Mister Parid's Neighborhood - dispersal behavior and flocking dynamics of the social Black-crested Titmouse (Baeolophus atricristatus)
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The black-crested titmouse (Baeolophus atricristatus, hereafter BCTI) is a small, non-migratory passerine that has a tendency to form family flocking groups due to delayed juvenile dispersal each summer. A recently elevated species, separated from its sister-taxon, the tufted titmouse (Baeolophus bicolor), the BCTI is an ideal model species for studying family-flocking dynamics. The three goals of my project were to: (1) determine whether intrinsic factors, such as weight or sex, or extrinsic factors, such as territory size or habitat composition, affect the natal dispersal of the BCTI; (2) determine whether BCTI flock or territory size varied across seasons; and (3) examine the social dynamics of familial related BCTI individuals.
Between 2013-2015, 263 individual BCTI were captured and color-banded at the Freeman Center in San Marcos, Texas across a study area comprising approximately 1,400 ha where family flocks were resighted and monitored. Over 800 hours of focal observations were recorded. Using a generalized linear mixed effects model and Fisher’s exact test, the intrinsic factors weight (p < 0.01) and sex (p < 0.01) of juvenile BCTI appeared to influence which individuals in a brood would delay their dispersal. Through another generalized linear mixed effects model, the predictor Julian date was found to be a significant predictor for annual flock size (p < 0.001), with territory size increasing an average 3.4 ha (p < 0.01) between the breeding season (March-July) and the non- breeding season (August – February). Juvenile BCTI that delayed their dispersal often establish territories adjacent to their parents the following year, thus creating kin- structured neighborhoods. Social interactions between related individuals on neighboring territories were almost always passive as opposed to aggressive (p < 0.05), potentially leading to an increase in inclusive fitness for both individuals involved.
Family flocking dynamics of the BCTI are more complicated than previously thought, and future research may yield insight into how this species evolved its current social structure. With the ever-growing threat of habitat fragmentation, the importance of the family unit and social dynamics to the survival of the BCTI may be key for management of this species.