An Experimental Study of Clutch Size of the American Coot

MUCH information on laying behavior has appeared in the literature since Phillips (1887) performed his classical experiment by removing eggs daily from a flicker's nest. Lack (1947, 1948a, 1954) suggests a variety of factors that might influence clutch size, and he believes all factors are related to the maximum number of young a pair can rear successfully. Lack's ideas are based on studies of closely related species that nest over wide geographic areas. Not all workers agree with Lack's theory on food as the mechanism for controlling clutch size. Skutch's (1949) observations on tropical nesting birds indicate that these birds rear fewer young than they can nourish. Wynne-Edwards (1962) proposes that populations are regulated by social behavior before food resources are depleted. The differences of opinion existing between workers may be attributable to different mechanisms that are effective in regulating animal populations in varying degrees under a variety of conditions (Weins, 1966). Experimental studies of clutch size have been conducted on oceanic birds that are determinate layers. Clutch size has been manipulated in Laysan (Diomedia immutabilis) and Black-footed Albatrosses (D. nigripes) by Rice and Kenyon (1962) and in North Atlantic Gannets (Sula bassana) by Nelson (1964). I know of no reports in the literature that describe responses to manipulations of clutch size of indeterminate layers after the clutch is complete and incubation has started. American Coots (Fulica americana) are indeterminate layers (Sooter, 1941) and both sexes normally incubate before the clutch is complete. The eggs hatch over a period of several days, and both sexes brood and feed the young. To gain an insight into the mechanism controlling clutch size in coots, the clutch was changed by adding or removing eggs during incubation after the clutch was complete. The manipulations of clutch size provided information on the number of eggs coots can incubate successfully and the number of young they can rear successfully.