BROOD AMALGAMATION IN THE BRISTLE-THIGHED CURLEW NUMENIUS-TAHITIENSIS- PROCESS AND FUNCTION

Alloparental care in birds generally involves nonbreeding adults that help at nests or breeding adults that help raise young in communal nes ts, A less often reported form involves the amalgamation of broods, wh ere one or more adults care for young that are not their own. We obser ved this phenomenon among Bristle-thighed Curlew Numenius tahitiensis broods in western Alaska during 1990-1992. Amalgamation of broods gene rally involved the formation of temporary and extended associations. T emporary associations were formed by the incidental convergence of bro ods soon after they left their nests. During this period, parents defe nded distinct brood-rearing areas, were antagonistic to conspecifics a nd remained together for less than 3 days, Extended associations forme d when chicks were 1-2 weeks old, Here, parents and their broods occup ied distinct, but adjacent, brood-rearing areas and moved around as a unit. Whether a brood participated in either temporary or extended ass ociations or remained solitary appeared to depend on brood density in the immediate area and on hatching date, When chicks were 3-4 weeks ol d, aggregations of up to ten broods formed wherein young mixed and par ents defended a common brood-rearing area, All broods (n = 48) that su rvived to fledging joined such aggregations. Alloparental care involve d only antipredator defence and was not associated with activities suc h as feeding and brooding, Most female parents abandoned their broods shortly after the young could fly and when aggregations were forming, The female parent of a pair always deserted its young before or on the same day as the male parent and, in every aggregation, one or two mal es continued to tend young for about 5 days longer than other male par ents, In most cases, adults deserted the young 2-6 days before the you ng departed the area when about 38 days old. Bristle-thighed Curlews a lso formed temporary associations with American and Pacific Golden Plo ver Pluvialis dominica and Pluvialis fulva, Whimbrel Numenius phaeopus , Bar-tailed Godwit Limosa lapponica, Western Sandpiper Calidris mauri and Long-tailed Skua Stercorarius longicaudus. Curlews and other larg er bodied species commonly attack-mobbed predators together, whereas s maller bodied species generally gave alarm calls and circled the preda tors, For all species, the intensity of antipredator defence by attend ing adults gradually decreased as young became older and aggregations formed, We suggest that amalgamation of broods among Bristle-thighed C urlew enhances predator defence, aids in the process of flock formatio n for migrating young, and allows females and some males to desert the ir young earlier.