The influence of social-grouping on territorial defense behavior in the black-crested titmouse (Baeolophus atricristatus)

Abstract

Status signals have evolved for individuals to avoid energetic and physical costs of resource defense. These signals reflect an individual’s competitive ability and therefore influence competitors’ decisions on how to invest in a fight. We hypothesized that the response of receivers to status signals will depend on the social context. During territorial defense, group members may provide support to a territory owner by participating in defense. We investigated whether the presence of juveniles—who group together with territorial males—alters the territorial male’s attack decisions and level of aggression in the black-crested titmouse (Baeolophus atricristatus). Crest-length in this species functions as status signal. We simultaneously presented two taxidermic male models in a territory: one with an unmanipulated crest and one with a modified shortened crest. Models were presented to males that had resident juveniles cohabiting on their territory, and to males without juveniles. During intrusions, juveniles actively defended against the simulated intruders by approaching and sometimes attacking. The presence of juveniles affected how territorial males responded to the status signals of the intruders: when juveniles were present, males were more likely to first attack the model with the unmanipulated crest (i.e., longer, and more threatening), compared to males residing without juveniles. This suggests that juvenile support alters the risk-taking decision of the territorial male. To our knowledge, this is the first indication that behavioral responses to a status signal depends on the presence of supportive group members. Significance statement Status signals can indicate relative quality of animals and can therefore be used to evaluate a competitor when deciding whether or not to fight over resources. The black-crested titmouse has been shown to use its crest length as a status signal during fights over food. In our study, we assessed if this status signal is also used in territorial defense, by conducting an experiment where we presented two taxidermic male models with different crest sizes to a territorial male. We also investigated whether juvenile presence influenced which model was attacked. In trials where juveniles were present, territorial males attacked the longer crested model significantly more often than in trials where territorial males were alone. This suggests that the presence of juveniles, which help the male defend the territory, allows the male to attack the more aggressive-appearing intruder.

Geographically consistent hybridization dynamics between the Black-crested and Tufted titmouse with evidence of hybrid zone expansion

“…Along with the possibility of divergent mate choice, and particularly female mate choice, female-female or male-male signaling could act as a selective agent (Murphy et al 2009a, 2009b, Tarvin and Murphy 2012, Brooks and Wimberger 2023). At least male Black-crested Titmice use their crests for signaling (Dixon 1955, 1978, Queller and Murphy 2017, Borger et al 2020); anecdotally female Black-crested Titmice do not (Borger et al 2020). Whether male or female Tufted titmouse uses crests for signaling is, to the best of our knowledge, as yet unstudied, although foreheads in Tufted titmice are related to status signaling (Moses and Ritchison 2000).…”

No genetic evidence for parent–offspring relatedness in post-breeding social groups of Black-crested Titmouse (Baeolophus atricristatus)

“…A larger group size might also be beneficial in territory defense (Davies and Houston 1981, Brouwer et al 2005, Mares et al 2012). In the Black-crested Titmouse, there is evidence that the subordinates are heavily involved in territory conflicts (Borger et al 2020) and that juveniles participate in predator mobbing (TGM, pers. obs.).…”

“…Males are highly territorial, defending confined breeding territories in spring and summer, and have larger home ranges in the winter (Brawn andSamson 1983, Rylander 2015). After the breeding season, 2 paired adults often form social groups with up to 6 juveniles, and such groups may persist until the following breeding season (Rylander 2015, Queller and Murphy 2017, Borger et al 2020. Cooperative breeding does not regularly occur in this species, although its occurrence has been reported (Rylander 2015).…”

“…This study was conducted on a site near San Antonio, Texas, USA (29841 0 15 N, 98819 0 49 W) during the post-breeding season, between 18 May and 12 July 2018. As part of a larger study of territorial defense (see Borger et al 2020), territorial intrusion experiments (STIs: Simulated Territorial Intrusions; with taxidermic models and conspecific vocalization) were carried out on 25 territories. During these experiments, 12 of these territories were defended by adults and accompanying juveniles (the remaining territories were defended by adults alone).…”