play a key role in either facilitating or preventing hybridization between some species. Within a hybrid zone, differences in habitat requirements could spatially separate two species to reduce hybridization. However, hybrid individuals might be best adapted to an intermediate type of habitat also occupied by the parent species in which case hybridization might be facilitated. Tufted Titmice (Baeolophus bicolor) (TUTI) and Black-crested Titmice (Baeolophus atricristatus) (BCTI) are sister species that hybridize within a narrow east-west contact zone that extends from central Texas into southwest Oklahoma. The zone corresponds with an ecotone that transitions from closed-canopy deciduous forests in the east, occupied by TUTI, to arid and open woodlands in the west occupied by BCTI. Furthermore, the ecotone encompasses areas impacted by urban and suburban development. For the first objective of my study, I tested whether landscape-level habitat characteristics found within the TUTI × BCTI hybrid zone influence the relative abundances of the two parent species and their hybrids. I predicted that the parent species would differ in their habitat associations and hybrids would associate with habitats intermediate between that of TUTI and BCTI and with human disturbed habitats. I used eBird data and high resolution GIS land cover data in a grid-based sampling design to analyze the habitat associations of the three titmice types throughout the hybrid zone, plus locations immediately outside the h zone only occupied by the parental species Logistic regression and AIC model selection revealed that the relative abundances of the three titmice types were most closely associated with percent evergreen forest, mixed forest, and shrub cover. BCTI was associated with higher percentages of evergreen forest, and lower percentages of mixed forest and shrub cover, whereas TUTI had the inverse associations. Further, PCA revealed that hybrids tended to occupy a habitat that was intermediate between that of TUTI and BCTI. Anthropogenic disturbance did not appear to influence relative abundances of BCTI, TUTI, or hybrids. The second objective of my study was to determine if there are differences in the habitat associations of titmice belonging to different phenotypic classifications. I used camera traps and conducted bird surveys at select locations within the hybrid zone to identify titmice using Dixon’s hybrid index applied to plumage characteristics. Habitat associations were analyzed similar to the eBird data but at a finer spatial scale. Locations occupied by only pure BCTI phenotypes had more canopy cover and evergreen forest than locations with hybrid phenotypes. My study demonstrates that within the hybrid zone, BCTI and TUTI have different habitat associations. However, hybrid individuals appear to occupy an intermediate type of habitat shared by the parental species.
tufted titmice, black-crested titmice, hybridization, habitat associations
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