Check out this absolutely wild video of a newly hatched cuckoo bird pushing the other unhatched eggs over the edge of the nest, destroying their chances for survival. When I saw it on the subreddit "Nature is Metal," I was floored, and needed to know more. I learned that this is called "brood parasitism," which is a reproductive strategy where brood parasitic birds lay their eggs in the nests of other, smaller birds, thereby passing along the cost of raising their own young to the host bird mother. Nature explains:
For the parasite, benefits include increased fecundity due to greater allocation of resources toward mating and producing more eggs rather than defending nests, incubating eggs, and feeding young. For hosts of brood parasitic birds, the costs of parasitism range from diminished nestling growth rate, due to competition with larger and more competitive parasitic offspring (cowbirds, whydahs), to total loss of breeding by the abandonment of parasitized broods (cowbirds, cuckoos), the eviction of all host eggs by the early-hatching parasites (cuckoos), or the killing of host hatchlings by parasitic hatchlings (cuckoos, honeyguides) (Kilner 2005; Servedio & Hauber 2006). These costs exert reciprocal natural selection on parasites and hosts, such that in many cases host-parasite interactions result in escalating coevolution between intimately tied and interdependent species (Langmore et al. 2003). In turn, many hosts are able to discriminate against and reject foreign eggs or chicks based on visual, acoustic, or multimodal sensory cues (Cassey et al, 2008). The eggs of many brood parasites, for example, mimic those of their hosts (to deceive hosts to accept), have harder shells (to impede rejection by puncture), and require slightly shorter incubation times (causing a size advantage for parasitic nestlings) (Davies 2000) (Figure 1).
Nature is metal, indeed. If you want to learn more, here's a great primer video that further describes this adaptation, which has "evolved independently seven times in around 100 species of birds, and these birds can exploit over 950 host species."
