A brood parasite lays her eggs in another species' nest and fools the host parents into raising her offspring as their own It's an ingenious strategy, but it's also ruthless and cruel—in some cases, the parasites kill their adopted "siblings" once they hatch It's survival of the fittest at its most calculating, and about 100 different bird species are known to engage in this behavior
 
 It's one of the strangest things you'll find in nature Brood parasitism happens in birds, fish and insects A bird that is a brood parasite will lay her eggs in the nest of a host species, usually a closely related one or even a mate The parasite will then fly away and never see her offspring The hosts will raise the foreign chicks, and the parasite gets all of the benefits of raising offspring without the costs like gathering food, defending the nest against predators, or even caring for her own young
 
 But the host species don't take this assault lying down Over time, they've evolved brilliant defenses against these crazy tactics It's a classic example of what biologists call an evolutionary arms race The parasite species evolves an adaptation that helps it get past the host's frontline defenses, and then the hosts respond with their own counteradaptation And so on, in an ever-escalating cycle
 
 The arms race starts when the parasite lays her egg It has to look a lot like the host's eggs so that the host doesn't recognize it as an invader The eggs also have to have thicker shells and shorter incubation periods so that the invading offspring can hatch before the host's own offspring And the parasites have other tricks up their feathers, too they may imitate the color and shape of the host's eggs, and they might be a little bit larger to help them blend in
 
 After all of this, if the host doesn't spot the invading eggs or rejects them, she risks damaging her own eggs And if she does, the brood parasite could retaliate and destroy her whole clutch of host eggs—something that has been observed in Brown-headed Cowbirds attacking Prothonotary Warbler nests
 
 The asymmetry of the arms race is what makes brood parasitism so fascinating But  https//nestvuicom/yen-vien-baby/  's still unclear why some hosts choose to accept or reject the intrusion of a neighboring species' offspring into their family tree A recent study looked at the genetic evidence for this The authors found that brood parasitism has evolved independently in multiple bird lineages, but it's typically accompanied by profound changes in life history and ecology They suggest that it may be hard to understand why some species would choose to be brood parasites unless you view the arms race as a "landscape of peril" analogous to the landscape of fear that we know about prey responding to predators The study also highlighted some of the complexities of interpreting patterns of nest rejection behavior and provided a framework for future work