Will Great Tits become Little Tits ?

Abstract

The mean size of Great Tits Parus major in our study at Ghent shows a significant decreasing trend in the period 1962 to 1975. Since it has been shown that in the Great Tit the heritability of tarsus length, a measure for size, is rather high, we believe the observed trend to be a directional micro-evolutionary change. After investigating several possible causes for the size decrease, we suggest that the observed change is caused by a shift in the equilibrium between selective pressures favouring large and small individuals. Before our study, breeding numbers of Great Tits were limited through the lack of suitable nest-sites. We think that more of the large males could reproduce in mat situation. When we provided nestboxes in surplus this selective pressure was relaxed, and since small females must produce more surviving offspring, this further shifts the balance. We argue that during the evolutionary history of the Great Tit there was no lack of suitable nest-sites in natural habitats, that by providing nest boxes we restore the natural situation, and diat therefore the decrease in size should level off.