Seasonal Influence on the Strategies of Growth and Development of Canvasback and Lesser Scaup Ducklings

Abstract

Abstract Eggs of a seasonally early-nesting species of diving duck (Aythyini), the Canvasback (Aythya valisineria), and a late-nesting diving species, the Lesser Scaup (Aythya affinis), were collected from south-central Manitoba. Ducklings were raised in captivity so that the pattern of growth and development of both species could be examined. Rates of growth of external components were determined in order to test the hypothesis that late-nesting species (scaup) should develop the ability to fly at a younger age and gain weight more slowly than early-hatched species (Canvasback). If Lesser Scaup acquire flight earlier, we predicted that their digestive organs would grow and develop relatively more quickly than those of Canvasbacks, enabling them to process a large amount of food at a younger age. As growth rate is related to adult body size, rates of growth of the smaller Lesser Scaup were scaled to Canvasback size at the asymptote of the juvenile growth curve. Lesser Scaup fledged when they were 6 days younger than Canvasbacks, yet there was no apparent tradeoff in total body growth, as both species fledged at approximately 92% of juvenile asymptotic weight. Flight and leg muscle growth, however, did not differ statistically between species, nor did the change in water content, an indicator of tissue maturity. This is consistent with the idea that tissue maturity, and not the ability to fly, limits growth rate. As we predicted, growth of digestive organs was significantly more rapid in Lesser Scaup than in Canvasbacks. Culmen and tarsus of Lesser Scaup also grew faster than those of Canvasbacks, and we suggest this enables scaup to become efficient foragers more rapidly. The variation in growth rates between species was partially due to the difference in body size. After accounting for this, however, we found that several organs grew faster in Lesser Scaup than in Canvasbacks. Thus, the pattern of development may be influenced by the timing of nesting.