Breeding biology of the Roller Coracias garrulus in farming areas of the southwest Iberian Peninsula

We studied the breeding biology of Rollers nesting in nestboxes installed i n farming areas of Extremadura (southwest Spain). Laying dates of Rollers i n Extremadura are the earliest in the Palearctic region. Clutch size was la rger than in northern populations. In the studied population, yearly variat ion in clutch size could be explained by yearly variation in the timing of reproduction. Reproductive rates were higher than in more northerly populat ions. A seasonal decrease in breeding success of Rollers in Extremadura was caused by a reduction in clutch size and the percentage of hatched eggs, a nd a seasonal increase in mortality rate. The most successful clutches were those with six eggs although clutches with four and five eggs were the mos t frequent in Extremadura.