Marion Petrie, Rianne Pinxten, Marcel Eens (2008). Moorhens have an internal representation of their own eggs Naturwissenschaften DOI: 10.1007/s00114-008-0486-5
Intraspecific brood parasitism is a reproductive tactic which is known to be used by moorhens and many other bird species. Brood parasitism refers to the situation where one bird lays an egg in another birds nest. This provides reproductive advantage that the individual successfully propagates their genes, yet does not have the burden of raising the young themselves. They in turn can focus their efforts on other offspring, gathering more food, migrating, etc. Some species are well known for their brood parasitism such as the Brown-headed Cowbird. The cowbird lays its eggs in the nests of other bird species. This is believed to be the result of the cowbird lifestyle of following the great bison herds of North America. To keep up with the bison, the cowbirds could not stay in one place to raise their young.
In the case of the Moorhen,we are referring to Intraspecific brood parasitism which refers to the situation where a bird lays an egg in the nest of another bird of the same species. In previous studies it has been shown that up to 40% of Moorhen nests receive eggs from other females, illustrating that this is indeed a significant issue for a Moorhen.
Since raising the young of other birds places high demands on an adult, it can interfere with the survival of their own offspring. It therefore benefits an individual from an evolutionary fitness perspective if they can distinguish eggs that are not their own and abandon them.
The study focused on whether adult Moorhens could recognize eggs which were placed in their nest that were not their own. If true this could be a significant defense against intraspecific brood parasitism. To test this, the researchers would collect eggs and swap them in various nests. A particular hen would either receive an egg that was collected from one of their earlier broods or an egg from another hen. The results show that the Moorhen indeed has some internal recognition capabilities, although does not identify the specific mechanism. In 7 out of 9 cases, the Moorhen would abandon the nest if their first egg was replaced by an egg from another female. Only 1 out of 9 abandoned when the egg was replaced with one of their own from an earlier brood.
While not part of this study, it is interesting that Moorhens seem to readily tolerate parasitic eggs after they have laid their second egg in the nest. Clearly room for additional studies.