JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Abstract. We determined the time of day at which eggs were laid and the laying interval (time between laying of successive eggs in a clutch) in the Hudson Bay race of the Common Eider (Somateria mollissima sedentaria), at La P6rouse Bay, Churchill, Manitoba (58?24'N, 94?24'W). Nests were found at the one-egg stage and were subsequently visited three times daily. Analysis of the nest contents at each visit allowed us to estimate mean egg-laying times as well as the mean time at which eggs were lost to predators. The estimated mean egg-laying hour was 13:49 (CST, 95% CL 12:30-15:06). We detected no selective advantage to laying at this time based on the timing of egg predation. The average egg-laying interval was 27.7 + 3.4 hr. Laying intervals decreased with increasing clutch sizes. For clutches of four and five eggs, the estimated interval between the last two eggs was significantly longer than that for intervals between all other eggs, all other comparisons between intervals were not significantly different. If last-laid eggs were excluded the mean laying interval for all eggs was 26.1 + 4.3 hr, confirming that the last egg in a clutch takes longer to produce. We suggest that longer laying intervals of last-laid eggs may be related to hormonal changes associated with the onset of incubation. University of California Press and

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. N, 94024'W). Nest parasitism was detected by three methods: (1) multiple eggs laid in the same nest on a single day, (2) eggs laid before or after the host's clutch was laid, and (3) large withinclutch variances in egg size and color. It was determined that 42.4% (n = 153) of completed clutches were parasitized. Parasitic eggs were laid significantly earlier in the host's laying sequence than expected by chance: 65% of parasitic eggs were laid on the first two days of laying. Number of parasitic eggs laid, as a proportion of all eggs, did not change significantly throughout the laying period. The probability of parasitic and host eggs hatching was not significantly different from that in unparasitized nests. Hosts did not reduce their clutch size in response to parasitism, when data were controlled for initiation date, nor did they hatch any fewer of their own young for a given clutch size than unparasitized nests. Parasitized nests were found in areas with higher densities (number of neighbors within 10 m) at initiation. Parasitism in this species does not appear to be a salvage strategy and may be part of a mixed or conditional strategy. University of California Press and