Home & Garden Columns

Partnerships across species lines aren’t all that uncommon in nature. Where Darwin saw evolution as a process of deadly competition, the Russian aristocrat-anarchist Pyotor Kropotkin observed “mutual aid” everywhere-cooperative behavior not just within species, as in the beehive or wolfpack, but even between unrelated creatures. 

The association can be as tight as the symbiosis of the fungus and alga that form a lichen, or the ancient bond between two kinds of single-celled organisms that may have given rise to all complex life. Or it may be a transient connection, a chance opportunity taken. Commensal foraging is one of those loose mutualisms, in which a predator hangs out with another animal whose activities are likely to turn up lunch—“commensal,” after all, meaning “sharing a table.” 

The legendary hunting association between the American badger and the coyote is a good example of commensal foraging. The relationship is mostly to the coyote’s advantage: the badger, with its powerful forelegs and claws, may unearth rodents that the canid would otherwise never see. And if the prey escapes the badger, the coyote has a better shot at chasing it down. There’s really nothing in it for the badger, which would prefer to be left alone. 

Once in the inner Coast Range I saw a red-tailed hawk that appeared to be using a badger as a beater, hovering above the carnivore like a satellite in geosynchronous orbit. The badger was way too big to have interested the hawk as prey, but whatever it flushed might have been tasty. 

Some of the best-documented commensal-foraging relationships involve other birds and mammals: warblers with armadillos, falcons with maned wolves, herons with manatees. One heron, the cattle egret, even gets its name from its tendency to follow large ungulates around. These chunky white birds, native to Africa and Asia, have colonized the Americas and Australia as well, exploiting the cattle niche wherever they’ve gone. Singly or in flocks, they trail along behind the cows—or, depending on the continent, rhinos or water buffaloes—and snatch insects and other prey disturbed by the hoof traffic. In the absence of cattle, they’ll follow tractors. 

Cattle egrets used to turn up in the Bay Area more often. There’s still a large population in the Imperial Valley, but they’ve become scarce north of there. I believe the lone bird that used to winter at Lake Merritt’s Rotary Science Center failed to return this season. 

It’s rarer for the native North American egrets to engage in commensal foraging. I’ve heard of observations involving snowy egrets, mainly in the Southeast. But the great egret hadn’t been caught in the act until November 2006, when Garth and Heidi Herring, apparently visiting from Florida, stopped to observe a couple of small herds of black-tailed deer at Bodega Head. What they saw was recently published in the journal Western Birds. 

The Herrings saw a great egret, which had been loitering near the deer herds, fly into a herd and land near its center. Five minutes later, a second egret joined the second herd. Each appeared to select a deer, which it followed at a distance of up to six feet for the next half hour. The birds were observed plunging their beaks into the grass and acting as if they were swallowing prey, although it was impossible to identify what they had caught. Voles would be a strong possibility: great egrets and great blue herons are serious rodent-eaters.  

Note, again, that this is a one-sided association. The deer don’t benefit from the company of the egrets, although if there were still mountain lions and grizzly bears at Bodega Head, it might be a different story. Other birds do act as sentinels for their commensal associates. In this case, if anything the deer might be a bit inconvenienced by the tagalong birds. 

Whether it’s a one-off observation or a hint at more widespread behavior that we’ve just missed, this is an interesting example of behavioral flexibility on the egrets’ part. It reminds us that birds aren’t just automatons, driven by inflexible instincts. To some extent, they can improvise to take advantage of novel situations, a tendency that may reach its highest development in corvids and parrots. 

You also have to wonder if the egrets were reenacting a very old relationship with long-gone partners. Asia and African didn’t always have a monopoly on megafauna. It might have been really rewarding for a great egret to follow a mammoth around.  

Joe Eaton’s “Wild Neighbors” column appears every other Tuesday in the Berkeley Daily Planet, alternating with Ron Sullivan’s “Green Neighbors” column on East Bay trees.