Single-and multi-brooded species of birds differ in their seasonal patterns of clutch size. Single-brooded species start with a maximum clutch size that declines continuously as the season progresses, whereas the clutch sizes of multi-brooded species usually increase to a mid-season maximum peak and then decrease progressively until the end of the breeding season. Previous studies have shown that multi-brooded migrant species present seasonal patterns that are similar to single-brooded species at high latitudes but similar to multibrooded non-migratory species at lower latitudes. We studied the Greenfinch Carduelis chloris and Goldfinch C. carduelis populations in eastern Spain (Sagunto, Valencia) between 1975 and to compare seasonal variations in clutch sizes between years with early and late starts to the breeding season. The period over which clutch sizes increase was longer when the breeding season started earlier. The Goldfinch population showed no pattern of initial increase in clutch size when there was a late start to the breeding season: a late start shortens the season giving them less time to breed, and may also coincide with maximum food availability. Thus, the pattern of single-brooded species was observed. In the Greenfinch population, a trend toward the seasonal pattern of single-brooded species was also observed when the following indices were compared: clutch size increase, modal timing, initial slope and timing of maximum clutch size. We have also compared the seasonal patterns of clutch size of both species in eastern Spain with the patterns observed in Britain. Our results show that for both Goldfinches and Greenfinches, the non-migrant southern populations of Sagunto in eastern Spain do not tend towards a more multi-brooded seasonal pattern of clutch size than the migrant Goldfinches of Britain.Passerine birds use two tactics to maximize their number of offspring per breeding season. In singlebrooded species, the clutch size is at its maximum at the start of the breeding season and declines as the season progresses. Multi-brooded species, however, present a different pattern. Their breeding season is longer and females lay several clutches with a midseason peak of clutch size (Lack 1954, Klomp 1970. Therefore, in multi-brooded species, seasonal reproductive success is determined not only by the productivity of each brood but also by the number of broods raised (Bryant 1979, Desrochers & Magrath 1993. As a consequence, Crick et al. (1993) suggested that multi-brooded species start laying before the optimal moment, which accounts for the increase in clutch sizes until the optimal time, when clutch sizes are the largest. In addition, a tendency to start breeding earlier would be normal as it would allow a greater number of clutches per season. Furthermore, the same species might present both patterns across its distribution range if it is made up of non-migrant as well as migrant populations. The Eastern Bluebird Sialia sialis shows an initial increase in the size of clutches in the southern p...