Data from: ultimate drivers of forced extra-pair copulations in birds lacking penises: jackdaws as a case-study

Forced copulation is common, presumably because it can increase male
reproductive success. Forced extra-pair copulation (FEPC) occurs in birds,
even though most species lack a penis and are widely thought to require
female cooperation for fertilisation. How FEPC persists, despite a
presumed lack of siring success and likely non-negligible costs to the
male, is unknown. Using the jackdaw (Corvus monedula) as a case study, we
use SNPs to quantify extra-pair paternity rate through FEPC and evaluate
explanations for the persistence of FEPC in species without a penis. We
then collate evidence for FEPC across penis-lacking birds. Combining
genetic and behavioural analyses, our study suggests that the most likely
explanations for the maintenance of FEPC in jackdaws are that it provides
a selective advantage to males, or that it is a relic. Our literature
review shows that across birds lacking a penis, FEPC is taxonomically
widespread, yet little is known about its evolution. Broader
implementation of the approach used here, combining both genetic and
behavioural data, may shed light on why this widespread sexual behaviour
persists. Additional work is necessary to understand whether a penis is
needed for paternity through forced copulation, and to quantify the costs
of FEPC.