Seasonel and geographical variation in the clutch size of the Common Treecreeper Certhia familiaris

Abstract

The clutch size of the Common Treecreeper (n = 299 clutches) was studied in southern Finland in 1974-1983. The mean clutch size was 5.43±0.71(SD) and the mean date of laying was 27 April. One-third of the pairs also laid a second clutch . The seasonal variation in clutch size was peak-shaped. Age and size of the female did not correlate with clutch size . Seasonal variation in clutch size followed the same relation to calendar date in different years. The early breeding attempts produced more young to the next generation than expected . The strategy to start breeding early and to lay also a second clutch allowed the parents to produce more offspring than did those breeding only once in the middle of the season . The geographical variation is analyzed on the basis of literature and nest cards. The importance of three hypotheses (Food availability, Day-length, Reproduction effort) explaining the peak-shaped seasonal variation in clutch size is discussed. Acombination of the "Food availability hypothesis" and the "Day-length hypothesis" was most supported. Calendar effect and geographical variation in the clutch size gave evidence for the "Day-length hypothesis", but the "Food availability hypothesis" may still be more important.