Abstract
Ravens (Corvus corax) feed primarily on rich but ephemeral carcasses of large animals, which are usually defended by territorial pairs of adults. Non-breeding juveniles forage socially and aggregate in communal winter roosts, and these appear to function as ‘information centers’ regarding the location of the rare food bonanzas: individuals search independently of one another and pool their effort by recruiting each other at roosts. However, at a large raven roost in Newborough on Anglesey, North Wales, some juveniles have been observed recently to forage in ‘gangs’ and to roost separately from other birds. Here we adapt a general model of juvenile common raven foraging behavior where, in addition to the typical co-operative foraging strategy, such gang foraging behavior could be evolutionarily stable near winter raven roosts. We refocus the model on the conditions under which this newly documented, yet theoretically anticipated, gang-based foraging has been observed. In the process, we show formally how the trade off between search efficiency and social opportunity can account for the existence of the alternative social foraging tactics that have been observed in this species. This work serves to highlight a number of fruitful avenues for future research, both from a theoretical and empirical perspective.
Highlights
In the winter, common ravens (Corvus corax) typically forage over large areas on rich but ephemeral carcasses of large animals, which can be buried by unexpected snowfalls or consumed rapidly by other scavengers [1]
We proceed by describing how FD, a ‘gang’ foraging equilibrium – similar to the foraging behavior observed in the two Newborough sub-roosts – can be selected for over SR, which is equivalent to the recruitment-based foraging typical of juvenile common ravens in the main roost at Newborough and other roosts
Our formulation reveals that the typical juvenile raven foraging strategy of searching individually and recruiting from a communal roost can be vulnerable to invasion by gang-based foraging under a range of conditions (Figure 1), only a subset of these conditions is compatible with the long-term persistence of this foraging strategy (Figure 2)
Summary
Common ravens (Corvus corax) typically forage over large areas on rich but ephemeral carcasses of large animals, which can be buried by unexpected snowfalls or consumed rapidly by other scavengers [1]. The formulation is based upon the North American raven system, studied by Heinrich and co-workers for many years [1,4,5,7,8,9,10] This system is thought to represent the original native winter habitat of the common raven, in which non-territorial juvenile birds forage for carcasses over large snow-covered forested areas and form transient communal overnight roosts. The opportunity to share foraging information can be sufficient to drive this result, thereby confirming the logic of the ICH, while the benefits of foraging as a member of a group are not so necessary, but they are still likely to play an important role in the raven system
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