Territorial aggression in birds is widely observed and linked to sex, age, body size, physiology, seasonal cues, food resource, urbanization, and many other factors; rarely, however, are multiple predictors and response factors assessed simultaneously. Here we present an in-depth examination of environmental and social contexts influencing both aggression level and vocal production exhibited by 2 closely associated bird species when engaged in territorial defense. We tested aggression variations in Tufted Titmouse (TUTI, Baeolophus bicolor) and Carolina chickadee (CACH, Poecile carolinensis) during a breeding season in Northern Florida. During playback trials, we measured physical (closest approach, number of flights), vocal (number of calls), and temporal responses (latency to start, total time spent) of the most aggressive respondent, noted approaches by other individuals and species, and recorded all vocalizations produced during trials. Moreover, at each trial location, we characterized tree density (basal area and canopy cover) and background noise level. We also tested social factors such as aggression direction, territory density and the presence of larger heterospecific bystanders. Assuming that intra- would be greater than inter-specific aggression, we tested the hypotheses that social and environmental factors would modify the level of aggressiveness and the vocal production of respondents.
Results/Conclusions
The result showed that environmental factors mattered to vocal production. We found CACH vocalized more songs and calls when the habitats contain better food and nest resources (higher tree density). TUTI vocalized more notes per D-note call and more high-frequency squeal calls in noisier areas. Social factors mattered to both aggression level and vocal production. There was strong asymmetric interspecific aggression between two ecologically closed-related Paridae species and TUTI dominates CACH. The intraspecific aggression level was also much higher in TUTI than CACH. Territory density showed an important effect on TUTI vocal production. TUTI produced fewer songs and calls in habitats with higher territory density. Most interestingly, the presence of larger heterospecific (e.g. cardinals and woodpeckers) increased interspecific aggression and decreased the use of secret language (high-frequency contact calls) from TUTI. In conclusion, we suggested environmental and social context have strong influences on aggression level and vocal production in sympatric Paridae.