Sunday, June 27, 2010

Birds in June

(Robin's nest in a Syringa.  Why are the eggs of the American robin blue?)

I spent an hour this morning appraising the avian situation in my forest.  Male singing has changed over the past month in an interesting way.  Species that were quite vocal earlier in May are now fairly quiet, but others are singing constantly.  Dark-eyed juncos, chipping sparrows, song sparrows, black-capped chickadees, wood thrushes, all the woodpeckers, white-breasted nuthatches, eastern phoebes, American robins, gray catbirds, and blue-headed vireos are relatively inconspicuous now.  I assume that the frequency of bird song is correlated to the stage of the nesting cycle.  Males sing to keep other males at arm’s length and to attract females.  When the male or female (usually the female) is incubating eggs or tending nestlings, males tend to be quieter.  I am sure this is to avoid attracting predators to their territory, where the nest is located.

But other species are quite vocal.  Ovenbirds, red-eyed vireos, great-crested flycatchers, eastern wood pewees, and veerys are still waking me up early in the morning.  Because they returned from migration later than the group of silent species, some of whom are year-round residents, these late arrivals may be further behind in their nesting chronology.  If they are mated, then they must be at an earlier stage of the nesting cycle.

Some of this can be documented.  The phoebe nest hatched five nestlings about three weeks ago, and they are now working on their second brood of the season.  By the way, I covered this bird in a blog several weeks ago.  It turns out that this pair nested on a window ledge on the back of my house rather than on the light fixture on the front.  The chickadees that I described last week are now incubating eggs.  And I found a nest yesterday of an American robin in a Syringa shrub next to the house with four eggs.

But let’s review songbird nesting chronology a bit.  Males establish territories, sing, and, if fortunate, acquire a mate.  One or both adults build a nest, which is distinctive to that species (i.e., mud, moss, grass stems, twigs).  Bird nests are truly marvels of the animal world.  How birds actually build these structures amazes me constantly.  During this stage, they copulate, which is done by the male hovering in flight above the female; their cloacas touch, sperm is transferred, and voila. When the nest is complete, the female will start laying eggs.  Egg-laying occurs early in the morning, and the female lays only one egg per day.  Even the famous while leghorn chicken, which has been bred to do nothing but produce and lay eggs, can only lay one egg per day.

Clutch size varies from about 3-6 in temperate species, but the number is relatively fixed within a species.  One of the parents, usually the female, then begins incubating the clutch after the next-to-last, or penultimate, egg is laid.  Eggs do not begin developing until the heat from the female’s body is applied during incubation.  The last egg laid, which occurs one day after incubation starts, will hatch about 24 hours after the rest of the clutch; this “runt” of the litter is often the one not to survive because it is always one day smaller than its siblings.  Incubation takes about 10-14 days, depending on species, and then the real work begins.

One or both parents must then find food, and I mean a lot of food, to feed the hungry nestlings.  These morsels usually consist of insects or other invertebrates, which are high in protein.  Nestlings fledge from the nest after 10-12 days.  For large birds like hawks, incubation and the nestling period are about three times as long as for small songbirds.  If you have never found a nest of a small bird and followed it, you should do so.  The rate at which nestlings grow is truly astounding.  You can see the difference in size and feather development every 24 hours.  But here is a puzzle.  Those nestlings have to defecate several times per day, and yet you will see no feces in the nest.  Where is it?

Will you cause the adults to abandon the nest if you find it and check on it up close once or twice a day?  It depends.  If the adults are only at the nest-building stage, they may abandon that effort and relocate because they “think” a predator has found the nest.  Why continue if something is going to eat your eggs?  But once they have reached incubation stage, they will usually not abandon the nest.  Too much time and energy have now gone into that nest to just walk away.  So find an active nest, observe it until the babies fledge, and report to us here.  There are worse family activities in which you could be involved.

Once the young have fledged, many males will begin singing all over again in the hopes of attracting a new female who wants to nest.  And on it goes, throughout the ages—the stuff of which poems are made.