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.

Does touching a bird's nest cause the female to abandon it?

(Male Bobolink.  The male usually helps at the nest, but the female does most of the work.)

Did you grow up with your mother telling you not to touch that robin's nest because the mother would not come back and the babies would die?  Most of us did.  This has to be one of the most frequently uttered adages in all of nature lore.  The fact is, this is mostly myth.

During the 1980s, Eric Bollinger and I studied Bobolinks in upstate New York.  Bobolinks are a polygynous (i.e., males commonly have more than one female mate) species in the blackbird family.  The males have an incredibly long, bubbly song and their appearance is described as having  a tuxedo on backwards.  They are about the size of a sparrow.  Bobolinks build their grassy nest on the ground.  The female lays one egg per day until she has completed a clutch of five, begins incubating with the laying of the penultimate egg (next to last), incubates the eggs for 10-12 days, and then feeds her nestlings for another 10-12 days until they fledge.  Males usually help feed nestlings, but they are not as attentive as females.

Eric and I and our technicians located hundreds of Bobolink nests in those years, which are built on the ground in hayfields and meadows.  Once we located a nest, we placed a bamboo stick in the ground about a meter away from the nest with some colored plastic flagging on the top of the stick so we could relocate the nest at will.  Once found, we checked the contents of the nest every day to determine its progress and success.  When the nestlings were about 7 days old, we removed each one from the nest, collected a blood sample, measured it, placed an aluminum band on one leg, and returned it to the nest.  In some years, we removed the eggs and measured them before returning them to the nest.  In other words, we disturbed the nests a great deal during their three-week life, although we were careful not to trample the concealing vegetation around the nest any more than absolutely necessary.

Nearly 1,000 nests endured this harassment, and Eric and I learned a great deal about the behavior of  females because of it.  If we found a nest while the female was constructing it, she usually abandoned the nest.  If we found the nest when she had laid only 1-2 eggs, she often abandoned the nest.  Once the female had laid her full clutch of eggs and began incubating, she almost never abandoned, and if the nest contained nestlings, she would absolutely never abandon her brood.  The same seems to be true of most other birds as well.

Think of it this way: the more the female had invested in time and energy in the whole operation, the less likely she was to give it up.  Remember also, most birds have only a limited seasonal window during which they can successfully complete the nesting cycle.  In the case of Bobolinks, it takes a total of about 30 days from initiation of nest-building to fledging of their young.  In addition, they continue to feed their fledglings after they leave the nest for some period of time.  Bobolinks do not return from South America until early May and they start moving south again in August.  If they had to start over with the nesting cycle part-way through, they would barely have enough time to get those babies to a size and age where they could endure a long migration at the end of the summer.

Realize that the patterns I have described above probably apply to most songbirds in North America.  They may not apply equally well to tropical birds, which live in an area with many predators, and which always seemed to me to be extremely wary of predation threats.  Those species might abandon their nests more readily than temperate species.

So when your mother or grandmother tells you not to touch that nest because the female will not come back to it, you can say: "Well Mom, it goes like this".  There is a danger of attracting predators to a nest that you have disturbed, and where you have presumably left your scent.  Raccoons are very good at following these clues.  But as far as the female of the nest is concerned, she has invested too much for too long to walk (well, or fly) away easily.