What percentage of species, or what percentage of individuals in a species? I don&#x27;t know about the former, but in the latter case there are several species which are documented to form monogamous relationships with no extra-pair copulations (EPCs). That wikipedia page cites a couple of studies:<p>Anderson, D. J. and P. T. Boag. 2006. No extra-pair fertilization observed in Nazca Booby (Sula granti) broods. Wilson Journal of Ornithology 118:244-247.<p>Moreno, J., L. Boto, J. A. Fargallo, A. de Leon, and J. Potti. 2000. Absence of extra-pair fertilisations in the Chinstrap Penguin Pygoscelis antarctica. Journal of Avian Biology 31:580-583.<p>Lifjeld, J. T., A. M. A. Harding, F. Mehlum, and T. Oigarden. 2005. No evidence of extra-pair paternity in the Little Auk Alle alle. Journal of Avian Biology 36:484-487.