Stakeout: A Tale of Two Species

By Ryan Chew

Most everyone in suburbia knows the Canada goose, Branta canadensis. Its Quaker colors, distinctive honks, even the squishy pellets that litter area soccer fields are irritatingly familiar. Indeed, everything about the Canada goose seems so commonplace — as suburban as an “honor student” bumper sticker — that it’s a surprise to realize that important facets of its life cycle have remained a mystery.

Yet when goose populations exploded in our region in the 1980s and early 1990s, biologists had little idea what, if anything, might prevent them from multiplying until goslings spilled into every plastic kiddy pool. One local Christmas Bird Count that had consistently reported about 300 geese every year from 1972 to 1979 was suddenly tallying 2,000 geese in 1981 and more than 9,000 by 1993. Many scientists attributed the rise to the proliferation of turfgrass lawns and warm detention ponds, both of which made evading most predators a relatively easy task.

So when research biologist Charles Paine launched a study of geese in the northern and western suburbs of Chicago in 2000, he expected to find the population expanding rapidly. Instead, he discovered that growth had slowed to about 1 percent a year. Fewer than half of some 2,000 nests Paine monitored produced goslings. Paine ruled out death at other life stages. (By studying movement rates of juveniles and adults, he could determine that geese were not getting shot somewhere else, for example.) The population had stabilized because something was eating Canada goose eggs.

In the eyes of a typical nest predator — a raccoon, mink, or fox — a Canada goose is formidable. Already taller and larger than the raider, a goose defending her nest will stand erect, wings spread and beating, moving to shield the nest. If the display doesn’t ward off a predator, she’ll thrash it with her wings. Pairs of geese have been seen literally rolling stubborn raccoons wingbeat by wingbeat away from nests. A strike from the knuckle of the wing can break the skin of a researcher who approaches too closely, and a nip from the beak can raise a welt. It’s hard to see how such overmatched predators could hold the goose population in check.

As Paine studied the local geese, Stan Gehrt, a colleague and fellow scientist at the Max McGraw Wildlife Foundation, was radio-tracking coyotes in Cook County to see where they lived and how far they roamed. Though no one was taking an annual census of local coyotes, it was clear that they had been multiplying too, infiltrating some of the same suburban edge spaces where geese were expanding. The coincidence was suggestive, and Paine believed coyotes might be significant nest predators. Gehrt had tracked coyotes roaming through nesting areas, even observing a few attacks.

The problem is that coyotes aren’t known to raid bird nests. “We can’t open the stomachs of live animals,” Gehrt said, “so we collect scat to look at diets. Year-round, it’s dominated by mammals — rodents, rabbits, and even fruit if it’s available. That’s typical in all areas — rural and urban. If you look at scat, you never see eggs.”

Today, the coyote is a claimant to the role of keystone predator, a surprising change considering its historically more marginal niche. Smaller than the dominant timber wolves, which attacked coyotes on sight, the smaller canines learned to be wary, wily scavengers. Coyotes have profited in a modern landscape full of marginal habitat, purged of larger predators. They will visit dumpsters and hunt nuisance animals such as rats, but they also aggressively eliminate competitors such as foxes, and prey on animals as large as deer. “A dense deer population consumes the very cover that their fawns would benefit from,” Gehrt explains. “Coyotes will take up to 80 percent of the fawns in such areas.”

Coyotes are more social than their popular reputation. “They hunt as individuals, because their prey is small,” Gehrt said. “Then they join together after hunting, in rendezvous sites where you hear them howling, a pack, usually an adult male and female with some subordinate young adults from previous litters — getting together ‘after work.’ They mark and defend their territory from other coyotes as a pack.” In many landscapes, coyotes move through their territories day and night, but they’re very shy of humans, turning completely nocturnal in urban areas.

The coyotes’ wariness has made measuring their impact on nesting geese difficult. Since the standard method for studying predator diets — looking at scat — had never turned up evidence of egg consumption, it would take new techniques to confirm predation. Gehrt enlisted Justin Brown, a graduate student at Ohio State University. Fortunately, Brown turned out to be as wily as…well, as a coyote.

Beginning in the spring of 2004, Brown began placing cameras near goose nests to try to document coyote raids. Geese nest in aggregates, with as many as fifty pairs in close proximity. They’ll select sites near trees, as on islands at Busse Woods, or in open grassland, as at Buffalo Grove Forest Preserve. Whatever the nest site, a body of water must be nearby to provide refuge for the vulnerable goslings. Brown describes the nest as “just a little indentation in the ground, lined with whatever is available — sticks, grass, and leaves, enough to cover the eggs over when they leave the nest.” The female lays one egg — slightly bigger than a tennis ball — each day until the nest is full. She incubates them for about four weeks, while the male tends her and helps defend the nest.

Braving nips and beatings from the geese, Brown mounted motion-triggered 35 millimeter flash cameras a few yards from almost 60 nests, checking each every few days. The results in that first year looked like my childhood effort to capture the dolphin show at Sea World on film — lots of interesting things happening just off camera. Brown’s trigger-action cameras produced many blank images that might have been set off by predators on the margins of the focus area, by waving grass, maybe by the goose itself. He also got many suggestive images — the hind-quarters of a coyote escaping the frame, or the leg of a fleeing animal — but only a few showed unequivocal acts of predation.

The circumstantial evidence was piling up, but the research program was also affecting the results. At one colony, 11 nests were raided on a night when a camera captured a fleeing coyote, but the nest with the camera was left alone, and in succeeding nights, the whole colony was quiet. “We seemed to really be spooking the coyotes — the cameras make that focusing noise, and then the flash,” Brown said. “Raccoons didn’t seem to care. We’d have pictures of raccoons staring at the camera. But the coyotes wouldn’t come back.”

Brown also tried emptying goose eggs of the embryo, filling them with plasticene, and placing them back in nests. The doughy material would hold bite marks so that Brown could identify the predator. Again, Brown got mixed results. Some plasticene eggs showed raccoon tooth imprints. Others showed coyote marks — these eggs were often found quite a distance from the nest. Eighty percent of the eggs had “unidentified dentition,” weren’t taken at all, or were never found. With the fate of so many eggs unexplained, and a hint that the coyotes might carry eggs far afield, Brown still couldn’t state with certainty which predator was the predominant one.

For the 2005 nesting season, Brown settled on a new technique: infrared “spy” video cameras, which would capture the entire goose-predator interaction. The camera itself, golf ball size, would be mounted 15 feet from a nest, but the noisy recording mechanism would be up to 100 feet away, hidden in bushes. Infrared light would be imperceptible to the predators.

The video cameras quickly provided more definitive answers. Brown and the interns who helped him place cameras and retrieve tapes had noticed that Canada geese defend nests more aggressively early in the season. They were hard to scare off, even attacking the researchers on occasion. Later in the season, they spooked more easily. On video, Brown learned why. He witnessed successful defenses where geese warded off raccoons and opossums. He even watched an aggressive goose turn away a coyote by making itself look large, beating its wings and flying at the coyote, which decided it wasn’t worth the aggravation. “The goose bluffed his way through it,” Brown said.

The next night, however, the same coyote came back, killing the adult. “The behavior of the geese changed drastically,” Brown said. “Before, they’d be fighting us as soon as we got on an island. After the coyotes had killed a few adults, they’d run from us.”

Coyotes were taking as many as 20 eggs from a group of nests in a single evening. Brown found that the perpetrator was usually, but not always, an individual coyote. “At Busse, one coyote hit every single island,” he said. “I wasn’t expecting that, because we had at least six animals that might have been preying on nests. As one island got wiped out, we’d move the camera. We saw him at every island.” But a video from a nesting site at the Fermi National Accelerator Laboratory in Batavia, Illinois, showed a group of four coyotes raiding together.

Mainland nests were hit more quickly and more frequently than island nests, and the islands were more likely to be raided by single coyotes. Some nesting grounds went unmolested for a week or two, but eventually, they were attacked. At a Fermilab site, Brown saw nests and geese defending them, but he never saw any eggs. “I put a camera in to see what was going on,” he said. “The geese would lay eggs each day, and coyotes would come in every night.”

Some nesting areas took on a devastated look. “There’d be hundreds of broken eggs scattered on an island,” Brown said. “At Busse and at Buffalo Grove, pretty much everything was depredated. Our island with the most nests had 58, and not one was successful.”

To learn more about what was happening, Brown and his assistants numbered the eggs in monitored nests. Usually, the shell fragments were large enough for identification, which opened up a new mystery. Even after careful searches of very large perimeters, sometimes a couple hundred yards around in open terrain where it should be easy to spot egg fragments, some eggs couldn’t be found. During such a search, Alison Wellingham, an assistant on the project, noticed areas of disturbed earth. Probing a divot, she found some of the numbered eggs. Coyotes were digging caches to store them!

Turning the camera on some of the caches, Brown learned that coyotes returned to eat them, sometimes as many as three weeks after they were cached. This was usually as the nesting season was winding down, when fewer fresh eggs remained.

As the videos told their story, a complicated pattern emerged. Geese usually defended their nests from smaller predators, warding off raccoons in 44 of 45 recorded confrontations. (In the only successful raccoon raid, a determined and presumably very hungry raccoon endured more than a wing-beating — the goose stood on the raccoon’s back, flapping and pecking at it.) But when coyotes scared the geese away, small predators often took advantage of the opening. Almost all the successful attacks were initiated by coyotes, but raccoons ate almost half the eggs taken, and opossum and mink also shared the bounty. Raccoons rolled the eggs a short distance from the nest and crunched into them hurriedly, rushed and wary because of the larger predators. Coyotes cradled eggs in their jaws, carrying them as many as 150 yards before breaking them and carefully licking out the contents.

Coyotes were behaving as something more than alpha predators. They were serving as a foundation species, like alligators that thrash out watering holes in the soft Florida limestone, creating refuge for many other wetland species, or buffalo whose wallows were an important part of the presettlement grassland habitat. Coyotes, who routinely attack some rival predator species, nonetheless were opening up a niche for other midsized predator-scavengers.

It’s said that nature abhors a vacuum. In this case, nature is filling the vacant niche of goose predator with a vacuum cleaner that sucks down goose eggs on nightly sweeps. At least in some parts of Chicago Wilderness, explosive growth in the Canada goose population has returned to balance, and adaptation is leading to a new equilibrium: just recently, the groundskeeper at a local cemetery reported that geese, which had formerly nested pond-side, were now competing for space atop buildings and crypts. All it took was a coyote.