Size does not matter – no evidence for assortative mating by body size in the Mute Swan Cygnus olor

Finding a mate of high quality is of key importance for reproducing birds, and thus positive assortative mating is commonly observed in avian populations. Although assortative mating by age, body size, condition or ornamental traits was reported for many bird taxa, there is a scarcity of empirical evidence for such mating patterns in wildfowl. We studied mating patterns in the Mute Swan Cygnus olor from the Central-European population. We analysed four body measurements (total wing length, forearm length, head length, foot web width) in 91 different breeding pairs. Contrary to our expectations, we found no evidence for assortative mating by any of the collected biometrical measurements and by overall structural size (PC1 from all measured skeletal traits). Further, Mute Swans mated randomly by the size of bill knob, which is considered a sexually selected ornament in this species. We suggest that in the species with long-term pair bonds and monogamous breeding system, such as the Mute Swan, the benefits from large size of mates may be less important for positive reproductive output than other individual traits, e.g., age or previous breeding experience.