All in the family: The effects of urbanization and weather variability on the behavior, sociality, and population dynamics of a kin-structured passerine, the black-crested titmouse (Baeolophus atricristatus)
MetadataShow full metadata
There is much debate as to the proximate and ultimate causation of kin-structure in avifauna, as it is relatively rare among bird species. However, the research needed to understand causation of kin-structure begins with foundational work on population demographics and social dynamics. Without sound knowledge of dispersal behavior, survival rates, and reproductive metrics, one cannot begin to untangle how kin-selection favors the formation of family groups where members increase inclusive fitness through the passing of personal and shared genes. This is especially important considering how urbanization and shifting weather patterns may additionally effect species evolved to remain near kin.
One such species that forms extended family groups is the black-crested titmouse (Baeolophus atricristatus, BCTI), a non-migratory passerine in the Paridae family. My master’s thesis research revealed that in a rural setting, large juvenile male BCTI often limit their natal dispersal and establish territories adjacent to their parents the following spring, forming kin-structured neighborhoods over time. Even though this was an interesting and novel discovery for the Baeolophus genus, my work prompted additional questions as to why BCTI maintain kin-structure and how urbanization influences extended family relations, topics which I addressed in this dissertation.
Because BCTI are commonly observed in rural and urban landscapes within their range, I first studied if BCTI residing in an urban environment form kin-structured neighborhoods, and if so, to what extent. Additionally, I examined home range size, habitat composition of home ranges, and body conditional indices (BCI) of urban BCTI and compared them to a rural population. My study site was at four different locations in San Marcos, Texas, that all varied in degrees of urbanization. The rural site, where I also conducted my thesis research, was the Freeman Center (hereafter Freeman), a 1400-ha Texas State University property ~10 km from downtown San Marcos. The habitat at Freeman is a mosaic of native hill country vegetation with minimal man-made structures or roads. My urban sites included Texas State University campus (hereafter Campus), single-home residential neighborhoods in San Marcos (hereafter Residential), and public parks operated by the City of San Marcos (hereafter Parks). Thus, the rank of locations from highest to lowest in terms of urbanization is Campus > Residential > Parks > Freeman. My results revealed that BCTI in urban locations form kin-structured neighborhoods but to a lesser extent than their rural counterparts. Home range size of urban BCTI was influenced by habitat composition, where home ranges were larger on Campus (highly urbanized) and smaller in Residential areas and in Parks (low degrees of urbanization) compared to BCTI home range size at Freeman. Additionally, BCTI in urban and rural locations did not differ in BCI, but there was a high degree of variation of BCI within and among family groups.
Because there were differences in the extent of kin-structure neighborhood formation between urban and rural populations of BCTI, for my second study, I investigated if there were differences between the two in relation to productivity, nesting success, and timing of reproductive cycle. I also accounted for differences in habitat composition and weather variability across the breeding season. My results provided evidence that urban BCTI begin first clutch initiation ~9 days before rural BCTI, and average February low temperatures were highly correlated with nest initiation date at Freeman. Though overall productivity was not different between urban and rural locations, nest fate and daily survival rates of nests were much lower for Residential nests than nests in any other location.
Results from my second study suggested that additional population demographics of the BCTI may be influenced by urbanization, therefore I addressed this in my third study. I examined if apparent survival of BCTI varied between urban and rural populations, between sexes and ages, as well as between breeding and non-breeding seasons (summer and winter, respectively). Additionally, I assessed if apparent survival increased when adult BCTI held territories near kin in kin-structured neighborhoods. Overall, apparent survival estimates were higher for males compared to females, as well as higher for adults compared to juveniles regardless of location or season. Urban males had a higher apparent survival than rural males during the summer but a lower apparent survival during the winter. Moreover, urban females had a lower apparent survival than rural females in both seasons. Thus, rural BCTI apparent survival was more stable over time for both age classes and sexes, indicating that urban BCTI may be more susceptible to population fluctuations due to inclement weather and habitat inconsistencies. Interestingly, apparent survival for rural BCTI surrounded by kin was lower than that estimated for adults not surrounded by kin. Though this seems counterintuitive to kin-selection, I hypothesize that these estimates could indicate that once adults are established territorially, they tend to remain in kin-structured neighborhoods until they die, whereas adults not surrounded by kin may be younger, healthier, and still aiming to achieve kin-structured status.
Finally, in my fourth study, I investigated if there were potential inclusive fitness benefits for BCTI in kin-structured neighborhoods through the behavior of resource sharing. By using radio frequency identification (RFID) feeding stations, I was able to monitor a small population of BCTI equipped with passive integrated transponder (PIT) tags and could record when certain individuals aggregated to share food. Results revealed that both closely and distantly related kin may benefit through “quality over quantity” foraging bouts together (foraging time together versus frequency of foraging bouts together). Additionally, males accompany their mate to a high extent on foraging bouts during the breeding season as compared to any other season, likely indicating mate guarding behavior.
Overall, results from this dissertation indicate that though BCTI are still common and establish kin-structured neighborhoods in urban locations, they likely incur lower reproductive success and apparent survival due to certain stressors and threats that are not as prevalent in rural locations. Therefore, the differences in BCTI social dynamics and demographics between urban and rural populations may eventually lead to less kin-structure in urban areas over time. This in turn could create a positive-feedback loop, where less kin-structure leads to lower survival and recruitment, which again influences kin-structure. Because it does not appear that resource sharing is a highly kin-selected behavior among BCTI, future work should examine other behaviors that potentially maintain extended family cohesion, such as joint territory defense, predator vigilance, or the availability of empty territories for juveniles to inherit. Thus, by determining what behaviors or traits are most important to maintaining BCTI kin-structure, future conservationists may be able to directly increase apparent survival and reproductive success should BCTI populations begin to decline.