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For freeloader birds, careful counting is key

Aug. 24, 2009
Special to World Science  

A spe­cies of birds that free­load on oth­er birds by dump­ing their off­spring on them, em­ploys soph­is­t­icated count­ing skills to car­ry out the ru­se, a study sug­gests.

Da­vid J. White of the Un­ivers­ity of Penn­syl­va­nia and col­leagues in­ves­t­i­gated the habits of brown-headed cow­birds, who put their eggs in oth­er birds’ nests so that some­one else can do the job of rear­ing their big, de­mand­ing chicks.

A female brown-headed cow­bird. (Im­age cour­tesy Lee Kar­ney, USFWS)

Al­though sev­er­al an­i­mals have been found to have sim­ple count­ing abil­i­ties, in­clud­ing some pri­ma­tes, birds and dogs, there has been lit­tle re­search on just how an­i­mals use such abil­i­ties in the wild.

The cow­birds, com­mon through­out much of North Amer­i­ca, seem to em­ploy some fairly ex­act­ing arith­me­tic skills when de­cid­ing which host nests to tar­get and when, ac­cord­ing to White and col­leagues.

Most of the host birds par­a­si­tized by cow­birds lay eggs eve­ry day for a few days. They stop when they have about three to six eggs, af­ter which they start in­cu­bat­ing the eggs by sit­ting on them. How­ev­er, in­cuba­t­ion may be in­ter­rupt­ed from time to time with­out harm to the off­spring. So one can’t tell, just from see­ing a nest with a few eggs, wheth­er in­cuba­t­ion has be­gun or not.

In or­der to prop­erly syn­chro­nize egg-lay­ing with their hosts, cow­birds need to tar­get nests where in­cuba­t­ion has­n’t yet be­gun, White said; the im­pos­tor egg needs to be in­cubated along with the oth­ers. 

A cow­bird egg in a chip­ping spar­row nest. Cow­birds some­times dis­card host eggs when they add in their own. The cow­bird chick of­ten hatches be­fore the oth­ers and out­com­petes them in de­mand­ing food from the par­ents. (Im­age cour­te­sy U.S. Nat'l Park Svc.)

Al­so, White said, cow­birds seem to pre­fer nests that al­ready have at least three eggs.

But one can­not tell wheth­er all these con­di­tions have been sat­is­fied just look­ing at a nest once. The cow­bird in­stead has to fly by the nest a few times and check out the situa­t­ion. If a new egg is be­ing added eve­ry day, in­cuba­t­ion has probably not yet be­gun, so it’s safe to put in the egg. 

If a day or more has gone by with no new egg added, in­cuba­t­ion has prob­ab­ly be­gun. Then it’s too late for the free­load­ing par­ent to do its trick, which may al­so in­volve re­mov­ing one of the orig­i­nal nest eggs.

White and col­leagues con­ducted a se­ries of tests with ar­ti­fi­cial nests, in which they added fake “eg­gs” at dif­fer­ent rates in­to ar­ti­fi­cial nests. Cow­birds were quite will­ing to lay eggs in these ar­ti­fi­cial nests, so it was easy to use these to study their lay­ing habits, the group re­ported.

“Cow­birds avoided a nest if the num­ber of eggs that had been added was less than the num­ber of days that had elapsed,” the re­search­ers wrote, re­port­ing their find­ings in the July 29 on­line is­sue of the re­search jour­nal Psy­cho­log­i­cal Sci­ence.

“The abil­ity of fe­males to re­mem­ber egg num­ber and com­pare changes in egg num­ber across days al­lows them to se­lect nests most suit­a­ble for par­a­sitism,” they added.

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A species of birds that freeload on other birds by dumping their offspring on them, employs sophisticated counting skills to carry out the ruse, a study suggests. David J. White of the University of Pennsylvania and colleagues investigated the habits of cowbirds, who put their eggs in other birds’ nests so that someone else can do the job of rearing their big, demanding chicks. Although several animals have been found to have simple counting abilities, including some primates, birds and dogs, there has been little research on just how animals use such abilities in the wild. Brown-headed cowbirds, common throughout much of North America, seem to employ some fairly exacting arithmetic skills when deciding which host nests to target and when, according to White and colleagues. Most of the host birds parasitized by cowbirds lay eggs every day for a few days. They stop when they have about three to six eggs, after which they start incubating the eggs by sitting on them. However, incubation may be interrupted from time to time without harm to the offspring. So one can’t tell, just seeing a nest with a few eggs, whether incubation has begun or not. In order to properly synchronize egg-laying with their hosts, cowbirds need to target nests where incubation hasn’t yet begun, White said; the impostor egg needs to be incubated along with the others. Also, White said, cowbirds seem to prefer nests that already have at least three eggs. But one cannot tell whether both these conditions have been satisfied just looking at a nest once. The cowbird instead has to fly by the nest a few times and check out the situation. If a new egg is being added every day, incubation has probably not yet begun, so it’s safe to put in the egg. If a day or more has gone by with no new egg added, incubation has probably already begun. Then it’s too late for the cowbird to do its trick, which may also involve removing one of the original nest eggs. White and colleagues conducted a series of tests with artificial nests, in which they added fake “eggs” at different rates into artificial nests. Cowbirds were quite willing to lay eggs in these artificial nests, so it was easy to use these to study their laying habits, the group reported. “Cowbirds avoided a nest if the number of eggs that had been added was less than the number of days that had elapsed,” the researchers wrote, reporting their findings in the July 29 online issue of the research journal Psychological Science. “The ability of females to remember egg number and compare changes in egg number across days allows them to select nests most suitable for parasitism,” they added.