My dog and I rounded Anchorage’s Cheney Lake one morning in June and found a bemused jogger aiming his smartphone camera at dozens of Canada goose chicks occupying the walkway in one big cluster of fuzzy-wuzzy cuteness. A couple of sentries eyed us while several other adults rested among the goslings.
“Pretty tolerant, aren’t they?” I commented as I edged by.
“This is amazing!” he said. “I’ve never seen anything like it.”
The throng didn’t surprise me because I’d been counting my way around the neighborhood lake during daily walks. Two common loons, no chicks (yet); 17 mallard ducklings; two red-necked grebes; 10 Canada geese with, let’s see, 33 goslings.
It’s embarrassing to admit how large these fuzzballs had grown before I wondered why these geese had formed a giant gaggle in the first place. Did the parents mix up their offspring and settle for communal living rather than sort them out? Was this some sort of avian day care? Were the adults related, creating one big extended family?
As it turns out, Canada geese (Branta canadensis) take a flexible approach to the concept of the family unit. Most adults do create lifelong bonds, and many pairs form what we call a nuclear family: two parents who typically rear between two and eight offspring annually.
But geese also commonly employ another parental strategy known as brood mixing, in which they care for unrelated goslings. Researchers have reported that up to 50 percent of breeding goose pairs adopt chicks in some areas. Other families shuffle their broods in a variety of arrangements. In one study, some goslings had joined three different families by the time they could fly.
Other species of ducks and geese engage in brood mixing, and more than 150 bird species adopt chicks. “Posthatching brood amalgamation” is a broad term for this behavior. But not all broods are alike, according to researcher Ellen Kalmbach in a 2006 paper published in Ibis.
“‘Gang brooding’ (or gang broods) mostly refers to cases where several families join together with their offspring,” Kalmbach wrote. “Although typically not all parents will stay throughout brood rearing, the caring adults come from the group of original parents, which is not necessarily the case in crèches.” In crèches, different broods form a large group under the care of several adults that may include non-breeding or unrelated geese.
The Cheney Lake bunch was what Kalmbach calls a super brood. Researchers have reported super broods with 35, 36, even 40 goslings, some up to three weeks apart in age. A 1983 study documented broods with as many as 75 goslings. One photographer captured a picture of a single goose shepherding 23 chicks.
The basic question is: Why?
“This seemingly altruistic behavior seems contrary to natural selection,” noted Anthony Nastase and Dawn Sherry in a 1997 paper published in Animal Behavior.
Think of it this way: We all know that raising eight kids takes more time, attention, energy and money than raising two. From an evolutionary standpoint, why would an organism risk the survival of its own offspring (and genes) by devoting valuable resources to unrelated young?
One way to approach this problem is to consider the costs and benefits both short and long term. For Canada geese, the costs seem minimal because their offspring don’t demand much parental care. They are precocial, meaning they can walk, swim and feed themselves within a day of hatching, so parents mostly provide warmth and protection.
On the plus side, many studies show that large goose families are more dominant socially, so they feed in the best places on their wintering grounds. Geese in dominant groups also grow faster and are in better condition.
“It is clear that being part of a family is advantageous for juvenile geese and adults alike, and that being part of a dominant or large family is better still,” Halmbach wrote.
But a larger brood might also require adults to spend more time remaining vigilant, potentially reducing the time they spend foraging. Some studies have suggested this occurs, while others found that geese devote more time to vigilance without feeding changes.
Adult geese may adopt goslings to increase survival odds for their natural offspring, according to some hypotheses. Adding members may increase the brood’s ability to detect predators, or it could create a dilution effect by reducing the probability that predators will kill a natural gosling. Researchers also wonder if large broods display the “selfish herd effect,” a behavior in which the most dominant animals occupy the safest place in the group, usually the center.
Nastase and Sherry found that adopted goslings did tend to hang out farthest from the parents, although this didn’t seem to affect their survival rates. However, “natural goslings raised in mixed broods lived longer than adopted goslings or natural goslings in unmixed broods.” Though predator detection, reduced predation on natural offspring, and the selfish herd effect may indeed be among the long-term benefits of mixed broods, Kalmbach noted that the sample size in this study was small.
In general, published studies of brood mixing in several species of geese and ducks have produced contradictory results, and few researchers have tested their predictions experimentally, Kalmbach wrote. Nor does the published literature produce any consensus about the “ultimate explanation” for this behavior, she noted.
Parenting strategies by Canada geese do appear to be more than mere happenstance, judging from a recent study by Michael Conover of Utah State University. Experience may play a role, for one thing. Among the geese he followed, 80 percent with five or more years of parental experience formed gang broods, while only 29 percent of first-time breeders did so. The geese tended to stick with their approach, too. More than 60 percent of both “gang-brooders” and two-parent families used the same strategy the following year. Those that switched techniques had usually switched mates.
“These results indicate that gang brooding is a behavior learned as an adult,” Conover wrote in the May 2009 Condor. He also found that geese were just as likely to form coalitions with unrelated parents as with relatives.
Canada geese exhibit several traits that probably encourage brood mixing, Kalmbach observed, including precocial development; nesting in large colonies and synchronized hatching; social hierarchies; long-lasting pair bonds; and the fact that both parents care for their young throughout their first winter. Some researchers report that brood mixing occurs in crowded nesting areas where grazing is limited. Forming groups might be a way for adults to avoid fighting.
The Cheney Lake super brood seemed remarkably tolerant of the daily parade of dog walkers, joggers, bicyclists, anglers and excited children. After reading about gang brooding, I may have mistakenly assumed that the geese remained calm despite the constant nearness of people, when some studies suggest that high concentrations of geese combined with human disturbance may play a role in creating super broods. Given the widespread prevalence of Canada geese in urban areas across the country, you may find a super brood coming soon to a neighborhood near you.
—Sherry Simpson teaches creative writing at the University of Alaska Anchorage and is the author of The Accidental Explorer: Wayfinding in Alaska, and The Way Winter Comes: Alaska Stories, both published by Sasquatch Books.