On the Fly – Birds of the Beaver Dam

My friend who lives on the Alhambra Way part of the creek has a new pastime: watching for hooded mergansers. When he hears their guttural, froglike courtship calls, he can hang his head out the bathroom window and, if the creek is clear, watch them swimming—underwater—after their little prey. They like fish, aquatic insects, and crustaceans. He says they dart around like pinballs.

People have been seeing these spectacular little crested ducks all over the central part of the county this year, from Moraga to Martinez. Recently two pairs have been swimming around at Heather Farm Park in Walnut Creek, and as I mentioned a few weeks ago, half a dozen or more have been seen from time to time at the Martinez Regional Shoreline pond. I haven’t yet had the luck to see them on the creek, but I have high hopes.

The people at Worth A Dam, the group that has been advocating for the Alhambra Creek beavers since their 2006 arrival, have film of hooded mergansers at the beaver dam and great photos of other birds that are taking advantage of habitat improvements the beavers have made: kingfishers, cormorants, grebes, and egrets to name a few, and a couple of herons. Okay, technically egrets are herons, but I’m talking about green herons and black-crowned night-herons.

The green heron (Butorides virescens) is a study in camouflage. When one of these birds is standing on the beaver dam it blends in perfectly with the local color. You will not see it unless it moves, and neither will the fish it is searching for. But what a flashy bird this is when it’s not hiding, as you can see in the photo.

Green herons actually fish with bait and lures: they drop anything from twigs or feathers to bread crusts on the water to attract fish, and then they grab them. Biologists identified that fish in the picture as a Sacramento splittail, a species that hadn’t previously been documented in Alhambra Creek. Beaver ponds alleviate sediment and pollutants in waterways, and it’s nice to think that these little fish, which have been challenged by the environmental impact of humans, are getting some help from the beavers. No doubt some of them are getting past the herons.

The green heron has an astonishingly long neck but usually keeps its head hunched up next to its body. The black-crowned night-heron (Nycticorax nycticorax), on the other hand, has a short neck. Its legs are short too, and with its thick bill and beady red eyes it certainly lacks the aristocratic look of its relatives the egrets and great blue herons, but it is beautiful nonetheless. Night-herons fish by night and then head for the trees and bushes to sleep, but you might see them moving around during the day. Maybe because they are mostly night birds they seem unusual, but in fact they have a bigger range than any other heron—five continents. Locally, there are throngs of them at the Mountain View Sanitary District marsh.

Back at the beaver pond, songbirds too are finding their habitat improved. A 2008 study by the Wildlife Conservation Society found that where there are more beaver dams there are more songbirds. The dams and ponds recharge water tables and improve the health of streams. Taking out pondside trees, the beavers encourage low-growing plants; chewing willows and cottonwoods to the nubs, they stimulate new shoots on those trees. All of this creates cover for songbirds and nesting habitat for waterfowl.

In the late 1700s, after traveling fifty-five hundred miles up and down and across the continent, explorer, surveyor, mapmaker (and fur trader) David Thompson concluded that North America had once been “in the possession of two distinct races of Beings, Man and the Beaver.” By the time he said this, beavers were approaching extinction in the eastern part of the continent and soon to meet the same fate in the West. Imagine the environmental impact these determined little rodents must have had—and then imagine the impact of their near-disappearance.

It is true that beavers take their toll on vegetation, but plants have obviously adapted to withstand this, and other creatures have adapted to take advantage of it. Sometimes the rising water level of a beaver pond will drown a tree—but a woodpecker might then carve a nest in it, and other birds might later use the same nesting cavity: flycatchers, wood ducks, owls, kestrels, and mergansers among them.

Hooded mergansers usually head north to breed and are not often seen in Contra Costa County after April, but at least one study shows that they are expanding their breeding area into California. So who knows, maybe there’s a pair just waiting for the right opportunity to take up residence in Martinez. I will admit that I’d happily sacrifice a cottonwood or two in exchange for a view of merganser ducklings swimming up the creek.

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Links to some of the things in this column:

Worth A Dam

Photos of various birds at Alhambra Creek beaver dams can be seen on the Worth A Dam website and also on the “Alhambra Creek” Wikipedia entry .

Worth A Dam footage by Moses Silva, video by Heidi Perryman of hooded mergansers is on the mtzbeavers channel on YouTube. 

Some previous “On the Fly” columns about birds mentioned here: belted kingfishers; egrets and great blue herons; cormorants; hooded mergansers.