CLUTCH SIZE OF PASSERINES AT MIDLATITUDES - THE POSSIBLE EFFECT OF COMPETITION WITH MIGRANTS

The clutch sizes of the passerines of Israel and the Cape Province, So uth Africa, which lie at similar latitudinal range, were compared. Mea n clutch sizes in Israel and the Cape Province are 4.09 and 2.87, resp ectively. Mean clutch size of Israeli migrants is larger than that of residents (4.45 and 3.93, respectively), but no such difference exists in the Cape Province. It is suggested that the larger clutch size in Israel is a result of two factors: (1) the higher proportion of winter ing birds in Israel in comparison with the Cape Province and the prese nce of many transients there which may compete with resident birds and cause high winter mortality among them and (2) a higher proportion of migrants in the Israel avifauna, which suffer heavy losses during the ir trans-Saharan migration in comparison with Cape Province migrants, which travel shorter routes. The resulting reduced competition for foo d during the breeding season in Israel enables passerines there to lay larger clutches as predicted by Ashmole (1963) and Ricklefs (1980).