Seasonal declines in the fecundity of arctic-breeding sandpipers: different tactics in two species with an invariant clutch size

The breeding biology of Western and Semipalmated Sandpipers was studied for four years near Nome, Alaska. Despite a short breeding season and a putati vely invariant clutch size. there were seasonal declines in the fecundity o f both sandpiper species. Most females produced only one clutch ( > 98%), b ut often laid fewer than four eggs (7.6-34.5% of first nests). Clutches of 2-3 eggs were initiated significantly later(6-8d) and contained smaller egg s(2.9-3.5%,) than 4-egg clutches. Small clutches were not renesting attempt s, and were not caused by disturbance during laying or partial clutch loss. This is one of the first reports of seasonal declines in egg number among birds considered to have an invariant clutch size. We evaluated four explanations for the seasonal declines in fecundity. The data did not support a clutch size-egg size tradeoff or the nutrient-reallo cation hypothesis, but it was not possible to evaluate the cost-of-delay hy pothesis. Variation in parent quality was the most likely explanation for t he seasonal declines in fecundity. Females that were familiar with the stud y area bred significantly earlier in Western Sandpipers (4.5 d) but not Sem ipalmated Sandpipers (3.5 d, p = 0.07). Timing of clutch initiation was not related to female age in Western Sandpipers or to familiarity with a mate in either species. The two sandpiper species adjusted different components of fecundity. Weste rn Sandpipers typically laid four eggs, and In three of four years there we re significant seasonal reductions in egg size among 4-egg clutches. In con trast, Semipalmated Sandpipers laid significantly fewer eggs than Western S andpipers. but females laying 4-egg clutches did not produce smaller eggs i f they laid late in the breeding season. Semipalmated Sandpipers may reduce clutch size because their smaller eggs are close to a minimum threshold si ze necessary to produce viable precocial young in the arctic.