The copulatory behaviour of Grey Heron Ardea cinerea was observed in a
colony in Donana (SW Spain) during two years. A total of 262 pair cop
ulation attempts (PCAs) and 39 extra-pair copulation attempts (EPCAs)
(12.9% of all copulation attempts) were observed among 31 focal pairs.
84% of PCAs and 50% of EPCAs resulted in cloacal contact. Eleven male
s (35%) and 19 (61%) females were involved in EPCAs. There was a corre
lation between the time that female spent alone on the nest and the oc
currence of EPCAs during the presumed fertile period of the female. Th
e incidence of EPCAs observed at Donana was greater than reported for
the species in other localities, and could be explained by more opport
unities for promiscuous behaviour at high densities. Data provided by
this study support the conclusion that mate guarding and PC frequency
are mechanisms of paternity defense under the sperm competition hypoth
esis.