Is that a boy or a girl bird?

It's courtship season in the avian world, and male birds are pulling out all the stops to attract a female and begin raising a new generation.

For bird behavior watchers, this can be a fascinating time, but it can be challenging, too, since it's not always easy to tell the genders apart. Is that a male singing his heart out on a branch? Is that a female carrying what looks like material for building a nest?

Bird species where males look very different from females are easy. Think of bright red male cardinals and the subtler but equally beautiful females, or house finches, with red-splashed males but drab, brown females. And there's no mistaking male and female mallards and rose-breasted grosbeaks, either.

Among raptors, the females and males of many species have similar feather coats, but some give us a clue via size differences. This is true for sharp-shinned hawks, where a female may be a third larger than a male. And female great horned owls and bald eagles can be noticeably larger than their mates, but you'd need to see them perched together to discern this difference.

Among the birds visiting our feeders, blue jays and black-capped chickadees are nearly impossible to tell apart, because both males and females have identical coloration and markings.

"Chickadees are a little easier than jays because males sing much more than females do," says Duluth naturalist and ornithologist Laura Erickson, "but this isn't 100 percent accurate because female chickadees sometimes sing, too." (If we had a black light, like Canadian researcher Daniel Mennill, we could see the fluorescent pink shine on 'dee feathers that tells males from females.)

Another challenging backyard visitor is the catbird, with both genders showing a sleek gray body and rusty rump. Ditto the cedar waxwing, with females and males having identical, sleek Art Deco looks. Crow genders look exactly the same, as do Canada geese, great blue herons and sandhill cranes.

Sometimes bird behavior gives some clues. A bird performing a courtship ritual, such as passing food to another bird via the beak, is probably a male. The same is true for courtship flights by ruby-throated hummingbirds — the male pirouettes, the female watches. Among woodcocks, the birds twirling in an elaborate sky dance at dawn and dusk are male.

So where does this leave someone who really wants to know the sex of the birds he or she is seeing at feeders or out in the wild? In some species, we can easily learn to spot the differences. With others, such as many raptors, we can hope that we see a pair together to note any difference in size. But for a number of bird species, we're out of luck, something of no concern to the birds.

Because one thing we can be sure of is that they can tell males from females — the future of their species depends on such judgments.

St. Paul resident Val Cunningham, who volunteers with the St. Paul Audubon Society and writes about nature for local, regional and national newspapers and magazines, can be reached at valwrites@comcast.net

• Why are females of some species so much drabber than the males? One reason is that females spend so much time on their nest, and fading into the background helps hide them from predators.

• Why do some birds, like cedar waxwings, look exactly alike? They probably don't to other cedar waxwings, which can detect UV light and other signals.