Sunday, May 9, 2010

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.

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