The reduction in foraging and non-foraging group size in birds is compatible with the hypothesis that predation risk is an important component of sociality. However, the fact that sociality did not completely disappear in most species suggests that while anti-predation is a major component of sociality in birds, it is not the only one.
In many tallied species with negligible predation risk, social foraging increased foraging efficiency by increasing the chances to locate ephemeral food patches and/or by increasing prey capture rate while in a food patch (e.g. Fleming et al. 1992
; McMahon & Evans 1992
; Bélisle 1998
). This is particularly striking in the Swainson's hawk (Buteo swaisoni
), a large raptor with negligible predation risk, but that nonetheless roosts and forages in large groups, presumably in response to patchily distributed food (England et al. 1997
These results corroborate findings from an earlier study indicating that island species of birds with few predation threats also foraged in smaller groups (Beauchamp 2004
). The results on sociality in birds are thus in line with the general finding from relaxed selection studies that traits under relaxed selection persist in their current form or in an intermediate form when other functions are still operating (Lahti et al. 2009
). In the case of sociality in birds, it would appear that the foraging benefits associated with sociality are important in its maintenance.
The association between a relatively larger body mass and reduced sociality raises interesting evolutionary issues. Perhaps solitary species of birds did not evolve greater sociality because they already possessed a large size and were thus less vulnerable to predators. Alternatively, sociality was lost as greater size was evolved. Future work could identify the probable sequence of evolutionary events pertaining to the association between body mass and sociality. Nevertheless, the results do suggest a negative relationship between body mass and sociality in birds. In contrast, a positive relationship between sociality and body size was reported in rodents (Ebensperger & Cofré 2001
) but not in ungulates (Brashares et al. 2000
). While more work is definitely needed to address the coevolution of body mass and sociality, it is conceivable that larger body mass may deter predation to different extent in different taxa.
It is interesting to note that the decrease in sociality with negligible predation risk occurred in non-foraging as well as in foraging groups. Non-foraging groups, which in most cases include resting or roosting birds, have often been thought to form to reduce predation risk (Beauchamp 1999
), but the fact that non-foraging groups can persist in the face of negligible predation risk suggests that other functions for these groups are conceivable and currently operating. Such functions may include transfer of information about distant food sources (Ward & Zahavi 1973
). Nevertheless, when faced with negligible predation risk, more non-foraging groups than foraging groups consisted of solitary individuals suggesting either that avoidance of predation is more relevant in non-foraging groups or that current functions are fewer or acting less strongly. Future comparative in birds and other taxa as well will be needed to assess the generality of these findings.