Research
Black-crested Titmice
Baeolophus atricristatus
This species of titmouse exhibits deeply complicated social dynamics, specifically in reference to juvenile involvement in social grouping, and the ratio of males and females across the species as a whole. We are interested in the interplay between these dynamics, as well as the adaptive and biological causes behind each.
Juvenile roles in territorial grouping dynamics
Black-crested titmice exhibit unique grouping dynamics within a non-breeding territory, where a territorial pair of adults allow juveniles to join their social group during the 4-7 month non-breeding
period. Grouping juveniles – some related and some unrelated to the territorial pair – participate in cooperative territorial defense in tandem with adults. Given the aggressive nature of these juvenile-groups, we refer to them as ‘platoons’.
We found that juveniles within a platoon contribute to territorial defense to a similar degree, or even more actively, than adults. They responded to simulated territorial invasions (STIs) more quickly than adults, suggesting they have a higher incentive to defend the territory than adults.
Male taxidermic model on perch covered with a wire cage for STIs
This research helps characterize the nuanced role that juveniles may play in the unique groups that black-cresteds form in regards to territorial defense, and aids in the further investigation of these unique social groups as a whole.
Sex ratio bias towards males
Commonly, the ratio of males to females in bird populations is close to 50:50. Black-cresteds do not exhibit sexual dimorphism, meaning that, beyond certain behavioral differences, we cannot tell whether a bird is male or female based on sight alone. Thus, it is necessary to use molecular sexing techniques on extracted DNA in order to determine sex definitively.
Image of 33 PCR products run on a 2% agarose gel. Lanes with two bands indicate a female (heterogametic sex; ZW chromosomes); lanes with one band indicate a male (homogametic sex; ZZ). Empty lanes indicate that the DNA did not amplify.
After sexing over 200 blood samples from 2023 and 2025, we found that the ratio of male to female nestlings was significantly different from 1:1 in both the 2023 and 2025 samples, with male being the majority sex. There was no significant difference between the sex ratio of the 2023 and 2025 samples despite large environmental differences during the breeding seasons.
This is a significant finding that indicates that males are being selected for at a currently unknown developmental stage. We look to establish a primary sex ratio (sex ratio at the time of fertilization) in or to determine the development stage that the selection occurs. We wish to further investigate why this bias may occur, and by what biological mechanisms.
Stable hybridization zone with the tufted titmouse
Black-crested and tufted titmice are both species of titmouse. In areas where their ranges overlap, they are able to hybridize with each other. In East Texas, there has been a stable hybrid zone for over 10,000
years, but it remains unknown why this hybrid zone has remained so stable throughout this time.
We want to investigate the genetic relatedness between these two species in order to determine the mechanisms behind this stable hybridization, as well as to further our understanding of the titmouse genome through DNA sequencing.
Black-crested/tufted titmouse hybrid. Its brown forehead area indicates its hybrid nature.
PAST RESEARCH SUBJECTS
American Goldfinches
Carduelis tristis
Goldfinches have sexually dichromatic plumage, yet both female and male bill color changes from grey-brown to bright orange before the breeding season.
Keith Tarvin and I tested whether female bill coloration signals fighting ability to conspecifics, and whether bill color is evaluated by discriminating males during mate-choice.
Test of female-female status signaling
We tested whether caged females avoided feeding adjacent to female taxidermic models as a function of the model’s bill color, which was experimentally augmented or dulled. We predicted that
augmented-billed females would appear dominant and that females would feed at the feeder protected by the “wimpy” looking female.
We found an overwhelming pattern that 17 of 19 females avoided feeding near competitor females with colorful bills.
This suggest that female carotenoid-based orange bills are evolutionarily maintained through selection for intrasexual status signaling.
This research provides some of the first evidence that carotenoid-based ornaments in females can function as status signals used in contests over non-mate based resources.
Test of mate-choice signaling by females
experimentally altered bill color.
Males showed no preference for bill color and spent similar amount of time courting augmented-billed and control-billed females.
The lack of effect of female bill color on male behavior suggests that males are not the intended receivers of this signal.
To test for a male mate-choice function to female bill color, we gave caged males a choice of courting two live females with
Streak-backed Orioles
Icterus pustulatus
We investigated the role of female coloration in signaling status both within and between the sexes. This research was part of an NSF postdoctoral project with multiple collaborators.
We tested whether the carotenoid-based orange breast coloration of the female streak-backed oriole signals status during territorial interactions. To do this, we simulated territorial intrusions using taxidermic models and compared the roles of the sexes within pairs during territorial defense directed toward different types of simulated intruders. Females were more territorial than their mates during the breeding season, whereas males were more territorial than their mates in the nonbreeding season, contrary to patterns seen in studies of temperate zone birds. The coloration of simulated female intruders also influenced territorial responses: When presented with color-augmented female models, females responded with greater intensity than their mates, whereas the intensity of defense was similar for both sexes when presented with average-colored female models. The greater female response to more colorful intruders suggests that females perceive more ornamented females as greater threats to their territorial tenure or to their pair bond. These results are consistent with the hypothesis that female carotenoid-based coloration signals status in this species.
Turquoise-browed Motmots
Eumomota superciliosa
Both males and females of many avian species maintain elaborate plumage traits, and ‘elaborate monomorphic’ plumage can convey adaptive benefits to one or both sexes as inter- or intraspecific signals.
The selective forces maintaining elaborate monomorphic traits are often assumed to be similar for the sexes; however, in some species, elaborate traits confer different selective benefits to males and females. Male and female turquoise-browed motmots express an elongate racket-shaped tail, and my research has shown that the slightly longer male tail is maintained by a combination of sexual selection and natural selection for predator-prey communication, whereas the female tail is maintained by natural selection alone.
Pursuit-deterrent function to male and female tail
My research indicates that males and females use their tail in a pendulous left-to-right wag-display, which is performed in the presence of predators. Results from predator-presentation experiments indicate that the display is likely to deter ambush by communicating awareness of the presence of a predator (i.e., perception advertisement pursuit-deterrent signal).
When I experimentally presented predators to motmots, and when natural predators were observed, the wag-display was often performed in the absence of conspecifics (i.e., when only the predator is present). Furthermore, the wag-display does not appear to function as an alarm signal oriented to the mate or to general conspecifics because the display is performed by unparied and solitary birds in the same manner as it is performed by paired birds and gregarious birds at nesting colonies.
The wag-display is thus likely to communicate that the motmot is aware of the predator and is prepared to escape. This form of interspecific pursuit-deterrent signal provides a benefit to both the motmot and the predator; the display prevents the motmot from wasting time and energy fleeing, and the predator avoids a costly pursuit that is unlikely to result in capture.
Sexual selection function to male tail (but not female tail)
In addition to the naturally selected pursuit-deterrent function of the tail, my research has shown that sexually selected benefits are associated with the male tail, but not the female tail.
Males with longer tail-wires (barbless region of the central tail feathers) have greater pairing success, pair with females that lay larger clutches, and have greater fledgling success. In contrast, female tail length is not related to measures of performance and reproductive success. Additionally, there is no evidence for assortative mating for tail length, further indicating that females do not use their tails as sexual signals.
The tail is sexually dimorphic after controlling for differences in body size and the most dimorphic part of the tail is the wire, which is ten percent longer in males. The shorter female tail length is likely to represent the naturally selected optimum to efficiently signal during predator-prey communication, whereas sexual selection is thought to account for the extension of the male tail beyond this length.
Thus, two selective forces work in concert to maintain the elaborate monomorphic plumage in males, but that natural selection acts alone to maintain female tails.
Significance
This research highlights that selection should not be expected to act on signal characters of males and females in similar ways. Additionally, this research emphasizes that elaborate plumage characters do not always function as sexually selected signals, as is often assumed, but rather that multiple selective forces can work together to favor elaborate monomorphic traits.
painting by
Gabriel Willow