Are the costs of routine vigilance avoided by granivorous foragers?

Authors: Baker, D.J., Stillman, R.A., Smart, S.L., Bullock, J.M. and Norris, K.J.

Journal: Functional Ecology

Volume: 25

Issue: 3

Pages: 617-627

eISSN: 1365-2435

ISSN: 0269-8463

DOI: 10.1111/j.1365-2435.2010.01829.x

Abstract:

Routine vigilance is an important component of foraging for many species and can occupy a large proportion of this time. Vigilance can conflict with some aspects of foraging (i.e. searching) and, consequently, has the potential to reduce feeding rates by interrupting foraging behaviours. However, for animals that handle food in an upright posture vigilance can be compatible with this portion of handling and, therefore, any vigilance during this time will incur minimal time-costs to foraging. Several functional response models that incorporate vigilance have assumed that, (i) vigilance preferentially occurs during compatible portions of a foraging routine where no costs are incurred; and (ii) vigilance can be treated as a single discrete block of time related in frequency to the feeding rate, i.e. one vigilance scan per feeding event. To determine whether these assumption are appropriate we measured the vigilance behaviour of four species of granivorous bird, yellowhammer, tree sparrow, linnet and grey partridge, and assessed the quantity of vigilance compared to compatible handling time, the relationship between scan rate and feeding rate and the distribution of vigilance during compatible and non-compatible portions of foraging. The results show that there was frequently enough compatible handling time to accommodate routine vigilance, yet a high proportion of vigilance occurred during non-compatible components of foraging, thus incurring a time-cost. The frequency of vigilance bouts was higher than the feeding rate for three species and, therefore, routine vigilance was not just a by-product of the feeding rate (i.e. one scan per peck). Frequent head-down searching behaviour was recorded during handling suggesting that searching is still a prominent component of foraging even when prey is superabundant. We have shown that the assumptions of previous functional response models might underestimate the effects of vigilance on feeding rate by overestimating the proportion of compatible handling devoted to vigilance. Future functional response models must account for this distribution of vigilance between compatible and non-compatible behavioural states. We derive an example of such a model; however, more experimental research will be needed before we understand the factors that influence the distribution of vigilance. © 2011 The Authors. Functional Ecology © 2011 British Ecological Society.

Source: Scopus

Preferred by: Richard Stillman