Spring has arrived and so has nesting season

Spring has arrived. The weather has grown warmer, the hills are green and dotted with flowers, and the birds are singing. What are they so excited about? The nesting season, the time of year when birds pair up, construct nests, lay eggs and raise this season’s young.

It’s a little difficult to define exactly when California’s nesting season starts and ends, but we are undoubtedly in the thick of it now. The core baby season is perhaps April to July, with preparatory song and courtship beginning months before and second or third broods continuing to appear in August. Some birds fit less well into this outline, with owls and hummingbirds, for example, starting nesting as early as December. But the majority of Marin’s songbirds will wait until spring has truly arrived to hatch their young, aiming to align those extra mouths to feed with the peak abundance of insects and other natural foods.

So, what does the nesting season look like to an observant human? The most obvious phenomenon is what it sounds like: birds are singing. Song is the most notable preliminary to nesting proper, being used by birds both for mate attraction and territorial declaration (many songbirds only defend exclusive territories during the nesting season and are less combative in winter).

Around the neighborhood, some prominent voices in recent months include robins, house finches, goldfinches, mockingbirds, mourning doves and towhees. In woodlands, migrants like warbling vireos, orange-crowned warblers and Pacific-slope flycatchers join residents like titmice, wrens and juncos in bringing music to the trees.

By May, however, the dawn chorus is already starting to diminish compared to March and April. Once pairs are established and nesting is legitimately underway, most birds switch to a relatively lower-profile life, furtively bringing food to nest sites they would prefer to keep undetected. The impression of a slowdown in bird activity at this time of year can be partially due to this phenomenon, partially to migration (the crowned sparrows have left, for instance), partially to the breakup of noisy winter flocks, and partially — for yards with bird feeders — to the increased availability of natural food, which reduces birds’ hunger for sunflowers and suet.

Nesting birds move from song to secrecy. That doesn’t mean that there are no distinctive behaviors of the nesting season to observe; it just means you need to pay closer attention. First, you’ll see that more birds are in pairs. While some songbirds maintain their pair bonds all year round (California towhees, titmice and scrub-jays), many others do not, living individually or in larger flocks. Now almost all birds are paired up, with the exception of some individuals that have not found a mate.

Second, most birds are now spending a good portion of their days in some form of nesting-related activity. You may see a jay with a beak full of twigs for nest building, a towhee carrying the spoils from the bird feeder to a nest full of babies somewhere in the bushes, or red male house finches “courtship feeding” as they give food to their streaky brown female counterparts.

First stage: song. Second stage: nest construction and feeding nestling babies. Third stage: the babies emerge! While some early nesters have already fledged (flown from the nest), this is the phase of the season that we are entering into in May and June. For most songbirds, the young will be as large their parents by the time they are able to fly, but there can be some other clues that give away their newborn status. A few have distinctively streaky plumage, including juncos, bluebirds and spotted towhees, while all young songbirds initially show a visible “gape,” the unfeathered area around the edges of their bills.

If you don’t get a close look, however, then it may not be so much the fine details of plumage that indicate their youth, but their behavior. The classic sign of a recent fledgling is begging — a distinctive way of asking their parents for food, in which the baby bird crouches down, flaps its wings, opens its bill wide and calls plaintively. Small birds typically become independent from their parents a few weeks or months after fledging, so watch closely — the childhood of birds doesn’t last long.

Jack Gedney’s On the Wing runs every other Monday. He is a co-owner of Wild Birds Unlimited in Novato and author of “The Private Lives of Public Birds.” You can reach him at jack@natureinnovato.com.