Parental Investment in Clutch Size and Egg Size in the Ural Owl Strix uralensis

Abstract

Reproduction of the Ural Owl was followed in a population of about 140 pairs on an area of 2200 km2 of south-boreal coniferous forests in Finland. All pairs bred in nestboxes and almost every breeding female was ringed. We measured 331 clutches of 145 females in 1977-1984. The number of breeding pairs depended on the 3-4 year vole cycles. Only 24% of the females laid eggs during vole lows, while 76% laid in peak years. Average clutch size decreased from 3.48 in good through 3.01 in intermediate to 2.25 eggs in poor breeding seasons. The yearly median date of laying varied over 29 days, from 25 March 1983 (good vole year) to 23 April 1977 (poor vole year). Early laying seasons were preceded by warm autumns and mild winters with a good vole supply. Clutch size decreased by 0.09 eggs d-1 as the season advanced. The highest female components of egg size rose to 80-90% in poor and intermediate years, but decreased to about 65-75% in good years. Thus egg size among females differed most in poor breeding seasons and less when more food was available. According to the theory of life history tactics, individuals of long-lived species reproducing in fluctuating environments should adjust their reproductive efforts to the prevailing conditions. Ural Owls vary their clutch size and egg size (both within and between clutches) in relation to the quality of the breeding season. Mean egg size increases and within-clutch variation in egg size decreases as the prevalence (relative frequency) of a clutch increases. Assuming that nestling mortality is independent of clutch size, the most prevalent clutches will also be the most profitable. Observed within and between clutch variation in egg size may be explained as a female''s tactical response to the profitability of her investment. However, the possibility that the observed patterns are related to the variation in territory quality is not definitely excluded.