Heterocannibalism (in which non-kin get eaten) has been reported in seven of the 142 bird families and is most common in colonial sea birds like gulls. Here, the practice of consuming eggs or young is an integral part of foraging strategy and it can have a significant effect on bird populations. In one study of a colony of 900 herring gulls (Larus argentatus), approximately one-quarter of the eggs and chicks were cannibalized. Heterocannibalism also occurs in acorn woodpeckers (Melanerpes formicivorus). In this species, two female woodpeckers share a single nest and will even feed and care for each other’s young. But before that occurs, the nestmates may also destroy and consume each other’s eggs if one bird should lay an egg first. Presumably this is because the oldest hatchling would be the most likely to survive. To eliminate this advantage, the birds will keep eating each other’s eggs until they both lay their eggs on the same day, a process that can take weeks.
Sibling cannibalism, in which brothers and sisters get eaten by other brothers and sisters, is best known among the raptors, a non-taxonomic name for predatory birds like eagles, hawks, kestrels, and owls, all of which possess strong eyesight, powerful beaks, and sharp talons. The latter two characteristics exempt raptors from the gape limitations seen in many other birds and may help explain the increased frequency of cannibalism in these birds. In some species, sibling cannibalism is often the end result of asynchronous hatching, in which two eggs are laid with one of them hatching several days before the other (one can almost hear the acorn woodpeckers gasping in horror). As a result, the firstborn chick uses its extra bulk to win squabbles over food with its younger brother or sister. In instances where the parents are unable to provide their young with enough to eat, the firstborn will kill and consume its younger sibling. Researchers sometimes refer to these types of victims as “food caches,” as sibling cannibalism becomes an efficient way to produce well-nourished offspring (albeit fewer of them) during times of stress.
Something similar happens in the snowy egret (Leucophoyx thula), which commonly lays three eggs. The first two get a serious dose of hormones while still in the mother’s body. The third egg receives only half the hormone boost, resulting in a less aggressive hatchling. If food is abundant, the larger nestlings simply throw the passive chick out of the nest, but if alternative sources of nutrition become scarce, the smaller sibling is stabbed to death and eaten.
According to Koenig and fellow ornithologist Mark Stanback, filial cannibalism in birds has been reported in 13 of 142 avian families but is not well understood, perhaps because it is relatively infrequently observed. On rare occasions, birds like roadrunners (Geococcyx californianus) will eat undersized chicks. Similarly, barn owls (Tyto alba) are reported to consume their own chicks during extreme environmental conditions, such as when food is in short supply and the chicks are either starving or sick. It has been suggested that filial cannibalism of dying or decayed offspring can prevent infection and deterioration of the entire clutch. Presumably there are also benefits to getting rid of dead chicks before they attract legions of carrion-eating flies and maggots. In most cases, however, it’s the lack of alternative forms of nutrition that initiates the behavior.
“A lot of examples of cannibalism in birds are clearly associated with food limitation,” Koenig told me. “We’re basically talking about a lifeboat strategy, where the strong cannibalize the weak.”
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8 Insectivora and other traditional vertebrate classes like fish, reptiles, and birds are no longer considered to be valid units of classification, either because scientists have determined that species included in them aren’t as closely related as they once believed, or that species that should be in those classes aren’t.
3: Sexual Cannibalism, or Size Matters
Whoever authorized the evolution of the spiders of Australia should be summarily dragged out into the street and shot.
— Mira Grant, How Green This Land, How Blue This Sea
While it’s fairly common knowledge that the praying mantis is the co-holder (along with the black widow spider) of the title “Nature’s Most Infamous Cannibal,” fewer people know that the name praying mantis is shared by nearly all of the 2,200 species making up the order Mantodea.
The moniker comes from the curious manner in which the insects hold their forelegs while resting. As a result of this prayerlike attitude, they’ve become some of the most popular insects in mythology and folklore. Many of these mantid myths have religious or semi-religious overtones. In France, par exemple, they’re known as prie-dieu and are said to point lost children homeward. The Khoi people of South Africa regard praying mantises as gods, while Arab and Turkish folklore holds that the insects direct their prayers toward Mecca. Always looking on the bright side, Americans once believed that praying mantises blinded people and killed horses, this perhaps as a nod to the fact that rather than being used for prayer, the anterior-most limbs are actually modified into lethal, spike-covered weapons. Often well camouflaged, most species are ambush predators, lashing out with their “raptorial legs” to capture, crush, and secure their prey while a set of sharpened mouth parts slice, can-opener style, through the toughest exoskeleton.