Breeding biology of the Little Egret Egretta garzetta on the southern coast of the Bay of Biscay

Abstract

ABSTRACT Capsule: Annual reproductive parameters of Little Egrets Egretta garzetta in an Atlantic coastal colony showed strong variation in a 20-year study, mainly due to extreme events. Aims: To describe the breeding biology of Little Egrets in the Bay of Biscay and compare it with that of the Mediterranean basin. Also, to explore relationships between breeding parameters and colony size with some climatic indicators. Methods: Phenology, number of nests, clutch size, number of hatchlings, brood size, and hatching and breeding success were recorded over 20 years. Results: Median laying date of 214 nests was 1 May (range: 1 April–25 June) and 54% of the clutches were laid in the second half of April and the first half of May. Over the 20-year study period mean clutch size of 270 nests was 4.0, mean number of hatchlings was 3.1, and mean brood size was 2.3. Hatching success ranged from 46.1% to 100% and breeding success from zero to 100%. Number of nests was negatively associated with clutch and brood size. The highest clutch size and lowest brood size were recorded at the beginning of the season. Significant relationships were found between the number of hatchlings and the rainfall during the pre-breeding season, and between brood size and the summer rainfall. Conclusions: Reproductive parameters showed significant variation over the study period, which highlights the importance of using long-term data sets. Breeding occurred one month later than in natural colonies of the Mediterranean basin. Negative relationships between the number of nests and clutch and brood size suggest some degree of density-dependent effects. Ranges of clutch size, number of hatchlings, and brood size were within those reported in Mediterranean populations. The effect of rainfall on reproductive parameters was weak. Extreme weather and predation events caused low rates of hatching and breeding success that affected the growth of the colony.