Variations in clutch size and nest failure and its causes were studied in a population of Serins Serinus serinus breeding in orange groves in Sagunto, eastern Spain, during a nine year period (1985-93). Breeding success and mean clutch size decreased throughout the season. The main causes of nest failure were nest predation and nestling mortality. The main predator was the Montpellier snake Malpolon monspessulanus. Losses due to predation throughout the season ranged between 12% and 50% of the nests and predation increased until April, remaining at a constant level afterwards. The proportion of nestlings found dead in the nest increased through the season due to an increase in the number of chicks that died presenting the crop full of food. This suggests that other factors than food shortage (e.g. hyperthermia) must be the cause of the low breeding success at the end of the season.