After partial clutch loss due to predation plasma levels of prolactin, in mallards (Anas platyrhynchos) from a captive flock, decreased significantly from 26 to 11 ng/ml. These ducks also had a reduced hatching success of the remaining clutch. When clutches were experimentally reduced, decreases in plasma concentrations of prolactin were not observed and hatching success was normal. The extended period of reduced prolactin secretion, which follows partial clutch loss by a predator, probably leads to a deterioration in the quality of the incubation patch, which is prolactin dependent, and hence an inadequate heat and moisture transfer to the clutch with subsequent embryonic death. J. WILDL. MANAGE. 51(3):530-533 Disturbance at the nest site of waterfowl usually leads to decreased nesting success (Balat 1969, G6tmark and Ahlund 1984). Under natural conditions a primary cause of reduced nesting success is predation although other causes, such as human disturbance and temporary adverse environmental conditions, may be important (Kury and Gochfeld 1975, Bunnel et al. 1981). When nests of eiders (Somateria mollissima dresseri) with normal clutches are partially plundered by a predator the hatching success (of those birds that do not abandon) is reduced (Choate 1967). Increases in duck nesting success also have been reported when predation was temporarily reduced (Duebbert and Lokemoen 1980). The reason for the decrease in hatching success of the remaining eggs after partial predation, however, was not disclosed by these studies. Since a basic concept of wildlife management is the manipulation of factors that influence the inherent rate of increase of game populations, it is important to know the reason for the reduced nesting success of ducks if production rates are to be maximized. Incubation is a complex behavioral and physiological event in the breeding cycle of birds (Drent 1975). Prolactin has been demonstrated to be intimately involved in incubation of many birds, including waterfowl (Goldsmith and Williams 1980; Dittami 1981; Goldsmith 1982; Bluhm et al. 1983a,b; Hall and Goldsmith 1983). Although the exact role of prolactin has not been fully elucidated, it is believed to be absolutely essential for certain aspects of incubation, especially incubation patch formation and maintenance (Ensor 1978, Goldsmith 1983). Monitori g of plasma prolactin titers may provide a valuable indicator of likely success of incubation. H rein I report on a possible endocrinological mechanism responsible for the observed decrease in nesting success of ducks whose nests have been partially plundered by predators. I am grateful for assistance during the study or manuscript preparation given by H. Buhrow, J. P. Dittami, B. M. Knauer, D. S. Limberger, and J. A. Wozniak. A. S. McNeilly supplied the prolactin antiserum and the Natl. Inst. Health, Bethesd , Md. supplied the prolactin (NIHP-S-12) for use in the radioimmunoassay. MATERIALS AND METHODS Call ucks, a domesticated breed of an ancestral stock of wild mallards, were used in this study. This breed is similar to the mallard in size and time of breeding, molt, and behavior. The birds were kept as pairs in outdoor pens, approximately 6 x 6 x 2 m, that contained a pool (6 x 4 m) and a wooden nest box with straw for nesting material. Experiments were conducted at Seewiesen, West Germany (480N, 11PE) during 2 breeding seasons, 1980 (30 pairs) and 1982 (45 pairs), from late February to midJune. The pens were visited at least once a day. Females were considered to be incubating if they were in the nest box for >2 consecutive days, had stopped egg laying, and had lined the nest cup with down. All ducks used in this study were accustomed to human handling and also were used in a study on the endocrinological basis of incubation (Hall and Goldsmith 1983). Every 1-4 days during egg laying, incubation, and post-incubation the females were captured ' Present address: Department of Zoology, The University, Bristol BS8 1UG, England.