Eating More and Fighting Less: Social Foraging Is a Potential Advantage for Successful Expansion of Bird Source Populations

SIMPLE SUMMARY: Understanding why some animal species expand their distributions but others do not under the same global climatic fluctuations and habitat changes is essential to determine expansion mechanisms and predict alien species invasion. However, whether the source population of an expansive species has the potential to survive in a new environment after expansion is rarely evaluated. Social foraging is considered beneficial in obtaining food and increasing population establishment. However, competition will increase under the social context of group foraging. In this laboratory research, whether social foraging could promote individuals to consume more novel food in an unfamiliar environment, which can facilitate survival and population establishment, was tested in a source population of a successful expansive bird, the Light-vented Bulbul (Pycnonotus sinensis). Compared with the sympatric, nonexpansive relative species Collared Finchbill (Spizixos semitorques), bulbuls had longer latencies to consume novel food, but they increased eating times when transferred from solitary to group status and had higher possibilities of eating with their companions. In addition, more than two individuals eating together is significantly more frequent in bulbuls than finchbills. Therefore, social foraging can be a critical species-specific trait that is a potential advantage for expansion because it weakens competition among individuals. This study provides testable hypotheses to explain mechanisms of successful expansions and to predict alien species expansion and possibilities for control. ABSTRACT: Animals can expand distributions in response to climatic and environmental changes, but the potential expansive ability of a source population is rarely evaluated using designed experiments. Group foraging can increase survival in new environments, but it also increases intraspecific competition. The trade-off between benefit and conflict needs to be determined. The expanding Light-vented Bulbul Pycnonotus sinensis was used as a model to test mechanisms promoting successful expansion. Social foraging and its advantages were evaluated using lab-designed feeding trials. Consuming novel foods was compared between bulbuls and a sympatric, nonexpansive relative species, the finchbill Spizixos semitorques, from native areas at both solitary and social levels. Bulbuls increased their eating times when transferred from solitary to group, whereas social context did not affect finchbills. Bulbuls were significantly more likely to eat with their companions than finchbills when in a group. Thus, exploring food resources in a bulbul source population was facilitated by social context, indicating that social foraging is an important means by which birds successfully expand and respond to environmental changes. This research increases understanding of successful expansion mechanisms and will consequently help predict invasive potentials of alien species.