Year‐dependent sex‐biased mortality in supplementary‐fed Tengmalm's owl nestlings

Abstract

Summary In sexually size‐dimorphic birds, the larger sex requires more energy during growth, and often suffers from a higher mortality risk during growth than the smaller one when food is limited. Sex‐specific growth and mortality were examined in Tengmalm's owl Aegolius funereus (L.) nestlings, a species in which adult females are slightly larger than males. Nestlings were unambiguously sexed using molecular techniques. Nestling mortality in broods provided with supplemental food was compared with that in controls in an experiment carried out during two breeding seasons, 1998 and 1999. The natural food supply during the two years of the experiment differed. Although the spring vole abundance was similar for the two years, 1998 was characterized by a less severe decline in vole abundance during the previous winter, and also a greater increase in vole abundance during the summer than 1999. It was consequently inferred that the owls’ natural food supply was more limited in 1999. In 1998, female nestlings attained a 5% higher asymptotic mass than males, and fed nestlings were heavier than controls. No difference in rate of mass gain or wing growth was detected. Overall mortality was lower in fed broods than controls in 1999 only. Unexpectedly, female nestling mortality was unaffected by supplemental feeding during both years. However, in 1999, when natural food supply was more limited, male nestling mortality was reduced in supplementary‐fed broods. It is suggested that the females’ larger size gives them a competitive advantage against their male siblings when fighting over food items, and that when food is limited, males rather than females suffer increased mortality owing to lack of food. This study illustrates the importance of repeating food supplementation experiments to account for annual variation in natural food supply.