Focus On...Red-winged and Tricolored Blackbirds
The Red-winged and Tricolored Blackbirds are confusingly similar in plumage and strikingly different in their nesting habits. Few more fascinating examples of different biology concealed by similar morphology can be found among birds. The Red-winged is one of the most widespread species of North America, while the Tricolored is narrowly restricted to the Californian biogeographic region. Thus we in San Diego are in a great position to appreciate this remarkable pair of species.
The adult male Red-winged Blackbird, with his red epaulet on a black body, is one of the most familiar of American birds. Yet this simple fašade conceals variation of a complexity rarely equaled. When I give tours of the museum's bird collection and need an example of how many specimens may be needed to show what one species looks like, I always select the Red-winged Blackbird. The streaked females, of course, differ grossly from males, and the juveniles differ appreciably even from the females. Males take two years to achieve their definitive plumage, and the first-year males vary greatly among themselves in the heaviness of their streaking. Even though the Red-winged Blackbird molts only once per year, the fresh plumage of fall and early winter differs greatly from the worn plumage of late spring and summer, in both males and females, because the new feathers have pale or buff edges on them that, over the course of the year, wear off completely. Finally there is great geographical variation in the species in plumage color, plumage pattern, and bill shape. This variation is concentrated in California, the home of no fewer than seven subspecies of the Red-winged Blackbird.
The Tricolored Blackbird dispenses with some of the Redwing's complications. Though the Tricolor's sexual dimorphism is parallel, the immature males acquire a plumage very like the adults' in their first fall. The effect of plumage wear is less obvious because the feather edges are narrower and less contrasting to begin with, and there are no subspecies. Fortunately, only one subspecies of the Red-winged, neutralis, occurs in San Diego County commonly.
The complex differences between the two species in plumage may be presented best in a table. Easiest of all to distinguish are the females in fresh plumage. In fall the female Redwing, with her buff and chestnut feather edges, is a thing of beauty. At this season the female Tricolored is far more subdued. Over the year, however, the edges gradually wear off the feathers, and the differences between the two species recede as summer approaches. By midsummer, the streaking on the upperparts of the female Redwing may be practically obliterated. Even on the underparts the whitish streaks become narrower and less conspicuous. The female Tricolor, meanwhile, becomes almost totally black, with streaking remaining only on the throat. In San Diego we can be thankful we don't have to contend with the subspecies of the Redwing in the San Francisco Bay region, mailliardorum, whose females resemble those of the Tricolor far more closely and when worn may not be distinguishable from it from gross appearance alone.
Fortunately, at the time of year when they are most similar, the Redwings and Tricolors are nesting, and their profound differences in nesting biology readily identify them. The Redwing follows a strategy more or less traditional for a songbird, with each male advertising a territory and defending it from other males. Each male, though, may have a harem of several females nesting within his territoryup to 15 have been recorded. This leaves a surplus of nonbreeding, nonterritorial males that hang out in bachelor flocks, often at feedlots. Where the birds are nesting in marshes, the territories may be quite smallas little as 153 square meters, though 1600 is average, and territories are larger in other habitats.
The Tricolored Blackbird, on the other hand, follows the model of colonial seabirds. Males maintain no individual territories, and females may nest barely out of pecking distance of each other. The colonies may be enormousformerly of up to 200,000 nestsand a colony of 20,000 nests has been found packed into a marsh of only 10 acres. The birds within a colony are closely synchronized, so that all young hatch and fledge within a few days of each other. But different colonies may be quite unsynchronized, some establishing themselves two or three months after others. Some colonies, like that at Jacumba, are used year after year, but others, like those around Ramona, shift abruptly from one year to the next. When hundreds of birds are singing from a colony at once, the effect is surreal and identifies the birds as Tricolored Blackbirds long before the observer can focus on a single bird. The Tricolored Blackbird is unique among North American songbirds and one of California's remarkable natural treasures.
Unfortunately, the Tricolor's intensely social yet nomadic habits and preference for nesting in freshwater marshes put it at risk. Numbers throughout the species' range have declined seriously, and this decline has been proportionately greater in coastal southern California than elsewhere. Large colonies may be necessary for adequate predator defense and proper social stimulationthe Tricolored Blackbird has been called California's Passenger Pigeon. When I was a child in the 1960s, I regularly saw flocks of thousands of blackbirds, mostly Tricolored, commuting north-south over my parents' house in East San Diego. Until about 1980, the flamingo pond at the zoo was one of the easiest places to see Tricolored Blackbirds. Now our atlas results show them relegated largely to scattered colonies inland. The county population now is probably no more than 10,000 birds, which could be seen in a single day 40 years ago. We now have a good idea, thanks to the atlas, where the remaining colonies are located. But because of the species' nomadism, not all colonies are active every year, and to sum the colonies over the entire atlas period would overstate its abundance.
The Red-winged Blackbird is clearly a much more adaptable and therefore widespread species. Most squares on the coastal slope have small ponds or riparian strips enough for at least a few. Still, it is not really an urban adapter, as revealed by the several squares in metropolitan San Diego where the species is lacking. The distribution spills down toward the desert along San Felipe Creek as far as the mouth of Sentenac Canyon in square J23 and Carrizo Gorge in R27. It's still an open question whether the birds nesting in the Borrego and Vallecito valleys are the coastal subspecies neutralis or the thin-billed, pale-femaled subspecies sonoriensis common in the Imperial Valley. Scattered records from more remote places in the Anza-Borrego Desertas well as a couple of old fall specimens of sonoriensis from Jamachasuggest that the isolation of these subspecies isn't total.
Sketches by Nicole Perretta