This post was contributed by James L. Hayward, corresponding author of a recent paper in The Wilson Journal of Ornithology describing the occurrence and impact of egg cannibalism on a Glaucous-winged Gull colony.

Over the course of many field seasons at a colony of several thousand Glaucous-winged Gulls at Protection Island National Wildlife Refuge, Washington, I noticed large accumulations of broken eggshell littering a few of the nesting territories. I watched the owners of these territories invade the territories of fellow residents, grab an egg, fly the egg back to their own territories, and eat the contents. Eggshell fragments from the stolen eggs accumulated on the territories of these egg cannibals.

A cannibal is any animal that, like these gulls, kills and eats members of its own species, and their victims can be at any stage of the life cycle, including the egg stage. Members of at least 1,300 species are known to engage in cannibalism (Polis 1981), although the real number is certainly much higher. Cannibals are found among zooplankton, arthropods, fish, reptiles, birds, mammals, and other animal groups. Once thought to be an aberration, cannibalism is now considered a normal life history trait of many species (Fox 1975, Elgar and Crespi 1992).

Ornithologists have long known that gulls cannibalize their neighbors’ eggs, a behavior reported for at least nineteen gull species. Most such reports, however, have come from incidental observations during research into other aspects of gulls’ lives, and prior to the research I describe below, gull egg cannibalism was never well studied. I wondered about the extent and impact of this behavior on a breeding colony, so I decided to examine this phenomenon at Protection Island. Three undergraduate students, Ashley Polski, Karen Osborn, and Elliott Joo, two graduate students, Athena Mitchell and Amanda Sandler, and a mathematical ecologist, Shandelle Henson, worked with me on the project. Our results provide the first comprehensive assessment of egg cannibalism by gulls.

Only males on the Protection Island colony steal eggs. A cannibalistic male locates an egg to steal in one of two ways. He may fly slowly over an undisturbed colony looking for a momentarily unprotected nest, or he may wait for an eagle to fly over, disturbing residents in large areas of the colony so that they fly up from their nests and leave their eggs exposed. In either case, the cannibal capitalizes on the opportunity, quickly lands by the exposed nest, and grabs an egg.

The cannibal then flies the stolen egg back to his territory,gently holding the egg in his bill or swallowing it whole into his crop. Once back on his territory, the cannibal drops the egg or regurgitates it from his crop, pecks it open, and devours the slimy contents. If his mate is present, she typically begs to be allowed to participate in the feeding. Most of the time the male shares his loot with his mate, but occasionally he pecks at her and refuses to share.

Eating just two eggs per day just about fulfills the energy requirements of an adult gull (Hayward et al. 2014), but some cannibals in our study stole more than two eggs per day. In June 2014, for example, one male stole 81 eggs in 30 days, and another took 75 eggs during the same period. Unlike most colony residents, super egg predators like these spent little time looking for food off the colony.

We compared the contents of regurgitated food pellets of non-cannibals and egg cannibals. (Gulls regurgitate pellets of undigestible material just like owls.) Not surprisingly, pellets from cannibals were significantly more likely to contain fragmented eggshell than pellets from non-cannibals, but pellets from cannibals were also significantly less likely to contain fish remains than those of non-cannibals. This suggested that cannibals were substituting eggs for fish as their primary protein source.

Only about 1 in a 100 of the territories on the Protection Island gull colony were defended by egg cannibals⁠—yet these few cannibals stole 1 out of every 9 eggs produced in the entire colony in both 2014 and 2015! Despite the fact they are few in number, egg cannibals impact the colony in disproportionate ways, and several birds at Protection Island exhibited this behavior over multiple years.

How did the reproductive output of egg cannibals compare with those of fellow colony residents? Egg cannibals produced significantly fewer eggs than non-cannibals. Moreover, the proportion of eggs taken from nests of cannibals by other cannibals was significantly higher than the proportion of eggs taken from nests of non-cannibals. So, egg cannibalism appears to be a feeding tactic used by less attentive and less successful breeders.

During our research, egg cannibalism became more common when the sea surface temperature increased. When sea surface temperature is high, as during El Niño events, plankton and fish, primary food items for gulls, drop to lower, cooler levels in the water column. Unlike many seabirds, gulls can’t dive, so food is more difficult to obtain during these times; eggs, nutritious and conveniently abundant, become a more frequent food source for rapacious colony residents (Hayward et al. 2014).

It seems, however, that gulls have evolved a way to reduce the impact of cannibalism on their reproductive efforts. During years of low sea surface temperature and low egg cannibalism, egg-laying occurs during a relatively short period, which overwhelms would-be egg and chick predators from outside the colony with a glut of food; the chance that a given egg is predated by bald eagles is reduced. By contrast, during years of high sea surface temperature and high egg cannibalism, the egg-laying period lengthens and females nesting close together tend to synchronize their every-other-day egg-laying until they achieve the typical three-egg clutch of gulls; by laying their eggs on the same day as their neighbors, the chance that a given egg is predated by a cannibal neighbor is reduced (Henson et al. 2010). In other words, gulls switch between two breeding tactics, depending on if the threats to their offspring are from outside (eagles) or inside (cannibals) the colony (Weir et al 2020). Under the lead of one of our colleagues, Gordon Atkins, we found that females tended to avoid copulation on egg-laying days but welcomed copulation on the intervening days. The very loud, pulsating copulation call of the males functioned as the synchronizing signal (Atkins et al. 2017, 2021).

Our results raise many unanswered questions. For example, are some gulls genetically predisposed to cannibalize their neighbors’ eggs, or is this primarily a learned behavior? Are eggs with certain colors and pigmentation patterns more vulnerable to cannibalism than others? Do egg cannibals exhibit other forms of antisocial behavior outside the breeding season? Will increasing sea surface temperatures associated with climate change increase the incidence of egg cannibalism on gull colonies? Hopefully these and other questions will be answered by future research.