DUCKLING MORTALITY IN BARROW GOLDENEYE AND BUFFLEHEAD BROODS

We compared duckling mortality patterns in Barrow's Goldeneyes (Bucephala islandica) and Buffleheads (B. albeola) in the aspen parkland of central British Columbia, Canada. Hatching phenology of both species varied between years in relation to spring temperatures. Buffleheads hatched on average a few days later than Barrow's Goldeneyes each year. Mortality estimates for ducklings differed by as much as 55%, and depended on the technique used. Estimates of mortality rates per brood were higher and more variable than those calculated per duckling. The Mayfield estimate of mortality per duckling day, adjusted for duckling age, was the most accurate. Mortality rates of Barrow's Goldeneyes and Buffleheads varied similarly between years. Mortality patterns in relation to duckling age, however, varied between species and between years. The highest mortality usually occurred in the week after hatching. Hatching date influenced mortality rates in 3 of 5 yr, but there was no consistent pattern. Barrow's Goldeneye ducklings suffered higher mortality than Bufflehead ducklings in 4 of 5 yr. This pattern was reversed in 1984, when the largest number of Barrow's Goldeneye broods occurred. Duckling mortality was higher on ponds with several broods than on ponds with single broods, which implies density-dependent mortality. This may have been due to intra- and interspecific aggression. The high mortality of Barrow's Goldeneye ducklings in 1983 was followed by lowered female recruitment in 1985. Because aggressiveness by Barrow's Goldeneyes has more impact on conspecifics, Barrow's Goldeneyes and Buffleheads can coexist at low to moderate densities of goldeneyes. At high goldeneye densities, Buffleheads may be excluded from some ponds.