Using net sacks to examine the relationship between egg size and young size in Common Pochards

ABSTRACT.   Investigators studying the relationship between egg size and young size often face difficulties in assigning particular young to particular eggs. We present a new method based on the use of separate net sacks for each egg that allowed us to study this phenomenon in Common Pochards ( Aythya ferina ) without excessive disturbance of breeding birds. We found a positive isometric relationship between duckling body mass and estimated fresh egg mass ( P < 0.001). However, we found no relationship between estimated fresh egg mass and structural measurements of ducklings (tarsus and bill length, both P > 0.29). In addition, we found a weak positive relationship between estimated egg mass and the size of ducklings ( P = 0.049). Greater hatchling mass typically means a larger yolk supply, and more yolk may provide energy crucial for survival during the first few days after hatching. In addition, ducklings with longer tarsi may be better swimmers and better at finding food and escaping from predators. Although our method has certain limitations and investigators should consider risks when applying it in over-water nests or in nests with large clutches, we believe it is a reliable way to assign young to particular eggs in waterfowl and possibly other bird species.