INBREEDING IN AN ISLAND POPULATION OF THE GREAT TIT

populations under natural conditions. Although cases of inbreeding have been reported from population studies in birds, where nestlings were ringed and parents identified on the nest, the numbers involved were too small to allow more than a qualitative statement on the effect of inbreeding (Richdale, 1957; Bulmer, 1973; Greenwood et al., 1978). Yet inbreeding avoidance, presumably selected for by inbreeding depression, is used to explain a wide variety of phenomena (see for instance Bateson, 1978; Bengtsson, 1978). There is clearly a need for quantitive data on inbreeding in natural populations (May, 1979). The data from the long-term population studies of the Great Tit (Parus major) carried out at the Institute for Ecological Research at Arnhem (The N etherlands) allow us to give quantitative estimates of the occurrence and the effects of inbreeding. We will compare the occurrence and the detectability in a mainland and in an island population. Reduced hatching success will be shown to exist both for inbred zygotes and in clutches of inbred females. The deleterious effects of inbreeding may have been recognized since the start of civilization, possibly based on experience in breeding livestock and the occurrence of congenital defects in the offspring of relatives in man. The basis of these deleterious effects could only be understood after the rediscovery of Mendel's laws. The discovery of recessive lethal genes in mice and the realization that inbreeding promotes homozygosity followed soon. Because the relative difference between the proportion of homozygotes produced by inbred and by outbred pairs is larger when the frequency of the gene involved is smaller, the noxious effects of inbreeding can be explained by the increase in the level of homozygosity of rare recessive deleterious genes. The data available for man, domesticated animals and plants show that harmful effects of inbreeding are present in all crossbreeding organisms (Lerner, 1954; Cavalli Sforza and Bodmer, 1971; Wright, 1977) and suggest that deleterious recessive genes are numerous. The general occurrence of deleterious recessives was confirmed by studies in Drosophila species. With the help of chromosomes marked with dominant genes Population Study of the Great Tit and carrying inversions to suppress crossThe Great Tit is a common hole-breedjng-over, chromosomes from wild popuing passerine that readily accepts nestboxlations were made homozygous. Up to es and can therefore be studied with rel40% were found to carry recessive lethal ative ease. In the study areas nestboxes genes (Dobzhansky, 1955). The technique are provided and these are inspected which allows such a direct demonstration weekly during the breeding season. Date is presently only available in Drosophila. of first egg-laying, clutch size, date of Only in humans has inbreeding been hatching, hatching rate and nesting sucstudied in any depth in situations with a cess are observed or deduced from obsernatural partner choice. However, in man vations, using age estimates of the nestrelatives seem to be taken as partners lings and the knowledge that one egg is more naturally by some people than by laid every day. Eggs that are still present others (Bittles, 1977). Little evidence exwhen there are nestlings at least two days ists on inbreeding depression in animal old (in first clutches, four days in later