DIMORPHISM IN BIRDS

                                                 

Female Northern Cardinal

 - Photo courtesy of Sharon Milligan

If you would like to see more articles and photos, click on the blue title

Weekend rains and thunderstorms, some of them coming up from the Gulf of Mexico, then followed by a shift of wind direction, from south to north, set the scene early this week for the best migrant-trap birding we've experienced this spring.

Much of it spilled over into our back yards. Those with an eye for birds should have seen a fair share of flaming red Scarlet Tanagers, Rose-breasted Grosbeaks with ascots of hot pink and sunny orange Northern Orioles, to name a few of the paint-pot birds for which we yearn and the sight of which brings great joy into our lives.

As much as anyone, I revel in the parade of gloriously decked-out birds.

Perhaps you've noticed that these handsome sports are males of the species and that you hardly ever see their female counterparts, or if you do, you don't recognize them for what they are.

Just about all the males of our neotropical songbird migrants (translate that to mean the vast majority of insect-eating birds that winter in Central and South America) make the trans-Gulf flight in advance of the females, which follow one or two weeks later.

You may have experienced that earlier in spring, when a bunch of Indigo Buntings, all of them bright blue and easy to recognize as males in high breeding plumage, may have grounded in your yard. It is only later on, when they may have been joined by the brownish females and younger splotchy-blue-on-brown males that you might figure out there is a sequence to the migration of any given species: adult males first, then adult females and the not-quite-ready-for-primetime players, young birds hatched last summer.

But who looks at brown birds when the stage is dominated by bright blue ones?

When male birds are disarmingly attractive, and the females drab by comparison, that is a form of sexual dimorphism (two distinct forms, or morphs, of the same species). One of the most familiar examples of sexual dimorphism in plumage occurs in those Northern Cardinals that hang out around the feeder. Males and females are recognizably different from each other. In many other species (Blue Jays, for example) there is nothing in their outward appearances to indicate which is male and which is female.

Plumage differences between the sexes range from minor to major. For instance, the female Prothonotary Warbler is slightly less bright than the male, a difference which is noticeable when they are seen together. Certain other birds -- the Red-bellied Woodpecker, for example -- have more extensive patches of color in their plumage. He has a full satiny-red cap; hers is more of a half-cap. And others are so different that they may go unrecognized as the same species.

One of the most extreme examples of sexual dimorphism occurs in the Rose-breasted Grosbeak, affectionately known among birders as the Rosie Gross. On the one hand we have the male, decked out in jet black and pure white with an incredible ascot of hot pink at his throat, like a gent in formal attire. On the other hand, it is the female who really deserves to be known as Rosie Gross, for she is streaked with brown and white, appearing to be an overgrown sparrow, and she tends to shy away from the spotlight, often hiding behind leaves as if knowing how she would suffer by comparison.

In short, he is highly visible as befits his role as master of his own territory. She is inconspicuous, as befits her role as nest-keeper and general all-around drudge.

If you really want to get to know the females of such sexually dimorphic species, you should know what to look for. For example, everyone -- even those with next to no birding experience -- would recognize an adult male Scarlet Tanager or Northern Oriole on sight. These same people would be likely to ``tune out'' on these birds' female counterparts. The female Scarlet Tanager is yellow and black; the female Northern Oriole is yellow, white and brownish gray.

Notice that in a field guide, if the sexes look essentially alike, the female is not illustrated. When there are recognizable differences, the female is well treated. The printed text is invaluable to those attempting to establish familiarity with an unknown female (no double entendre intended).

The female songbirds that seem to stump most people include the Northern Parula, Black-throated Green, Blackpoll, Black-throated Blue, Cerulean, Cape May, Chestnut-sided, Bay-breasted, Blackburnian, Yellow, Wilson's, Hooded, Nashville, Connecticut and Mourning warblers and the American Redstart and Common Yellowthroat. The foregoing are warblers all.

Also, all members of the blackbird family, including the orioles, Summer and Scarlet tanagers, Dickcissel, Blue, Rose-breasted and Black-headed grosbeaks, Indigo and Painted buntings and the finches, House and Purple, and the American Goldfinch.

Don't you agree that it's time to give the distaff members of the bird community some recognition? That begins by knowing who they are. You have another week or two to get started with identifying migrant females, and the experience will be invaluable when, come autumn, virtually all of this summer's hatch will resemble their female parents. And then the fun and frustration really begin.

This article was published in April 1995