Shy collared flycatchers are less likely to get caught in ecologists' traps. Credit: Juniors Bildarchiv / Alamy

Ecologists may be underestimating species population numbers because only bolder animals are caught in their traps.

At a field site near Budapest, László Garamszegi, now at the Doñana Biological Station in Seville, and his colleagues placed caged female collared flycatchers on unoccupied nest boxes that had a male bird in attendance. They targeted 41 males, 33 of which responded to their female. Male courtship activity was actively monitored. Subsequently, white pieces of paper were stuck to the nest boxes to introduce novel objects into the birds' environments. Some males were distracted by the novelty and reduced their courtship, whereas others continued courting their caged female undeterred1.

Garamszegi and his team went on to measure the same males for the degree of aggression they showed a caged rival male, and for the risks they took in the presence of a human observer, considered to be the equivalent of a potential predator. In both instances, males whose courtship had not been affected by the white paper also demonstrated more intense fighting behaviour against rivals and took higher risks in the presence of approaching humans.

If this is widespread amongst many species and the different traps used by ecologists it could indicate that past measurements of wildlife abundance are flawed. László Garamszegi , Doñana Biological Station

The most startling aspect of Garamszegi's work was the ultimate discovery that the more courageous and more aggressive collared flycatchers were also twice as likely as shy members to get caught in traps that the researchers set after carrying out the behavioural tests.

"If this is widespread amongst many species and the different traps used by ecologists it could indicate that past measurements of wildlife abundance are flawed," says Garamszegi.

Blessed are the meek

Birds are not alone in exhibiting bold and shy characteristics. Fish have personalities too, says Peter Biro at the Institute for Water and Environmental Resource Management in Sydney, Australia. In a study published last year2, Biro and his colleague John Post at the University of Calgary in Canada demonstrated that bolder trout, which tend to be unafraid of new objects in their environment, are also the most likely to be caught.

The bolder fish were also faster growing than average, suggesting that the trout-fishing industry could be unwittingly removing bolder, faster-growing individuals from fish populations.

There is nothing more important to marine fishery biologists than estimating the abundance of a species of concern. William Bayliff , Inter-American Tropical Tuna Commission

"This idea [of personality creating a skew] was brought up years ago, but forgotten or ignored, until a few studies accumulated to suggest it might be widespread," says Biro. "[This is] a problem for all studies that sample animals, violating the basic statistical tenet 'thou shalt collect random samples'."

After reading Garamszegi's research, William Bayliff at the Inter-American Tropical Tuna Commission in La Jolla, California, was particularly struck by the consequences that this could have for ocean ecosystems.

"There is nothing more important to marine fishery biologists than estimating the abundance of a species of concern, and if non-random sampling of animals is occurring because only individuals with certain personalities are being caught, then that needs to be widely communicated," Bayliff says.

At sea, researchers work out 'catch per unit of effort' data to determine how threatened a species is — that is, they measure how much time needs to be spent fishing for a specific species before a member of that species is caught.

Catch per unit of effort data are widely used, and are one of the factors behind the belief that many tuna populations are in grave danger, says Bayliff. However, if tuna have personalities, as trout and flycatchers do, the species might not be in as much trouble as is thought.

However, even if fish populations are being underestimated because of personality skew, this does not necessarily mean that conservationists have less to worry about. "If greater harvest of bold fish leaves us with meek fish, this is bad because they tend to grow and reproduce more slowly, creating an unproductive population," says Biro.