Female ducks recognize their kin and allow them to add eggs to their nests, but fight such attempts by non-relatives.

Many birds try to trick others of the same species into incubating their eggs to avoid the associated energy costs. Malte Andersson at the University of Gothenburg in Sweden and his colleagues studied this 'brood parasitism' by filming the nests of High Arctic common eiders (Somateria mollissima) for more than 4,100 hours.

They also analysed the proteins in egg albumen to determine the relatedness of the females that laid eggs in the nest, and found evidence for discrimination against non-relatives.

In 65 nests studied, 11 contained eggs from two different females. At eight of these nests there were fights, and the two females laying eggs in each nest were unrelated. At three nests no aggression was observed, and the laying females were significantly more closely related than in the other eight.

Behav. Ecol. http://doi.org/2gj (2015)