Individuals need to trade-off vigilance and foraging, but species differ in how they manage this. We explored the effects of both intrinsic and extrinsic factors on individual vigilance/foraging patterns in two migratory, gregarious bird species markedly differing in body size and foraging strategies, the Greylag Goose Anser anser and the Common Crane Grus grus, during the non-breeding period.

We show that vigilance in individuals of both species decreased with increasing group size and distance from the edge of the group. However, the two species varied how they allocated the freed up time. While cranes allocated the resulting time to foraging, the same did not occur in geese. In addition, we investigated changes in individual position in the group (i.e., peripheral vs. central or vice versa) and how they elicited a prompt behavioral change (i.e., vigilance vs. foraging or other activity). We also explored whether the time elapsed since arrival in non-breeding grounds and proximity to spring migration affected vigilance patterns.

We analyzed data using generalized linear mixed models (GLMMs). Analyses were conducted in three steps. First, we fitted species-specific models to explore the factors influencing indices of vigilance and foraging in the two species. Then, we fitted models to investigate the general differences in vigilance/foraging levels between the two species. We calculated the vigilance rate (VR), i.e. the no. of vigilance bouts divided by minutes of observation, the proportion of time in vigilance (PTV) and the proportion of time foraging (PTF), and used them as response variables. Eventually, we focused on repositioning events in the position of individuals within a flock, to evaluate whether reshuffling elicited any change in vigilance and foraging activity immediately after the repositioning. We analyzed presence/absence of vigilance and foraging behaviors after repositioning events (both active and passive due to rearrangement) in the observed individual’s position within a flock. Both in the first and in the third steps, we fitted separate models for each species, whereas in the second step we fitted one model for geese and cranes combined. In all steps, we used a multi-model selection approach, through which we ranked all possible candidate models, including the null model.

Results allowed to disentangle the role of multifactorial determinants of vigilance and foraging, in turn increasing our understanding of forces driving the evolution of behavior in group-living animals.

Our study was conducted in the Northern sector of the Maremma Regional Park (42°39′N, 11°05′E), a protected area in Tuscany (Central Italy). Behavioral observations were conducted between the end of October and the end of January, during two consecutive wintering seasons (2019/20 and 2020/21). Observations were carried out from vantage points 5-7 m above the ground, whilst the observer maintained a 200-500 m distance from the flocks in order to minimize potential alteration of birds’ behavior. Further information on behavioral observations and data collection can be found in section 2.2 of the paper. All the analyses can be replicated by following the statistical procedure and by using the software described in the Methods section of the paper.