Dwarf egg in Long-tailed Skua Stercorarius longicaudus. / Dvärgägg hos fjällabb Stercorarius longicaudus.

Dwarf eggs, e.g. eggs that are very much smaller than normal eggs, are occasionally found among normal eggs in nests of birds. Although they are not often mentioned in the handbooks, the frequency of dwarf eggs seems to be about one per thousand, or less. In 2001 I found one dwarf egg in a nest with two eggs of the Long-tailed Skua Stercorarius longicaudus at Ammarnas in Swedish Lapland. I did not measure the eggs but have estimated the relative size from photographs. The length was 81.5% and the breadth 74.2% of the normal egg. The volume was calculated to be about 45% of a normal egg. I have not found any note about dwarf eggs in this species. In the nest record material from the Ammarnas area I have found 90 clutches of the Long-tailed Skua with a total of 172 eggs, and among them the dwarf egg that I found is the only one. The sample is, however, too small to give the true frequency of dwarf eggs in the species.