The trees were leafing out, winter's cold blasts had turned into gentle breezes, and the robins had been back for weeks. But something was missing. I was listening for a distinctive bird song, a sound I've come to associate with sunlight and warmth.

And then on a mild, breezy day, as I walked down the hill on my way to the local lake, there it was, a series of loud, sweet whistles pulsing from a tree on the lakeshore. A warm wind brought in the first of the returning Baltimore orioles, one of the sweetest singers of the season.

Quite often this happens right after May Day (my bird notes often cite May 4), and once I hear those clear, liquid notes, I know that spring has truly arrived.

Designer birds

The male Baltimore oriole sings a melodious song to attract a mate and repel competitors (hear an oriole here: These are beautiful birds whose vibrant orange almost seems painted on, with a designer then adding some touches of black and just a bit of white, in a wing stripe. The female is drabber, as is often the case in the bird world, with a coat that's more yellow-orange and gray. What she sacrifices in aesthetics she gains in protection from predators, since she's less visible as she sits on the nest.

Not all male orioles are the same brilliant orange. Their feather color comes from the foods they eat, so a male that enjoyed plenty of lush, red berries and fruit during the summer will be a brighter orange than one that consumed less vibrant fruits.

Does this make a difference?

Researchers have found that female orioles, like females in many other bird species, are attracted to brighter males — the vividness of the male's feathers suggests his skill at finding food, a bona fide occupational qualification in nesting season. Having bright plumage has another benefit for male orioles: The brighter his feathers, the more intimidating he is to other males, conferring a benefit in holding on to a breeding territory.

Unlike robins and many other fruit-eating birds, Baltimore orioles show a strong preference for dark fruit, the ripest they can find. A mulberry tree in full fruit will host a wide variety of birds, but the orioles will take only the darkest, ripest mulberries, leaving the merely red ones for the squirrels. Similarly, they choose very ripe elderberries and only the reddest cherries and darkest grapes. Their affinity for grape jelly and orange halves at our feeders is well known.

Nest prep

At this time of year, male orioles are intent on courting a female and beginning a nest. They initiate furious chases after other males that enter their territory. I've sometimes seen a pair of males so intent on each other that they nearly fly into people, cars or street signs.

Oriole females are some of the most accomplished builders in the bird world, constructing such sturdy nests that these often still hang from branch tips the following spring.

She weaves her purse-like nest by first anchoring long strands of plant material over a small branch high in a deciduous tree, often a maple or cottonwood. Then she winds and pokes additional thin strands, which she pulls from bark and animals, and uses twine or fishing line when she can find these, around the original strips. Her rapidly jabbing beak tends to create knots and tangles in the material, forming a perfect droopy bowl to hold her eggs. She finishes by lining the inside of the nest with soft fibers and feathers.

With only a small opening at the top of the hanging nest, it provides good protection from predators, and the location, out at the end of a twig, foils most squirrel attacks. However, larger birds sometimes are able to snatch an egg or nestling when both oriole parents are away. Once the oriole family fledges, they never use the nest again.

Their gorgeous spring song is a short-lived phenomenon, a spring ephemeral. But once you stand below a tree where a male is singing his exuberant song, you can almost count on an encore, same tree next spring.

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 val​