In a Bird’s Nest, an Animal Behavior Puzzle

From The New York Times

Sometimes the scientists who study animal behavior solve puzzles and other times they uncover new ones. The war between mockingbirds and cowbirds is a case in point.

Cowbirds are brood parasites, meaning they lay their eggs in the nests of other bird species, thus unloading the messy and demanding business of chick-rearing. They also peck holes in the eggs of the host birds, destroying as many as they can.

Mockingbirds are a favorite target of this plan, and it seems to make perfect sense for them to viciously attack cowbirds when they catch them in the nest.

But when Ros Gloag, then a doctoral student at Oxford, and her colleagues in Argentina looked closely at the war between chalk-browed mockingbirds and shiny cowbirds, they found something unexpected, as they reported in the November issue of Animal Behaviour.

They stationed small video cameras near the nests of 40 pairs of chalk-browed mockingbirds. Over two breeding seasons they recorded more than 200 attacks on intruding cowbirds.

They were surprised to find that these attacks, which their videos show to be quite vicious, did not stop the cowbirds from laying eggs. The cowbirds would hunker down and let the much large mockingbirds deliver hammer blows to the head, but in matter of seconds they would lay an egg and flee.

How could such a failed strategy persist in evolution?

The answer, according to Dr. Gloag, now a postdoctoral researcher at Australian National University, was that the attacks did prevent the cowbirds from carrying out the second prong of their plan, destroying the mockingbird eggs already in the nest.

So the mockingbirds managed to save some of their offspring. They still end up raising unwanted baby cowbird guests. But, said Dr. Gloag, once the eggs hatch, the larger mockingbird chicks compete very well with the smaller cowbirds.

PERMALINK + VIDEO: The New York Times

When Spring Trips North Again This Year

Lago Desierto, Patagonia

In the far reaches of the southern hemisphere the days are perceptibly shorter now.  The oceans and lakes surrender some of the heat they’ve stored over the summer and the daily temperatures begin to drop.  Conversely, in the southern reaches of the Northern Hemisphere, the first signs of spring are upon us.  Daylight hours are sensibly longer, temperatures are beginning their annual climb and the earliest and hardiest of plants have sent out tentative green.

In billions of birds, these signs stir primeval urges.

For instance, today, just to the west and high above us, Sandhill Cranes, the trumpets in the orchestra of evolution Aldo Leopold called them, are rising on thermals, circling and calling, then stretching into their V formation and soaring north.

It seems too early, but January and February here have been among the warmest on record so the birds may know something we don’t.  Or maybe it has little or nothing to do with the air temperature; perhaps this instinct to move is triggered solely by the tilting of the earth’s axis toward the sun. Possibly it is some of both; ancient pathways laid down in their genetic code coupled with the short term knowledge that the earth is warming and it is therefore safe to leave earlier.

Sandhill Crane

There are other signs that winter is losing its grip.  The male House Finches that spent the winter with us are growing their bright red “attract a female” plumage, our feeders are being drained of seed more rapidly, at our store hummingbird feeders are selling even though it will be more than a month until the hummingbirds begin to arrive, and we are writing about the spring migration and looking for the hay fever medications.

Billions of birds migrate twice a year, some from the far tips of the southern hemisphere to the far northern tips of the northern hemisphere.  Passerines fly in great flocks to the boreal forest of Canada to breed.  Hummingbirds by the millions make the hazardous journey from Central America to North America.  Raptors of all types climb into the skies and head north. They’ve been doing it since long before there were humans to observe it, even long before the continents assumed their current shapes and sizes.  (For perspective, the first true birds appeared about 145 million years ago.  Modern birds had evolved and most survived the great K-T extinction at the end of the Cretaceous which wiped out the dinosaurs.  That was about 65 million years ago.  Homo Sapiens have been here for only about 250,000 thousand years.)

The earth 50 million years ago

Science is not certain exactly how they do it.  Some elegant experiments have tried.  Perhaps it has something to do with the position of the sun in the sky?  No.  Make the birds fly only at night and they unerringly find their way home.  Maybe they navigate by stars?  No.  Make them fly only during the day and home they go.  Subtle fluctuations in the earth’s magnetic field?  Possibly.  But put them in a screwed up magnetic field and they orient in the right direction to go where they are supposed to go.  Tiny maps in an onboard computer?  Could be. but scientists have played games with bird brains and still they go home.

Most likely it is a combination of all of the above.  They fly thousands of miles and return to the same square foot of earth they left months earlier.  Some can even do it even though they’ve never been there before.  Many bird species migrate without parents teaching their offspring where to go, yet the youngsters get there all the same.  They fly in such numbers that they are visible as huge clouds on radar screens.

Weather Radar Image of Migrating Birds

Of course, birds are stellar navigators.  Raise homing pigeons in a mobile loft and send them off to forage one morning and then, while they are gone, move their home to the next county and they will show up at the relocated loft in time for dinner.  Nobody knows for sure how they do that either.  Not all of life’s questions have been answered.

Wonder is an underrated emotion and, as we’ve often said before, calling someone a “bird brain” is not an insult.