Foraging success of the cattle egret Bubulcus ibis in relation to insect abundance, herd and flock size

Abstract

The success of foraging birds is attributed to a wide range of factors. For example, in the cattle egret Bulbulcus ibis, the presence of cattle, insects and other conspecifics is widely reported to enhance foraging success. However, few studies have considered the combined effect of insect abundance, herd and flock size on such commensalistic relationship particularly under the nomadic cattle management system practiced across West Africa. Thus, this study, compared cattle egret foraging success (prey capture rates and time interval between successive prey captures) between egrets foraging within cattle and those foraging solitary during the mid-wet season (June to July) 2013. Also, foraging success was related to insect abundance as well as herd and flock size. Study populations were randomly selected across 13-sites in Jos-east and north Local Government Areas of Plateau State, Nigeria. A total of 100 cattle egrets were studied using the focal observation technique. Abundance of insect prey per site was quantified using both sweep net and direct counts. Irrespective of the number of cattle at a site, cattle egrets foraging within cattle, significantly captured more prey and spent less time between successive prey captures than those feeding solitarily. Also prey capture rates increased significantly (P<0.05) with insect abundance for the non solitary egrets. However, there was no significant relationship between foraging rates with flock size (P>0.05) and insect abundance (P>0.05) for solitary feeding birds. These observations agree with previous studies that proximate benefits in terms of enhanced foraging success could be the reason for the strong association between the two organisms.

Keywords: cattle egret, foraging success, herd size, flock size