Data from: A brood parasite selects for its own egg traits

Many brood parasitic birds lay eggs that mimic their hosts' eggs in
appearance. This typically arises from selection from discriminating hosts
that reject eggs which differ from their own. However, selection on
parasitic eggs may also arise from parasites themselves, since it should
pay a laying parasitic female to detect and destroy another parasitic egg
previously laid in the same host nest by a different female. In this
study, I experimentally test the source of selection on greater honeyguide
egg size and shape, which is correlated with that of its several host
species, all of which breed in dark holes. Its commonest host species did
not discriminate against experimental eggs that differed from their own in
size and shape, but laying female honeyguides preferentially punctured
experimental eggs more than host or control eggs. This should improve
offspring survival given that multiple parasitism by this species is
common, and that honeyguide chicks kill all other nest occupants. Hence,
selection on egg size in greater honeyguides parasitising bee-eaters is
imposed not by host defences, but by interference competition among
parasites themselves.