Honeyguides and other brood parasitic birds are famous for tricking host parents by laying eggs that mimic their own. Honeyguide eggs mimic host eggs in size yet, surprisingly, bee-eater hosts are undiscriminating and readily accept mismatched eggs. This study shows that honeyguide egg size adaptation has probably rather evolved to trick other honeyguides, not host parents: honeyguides selectively puncture any mismatched egg already present in the nest when they lay their own, lest it be the offspring of another honeyguide female and brutally kill their own chick when it hatches.
Read the full paper in Biology Letters [Open Access], or Ed Yong’s excellent article about the study. Here are some other nice articles and podcasts online covering this study: The Behaviour, Ecology & Evolution Podcast | Take Part blog | Earth Times.