Tufted Titmice, Black-crested Titmice, and their hybrids occupy different habitats within their hybrid zone
Date
2022-12
Authors
Martinez, Carli
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Abstract
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.
Description
Keywords
Tufted Titmice, Black-crested Titmice, Hybridization, Habitat associations
Citation
Martinez, C. R. (2022). <i>Tufted Titmice, Black-crested Titmice, and their hybrids occupy different habitats within their hybrid zone</i> (Unpublished thesis). Texas State University, San Marcos, Texas.