Effects of Rice Field Phenology on Breeding Parameters of Heron Colonies in the East of the Iberian Peninsula

Rice fields have traditionally been considered key feeding grounds for many waterbird species, including herons. Studies show that field management and flooding cycles influence the reproductive parameters of these birds. L'Albufera de València, on the east coast of the Iberian Peninsula, has large areas of rice fields and is home to the third largest breeding colony of herons in Spain. Changes in water and rice field management that have occurred since the colony was last assessed (1990) may have affected the colony's reproductive performance. Clutch and brood size, and hatching, nesting and breeding success were estimated for Grey Heron Ardea cinerea, Purple Heron Ardea purpurea, Cattle Egret Bubulcus ibis, Little Egret Egretta garzetta, Squacco Heron Ardeola ralloides (Ardeidae) and Glossy Ibis Plegadis falcinellus (Threskiornithidae) in 2015–2017. The extent of flooding and cultivation of rice fields was assessed from February 2016 to June 2017. Reproductive parameters were related to the state of the rice fields. All reproductive output parameters were lower than those reported in the literature. In 2016, laying started earlier (at the end of April), when the fields were still dry and hatching started from May, after flooding. In 2017, peak hatching occurred in June, when the fields were sown. A significant relationship was found between breeding success and the area of sown fields, except for the Squacco Heron. We found that birds that initiated egg laying closer to the date of flooding had a relatively higher breeding success for the Little Egret, Glossy Ibis, Cattle Egret and Purple Heron. The success of nesting populations in this type of artificial wetland requires water management plans that synchronize flooding regimes and maintenance of flooded areas with the biological requirements of these species.