The effect of female body size on clutch volume of Tengmalm`s Owls Aegolius funereus in varying food conditions

Abstract

Body size mayinfluence offspring quality so that large females produce "fit" offspring. Tengmalm's OwlAegoliusfunereus is one ofthe most size-dimorphic birds of prey in the Holarctic region with females larger than males. The contribution of female size to egg size and clutch volume (number of eggs x mean egg volume of a clutch) was studied in western Finland during 1981-1990, when the abundance of the owls' main food (voles) fluctuated in 3-yr cycles . Clutch volume was positively related to female bodyweight, independent of laying date, which significantly affected egg size in good vole years. Female age, male age, and territory quality did not affect clutch volume. Clutch volume was not associated with female wing-length, but in the peak phase of the vole cycle, long-winged and heavy females produced significantly larger eggs than did short-winged and light females. The results indicate that females can benefit from large skeletal size only in years of food abundance, and that the nutritional condition of females was essential to reproductive success. Our results also suggest that the largeness of females may not be the main cause of the reversed sexual size dimorphism in Tengmalm's Owls.