Calonectris diomedea
Contrary to these empirical results, there has been a multitude of studies where EPCs or EPFs have been found in seabirds. Perhaps the most surprising EPFs have been found in the Procellariiformess because all members of this order only lay one egg per year and some do not even breed every year. Waved albatross show high rates (up to 25% of offspring are extra-pair young) of both EPCs and EPFs and behavioral observations have shown that many of the EPCs are forced by the extra-pair male. Studies of wandering albatross have shown over 10% of chicks are extra-pair young; an extremely surprising result since adult wandering albatrosses only breed once every other year when successful. Similar results were seen in black-browed ("Thalassarche melanophris") and grey-headed albatrosses ("T. chrysostoma") nesting on South Georgia Island. EPFs have also been shown in Antarctic petrels. Perhaps the most unexpected result was when EPCs and EPFs were documented in two burrow-nestings Procellariids, namely the short-tailed shearwater ("Puffinus tenuirostris") and Cory's shearwater ("Calonectris diomedea borealis"). In the short-tailed shearwater, EPFs occur because the female would have to leave her burrow to solicit an EPC. In the Cory's shearwater population from Vila islet, Santa Maria island, Azores, strong competition for nesting cavities/burrows may explain the occurrence of EPCs and EPFs, with small males facing higher risks of cuckoldry than large ones