Resource partitioning in colonial herons, with particular reference to the grey heron Ardea cinerea L. and the purple heron Ardea purpurea L., in the Camargue, S. France
There have been major changes in the relative abundance of the six species of colonial Ardeidae (herons and egrets) which breed in the Camargue, since 1968. Two species, the Grey Heron and the Cattle Egret, started nesting in the delta in the mid-sixties and have increased dramatically in numbers, whilst there has been a simultaneous decline in the breeding population of Purple Herons. Examination of counts and of the patterns of resource use by these species show that although Grey and Purple Herons overlap extensively in morphology, breeding sites (reedbeds), feeding sites and diet, competition during the breeding season cannot have been responsible for the observed changes, since numbers of Purples have remained stable in those areas where the greatest increases of Greys have occurred, whilst they have completely disappeared in some areas where there were no Greys. Instead, breeding sites have been destroyed to such an extent within the delta that they now limit the number of Purple Herons below the level set by winter mortality, which has been shown to limit breeding populations in other areas. For Purple Herons, the relationship between colony size and food resources was examined at a breeding-site where colony size was not limited by the availability of adequate breeding habitat. Adults exploited feeding areas near (<2km) to the colony in preference to those further away (max. 15km); near areas were the first to be filled and remained filled throughout the breeding season. Early breeders held feeding territories in areas adjacent to the colony, to the exclusion of later breeders. Breeding partners did not share the same feeding territories, nor associate in any way on the feeding areas. It is suggested that the increasing costs of flight-time and flight-energy incurred by adults foraging further from the colony may set an upper limit to colony size. Aspects of the breeding biology of the Purple Heron are described. The average clutch-size in the Camargue is the lowest reported for Europe, and declined significantly through the summer. Siblings differed considerably in size as a result of the asynchronous hatching of the clutch, and not of egg-size differences. The first two chicks in broods of three, and the first three chicks in broods of four, grew fastest and a high proportion of the youngest chicks in a brood died of starvation. It is argued that the number of young fledged is limited by the ability of the adults to deliver food to the nest. Changes in clutch-size are used to adjust brood-size to predictable changes in food availability, while brood reduction through sibling hierarchies provides a fine tuning at the time of peak nestling demand. The diet of Grey and Purple Herons was examined from nestling regurgitations; both species are highly piscivorous. The Grey Heron specialises on large Eels, Carp and Mullet whilst the Purple Heron takes Eels and a wide variety of other fish, plus some invertebrates. Mechanisms of prey selection were examined for the Grey Heron. Large Carp and similar fish are the most profitable prey for adults whilst small fish and species which possess protective spines, such as Catfish, are of low profitability. Young chicks cannot consume prey as large as can the adults, although this ability develops by the time they are ca 30 days old. Evidence is presented that adults select prey in relation to the age of their chicks. The results of this study are discussed in relation to the conservation of breeding heron populations in Europe. Recommendations for the management of breeding areas are proposed.