Low Repeatability of Laying Date and Clutch Size in Tengmalm's Owl: An Adaptation to Fluctuating Food Conditions

Abstract

The repeatabilities of laying dates (0.03) and clutch sizes (0.15) of individual female Tengmalm's Owls Aegolius funereus breeding in different years were positive and those of males negative (-0.21 and-0.11), but they did not differ significantly from zero. This suggests that there may be slight heritable variation in the clutch size of females, but none in their laying dates and in the breeding characters of males. Between-year shifts in laying dates of individual females averaged 7.3 ? 20.6 d (maximum 72 d) and in mates of individual males 12.9 ? 26.5 d (81 d). The corresponding figures for clutch size were 0.4 ? 2.0 (4) eggs for females and 0.2 ? 1.9 (5) eggs for males. The flexible adjustment of breeding characters to the prevailing food supply is adaptive, as owls that inherited a trait to lay small clutches cannot benefit from good vole years, whereas owls laying only large clutches cannot breed in poor vole years.