Focus On...Redhead and Canvasback
With two of five of the atlas project's breeding seasons now under our belts, we have at least one confirmation for almost all of San Diego County's regular nesting species. Perhaps the commonest species among the ten or so still unconfirmed during the last two years is the Redhead. Therefore, I selected it and its close relative the Canvasback to "focus on" in this issue of Wrenderings. These two ducks occur in San Diego County primarily as winter visitors, and though relatively easy to distinguish have been confused, making them a suitable topic as we embark on our second winter season.
The Redhead and the Canvasback, both diving ducks of the genus Aythya, share many characteristics. Males have reddish heads, black breasts, and largely grayish bodies. The shape of the head and bill, though, differs obviously: the Redhead's head is rounded; its typically duck-shaped bill breaks the outline of the head abruptly. The Canvasback's head has a peaked crown and a sloping forehead that grades smoothly into the long bill with a much deeper base. The Canvasback's triangular profile thus resembles more that of a scoter, eider, or Trumpeter Swan. The Canvasback's bill is uniformly dark; on the Redhead's bill, the black tip contrasts with the gray base and is often separated by a whitish zone. The backs and sides differ strongly, too -- the fine gray vermiculation on the Redhead is much denser, at a distance making its back look much darker. By contrast, the Canvasback's back often looks an almost gleaming white. But beware the varying effects of light: bright sunlight makes the birds look whiter; overcast skies make them look grayer. The birds' plumage is sometimes stained brown, obscuring the natural color. Early in the winter, shortly after the birds arrive, males may retain some juvenile or eclipse plumage. A Canvasback in the Museum collected on 9 November shows this clearly -- the duller back could lead to birds like this being confused with the Redhead. Always ensure that the identification implied by back color is consistent with that implied by the head and bill shape.
Females of both species are very drab, being almost uniformly plain brown with a vaguely paler face and throat. Fortunately, the differences in head shape, bill shape, and bill color apply to them as well as to the males. The back of a female Canvasback tends to have more whitish feathers with gray vermiculation, that of a Redhead, few or none, leaving the bird with a uniformly brown back. But the difference isn't as absolute as the illustration in the National Geographic Society field guide might lead you to believe. The Canvasback tends to have a richer brown head with a more distinct pale streak behind the eye, the Redhead a more uniform gray-brown head. But these subtle differences can be obscured by the rusty stain, from natural chemicals dissolved in water, to which waterfowl are susceptible. The female Ring-necked Duck, a freshwater species often found in the same places, should be readily distinguished by its distinct white eye ring, whitish face, grayish cheeks, and contrastingly white-ringed bill.
As the maps imply (updated maps to be posted here soon), the winter distributions of the two species aren't grossly different -- both are patchy and scattered, as expected in waterbirds dependent on wetlands and lakes. Still, closer inspection, bolstered by information on the birds' numbers and biology, reveals some basic differences.
The Canvasback, which occurs in southern California as a winter visitor only, is almost entirely a bird of fresh or brackish water. It is not a common bird over San Diego County as a whole; its winter range lies largely north and east of us. During the atlas' first winter season, it was found in large numbers at only two sites: O'Neill Lake in Camp Pendleton (E6), where Pete Ginsburg estimated a maximum of 275, and Buena Vista Lagoon (H5), where David Rorick counted 232 for the Oceanside Christmas Bird Count. The Canvasback occurs in smaller numbers on other lagoons and lakes in north-coastal San Diego County, including Batiquitos Lagoon (maximum 26 on 6 February by C. C. Gorman), though the recent opening of this lagoon to the tides likely makes it less attractive. Though the Canvasback has in the past been recorded rarely on Mission and San Diego Bays, note its absence from the coast south of Del Mar in our data so far.
On lakes in the foothills and mountains, the Canvasback is much more frequent than the Redhead. Small lakes and ponds seem to attract it as least as much as the large reservoirs. The largest counts in the foothills at mountains last winter were 12 near the mouth of Couser Canyon (E10) by Maryanne Bache and Kathy Aldern, 10 in borrow pits in the Sweetwater River near Dehesa by Arnold Young, 19 on Big Laguna Lake (O23) by me, and 13 at Jacumba (U28) by our blockbuster weekend team. Canvasbacks may use the large reservoirs irregularly, though they are frequent on many of these, such as Wohlford (H12) and Barrett (S19). Our remaining four years of winter surveys should identify the consistent sites. As a winter visitor, the Redhead is most numerous at places where the Canvasback is typically absent: the salt water of Mission Bay and the San Diego River flood-control channel. Barbara Moore estimated 200 in square Q8 (NE quadrant of Mission Bay) on 4 January 1998; I estimated 300 in R8 (SE quadrant of Mission Bay and upper end of flood-control channel) on 24 December 1997. Interestingly, these numbers are appreciably higher than known from these areas 20-25 years ago. The Redhead occurs spottily in other coastal wetlands, especially Batiquitos Lagoon (maximum 50 in J7 by Mona Baumgartel) and the Sweetwater River estuary (maximum 50 in U10 by Barbara Moore).
Farther inland, the Redhead is generally quite uncommon and apparently irregular. The tendency to prefer small ponds to large lakes is even more marked than for the Canvasback. The only reports of more than five birds are Geoff Rogers' of 10 on a pond west of Warner Springs (F18) on the Lake Henshaw Christmas Bird Count and Arnold Young's of 15 in borrow pits in the Sweetwater River bed near Dehesa. There was only one record each of either species last winter from the Anza-Borrego Desert: three Redheads and one Canvasback on 9 February in G24, Borrego Springs South, by Paulette Ache. Paulette informed us of the changes to bird distribution induced by newly built golf-course ponds in the last issue of Wrenderings.
Other than the Ruddy, the Redhead is the only diving duck known to nest in San Diego County, the southern tip of its breeding range. Fifty years ago the Redhead was known to nest only in the San Luis Rey valley, possibly only at Guajome Lake, where it still summers. Then in the 1960s the late Alice Fries discovered Redheads with ducklings at most of northern San Diego County's lagoons. Our atlas results show them still occurring irregularly in small numbers in the breeding season at Batiquitos Lagoon, San Elijo Lagoon, and the San Dieguito River estuary. Despite the intense attention Batiquitos and San Elijo receive, over the last two years, we have only one breeding-season record of the Redhead for each. The species is a bit more numerous at the San Dieguito River mouth, where the regular counts there, organized by Don Grine, recorded a maximum of 16 on 19 June 1998.
So whenever you see Redheads in summer, please watch carefully for their nearly uniform yellowish ducklings -- much paler and yellower than the chicks of any other species of duck nesting in San Diego County.
Philip Unitt, with sketches by Nicole Perretta