Nutrient reserves and clutch-size regulation of Northern Shovelers in Alaska

We determined patterns of nutrient-reserve use by female Northern Shovelers (Anas clypeata) nesting at Minto Flats, Alaska, and compared them with tho se of female shovelers nesting in the Prairie Pothole Region of Manitoba, C anada. Individual variation in somatic lipid was best explained by nest ini tiation date; females that initiated nests early had larger lipid reserves than females that delayed nest initiation. These results contrast with thos e from Manitoba, which showed that females used lipid reserves and stored p rotein during egg production. Incubating females from Alaska did not use pr otein or mineral reserves, but lipid reserves decreased significantly throu ghout incubation. Females in Alaska and Manitoba used lipid reserves simila rly during incubation. We conclude that endogenous nutrient availability do es not proximately limit clutch size during laying for this population of s hovelers, possibly due to the high productivity of wetlands in interior Ala ska and/or the long photoperiod that allow females to forage extensively. S uccessful completion of incubation or brood rearing may be an ultimate fact or that controls clutch size for this population of shovelers.