The Cattle Egret Bubulcus ibis as a near-global indicator of terrestrial pollution

Persistent organic pollutants (POPs) are omnipresent in the environment. Due to their physical and chemical properties, POPs bioaccumulate in higher trophic-level species. Birds inhabit many niches and trophic levels and share physiological characteristics with humans. Wild bird eggs reflect the pollution of the environment and exposure experienced by the female prior to egg formation. Bird eggs are relatively easy to collect, easy to handle and store, and decompose slowly. In addition, bird eggs can also be used as biomarkers by examining the eggshell pigmentation and thickness. Aquatic bird eggs have been used most often with less published on terrestrial birds that also suffer consequences of pollution. Here, we review candidate species whose eggs can be used as near-global indicators. We found the Cattle Egret (Bubulcus ibis) as a suitable near-global indicator of terrestrial pollution. It is the most widespread Ardeid with extensive natural expansion, not threatened, occurs in high numbers, breeds in colonies with other Ardeids, has a high trophic status, and eggs are relatively large and easy to collect. Eggs of Cattle Egrets from eight countries and four continents have been analysed for pollutants such as PCBs, DDTs, HCH, HCB, dicofol, aldrin, dieldrin, endrin, heptachlor, mirex, and endosulfan. Multi-country and trans-continental studies using Cattle Egret eggs would therefore be possible, tracking the effectiveness of interventions such as the Stockholm Convention.