Tuesday, March 30, 2010

The phoebe and the porch light

 (An Eastern Phoebe with an insect.  Is it the same bird nesting on my porch light year after year?)

Each year in late March, Eastern Phoebes (Sayornis phoebe) return to my property from having spent the winter as far south as Mexico.  Today, they returned.  I can always tell, because the male sings incessantly when he returns, and his favorite song perch seems to be at the corner of our house next to our bedroom.  The singing starts just before it is light, so spring phoebes and DrTom are on the same schedule, fortunately.  I love early morning.

Bird migration has always fascinated me.  I have been more interested in why birds migrate, than in how they do it.  The answers to the how question are truly astounding, and there are many good summaries of this.  Much of the early pioneering work on this topic was done at Cornell University by Bill Keeton, who used homing pigeons as his model.  And the Germans Kramer, Sauer, and Wiltschko are important.  Depending on the species, they might use visual landmarks like rivers during the day, or they use the sun’s location, or they navigate at night by orienting to the stars, or they use the earth’s magnetic field.  Bobolinks (Dolichonyx oryzivorus), for example, contain small deposits of an iron compound called magnetite in their skulls.  This is presumably used to detect the weak forces of the earth’s magnetic field to help them migrate between North and South America.

Eric Bollinger and I published a number of papers in the 1980s on Bobolinks and the behavior known as breeding site fidelity, or breeding site faithfulness.  This is the tendency of individuals to return to the exact location where they bred the year before.  It turns out that this is a common phenomenon in migratory songbirds: adults often return to the exact location where they bred the year before, but their babies rarely return to the place where they were born.  In Bobolinks and most songbirds where this has been studied, adults tend to return to the site where they bred the year before if they were successful in producing babies at that location.  If the nestlings had been eaten by a snake or a skunk, for example, or the nest was destroyed by farming equipment, then those adults tend not to return to the same location the following year.  It appears there is a simple Darwinian algorithm operating in those pea-sized brains: if I was successful in producing offspring, return; if I was unsuccessful, do not return.

So, every year since 1980 we have had a pair of Eastern Phoebes near our home.  But the observation is more remarkable than that.  Phoebes originally nested on ledges beneath an overhang, probably rocky cliffs.  Houses, however, are a great substitute, because of the overhanging eaves and the existence of some kind of platform beneath that overhead protection—like a window ledge.  At our home, phoebes almost always use the light fixture next to the front door.  (They also use a window ledge on the back of the house.) This is convenient for me, because every morning during the breeding season, I step outside, reach my hand up and into the nest, count the number of eggs or nestlings by feel, and then resume drinking my Cafe Britt coffee (which, by the way, you can buy on this site).  Although I have never formally studied phoebes, this would make for pretty easy field work.  The bottom line is that nearly every year, the nest over our light fixture successfully fledges 4-5 young.

Now, I have never banded the phoebes at my house, and this is unfortunate.  I am missing a lot of the biological story, because I do not know if these are the same individuals that return to my property each year.  But for 28 years, phoebes have nested on this light fixture and yet these birds probably live only a few years—they can not be the same individuals during all of that time.  This means that new birds sometimes settle near my house, start looking for a suitable nest site, see the light fixture under that overhang, and a “CFL light bulb” goes off in their little head.  (Research has proven that light bulbs in bird heads are fluorescent and not incandescent).  Each succeeding generation of phoebes spots that nest location and simply can not resist it, in spite of the fact that every time we enter or leave the front door, the attending adult is flushed off the nest.

As you can see, my original interest in site fidelity has blended with a fascination for this incredible innate focus by the bird on a suitable resource, in this case a nest site.  I am sure that exactly the same consistency and skill go into locating and capturing food—phoebes mainly eat flying insects like moths.  Many thousands of years of natural selection have honed these abilities into a razor-sharp performance, which ensures their survival and successful reproduction.  For me, spring has not really started until I hear that simple, yet distinctive song of the phoebe.  My coffee is ready, so all I need now is this year’s nest.