BIOLOGY OF THE IMMIGRANT CATTLE EGRET ARDEOLA IBIS IN GUYANA, SOUTH AMERICA

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Abstract
Summary: The success of the immigrant Cattle Egret Ardeola ibis in Guyana appears to be due mainly to its breeding biology, combined with man's activities in clearing and draining land for stock and by his selective shioting.Long.‐established heronries are beine taken over bv the Cattle Egret. In Aurmst 1959 during the principal nesting season in the main rains 90% of the 1786 heron nests in the Botanic Gardens Georgetown, belonged to Cattle Egrets, the others to Little Blue Herons Hydranassa caerutea, Black‐crowned Night Herons Nycticorac nycticorux, Snowy Egrets Egretta thula, Tricolour Herons Hydranassu tricolor, and Streaked Herons Butorides striatus. In the December rains the same year another 1700 Cattle Egret nests were in use here, but no indigenous herons were nesting. These were, however, joined in January by Black‐crowned Night Herons, Snowy Egrets, Little Blue and Tricolour Herons.In addition to having two peak nesting periods a year, instead of one as the indigenous herons generally have, the Cattle Egrets react faster to the onset of the rains and obtain the best nest‐sites, and they are helped by the tendency of the indigenous herons to display more aggressively against their own species than against the intruder; also now that Cattle Egret numbers are high they can find mates very rapidly. Both indigenous herons and Cattle Egrets have one to three young per brood in Guyana, and the nesting period lasts about two months.The peak nesting time varies from year to year with the advent of the rains. Nesting appears to be stimulated by, and its success related to, rainfall. A cycle of wet years in Guyana preceded the arrival of the Cattle Egrets in North America in 1952. In Guyana their dispersal inland was restricted, probably by the forest, until 1961.