Lower annual fecundity in long-distance migrants than in less migratory birds of temperate Europe

We use widely supported handbook data on annual fecundity (clutch size 9 annual number of normal broods) to obtain indications related to the relative costs of long-distance migration compared to lower levels of migratoriness. Our examples show that the yearly production of eggs in congeneric passerines of similar size from temperate Europe is lower in long-distance migrants than in their less migratory relatives. The same tendency shows up in a more heterogeneous sample of non-passerines. In most passerines and in one among five pairs of non-passerines, this is due to longer breeding periods allowing a higher number of clutches in the less migratory species in spite of a tendency towards larger clutches in the passerine long-distance migrants. If both migratory types have only one clutch (as in one species pair of the passerines and in four non-passerine pairs) the trend towards larger clutches was reversed between the two types. The higher fecundity of the less migratory species suggests that wintering under harsh conditions may be more expensive than trans-Sahara migration among similar species. In keeping with this result, Ortolan and Rock Buntings (Emberiza hortulana and E. cia) breeding syntopically (and, in the 1980s, still in relatively stable populations) in an inner Alpine valley provide a well-studied example illustrating the high longevity of the migrants compared to the relatively short life span of the residents.