Dwergei bij Buizerd Buteo buteo

In 2001 a runt egg was added to a clutch of three normal-sized eggs, at least three weeks after the first clutch was completed. The first three eggs didn't hatch and only one of them contained a 1-week old embryo. The volume of the runt egg amounted to 59% of the average of the other three eggs in the nest (Table 1) and 54% of the average of 55 other Common Buzzard eggs found in the same region in the same year. Unexpectedly, it hatched whereas none of the three ‘normal’ eggs did. The chick had a comparatively light mass, and remained small and thin for its age until fledging (no data afterwards). Given its mass and measurements it must have been a male (at 36 days of age: 620 g and minimum lateral metatarsus width of 8.1 mm; compare with growth curves in Bijlsma 1999). During my raptor studies in The Netherlands since the early 1980s, I did not find any runt eggs among c. 1000 eggs of Goshawk Accipiter gentilis and c.1300 eggs of Sparrowhawk A. nisus. Among c. 1800 eggs of Common Buzzard I found a single runt egg; a probable second runt egg of Common Buzzard was not measured. Such frequencies of runt eggs are much lower than those summarized by Crick (1995), indicating that nest record schemes and egg collections probably provide biased samples of the frequency of occurrence of this phenomenon.