INCUBATION CAPACITY AND CLUTCH SIZE DETERMINATION IN 2 CALIDRINE SANDPIPERS - A TEST OF THE 4-EGG THRESHOLD

Several groups of vertebrate taxa, including shorebirds, are unusual i n that they produce a fixed number of offspring. The aim of this study was to examine whether the incubation capacity of western sandpipers (Calidris mauri) and semipalmated sandpipers (C. pusilla) limits their maximum clutch size to four eggs. Experimental enlargement of clutch size had no effect on rates of nest abandonment, nest attendance or lo ss of body mass by incubating sandpipers. The duration of incubation w as significantly longer for enlarged five-egg nests, and there were tr ends towards increased partial clutch loss and asynchrony at hatch, bu t overall hatching success was unaffected by experimental egg number. I conclude that small, calidrine sandpipers with biparental care are a ble to compensate for an additional egg in an enlarged nestbowl, despi te the constraints of conically shaped eggs and two brood patches. Pos sibly, shorebirds do not lay more than a fixed clutch size of four egg s because selection on factors acting during egg production or brood-r earing is more important in regulating offspring number.