Over The Horizon

Begging for Trouble

James Briskie

Anyone who has watched a bird feed its young will have been struck by the noisy spectacle as the nestlings stretch their necks, rise on back legs and call vigorously. Such displays can be energetically costly, but since vigorously begging young are fed preferentially, these costs may be compensated by the extra food a nestling obtains -- it pays to speak up and demand a bigger piece of the pie. However, recent work in the US suggests it pays not to speak up too much, as begging may inadvertently attract predators.

The world is a dangerous place for young songbirds: about half of American robin nests are lost to predators. In the hermit thrush, up to 90% of nests fall victim. The loss of nests to predators is often the biggest cause of nest failure. For a chick, begging too vigorously may court disaster. A chick thus faces a dilemma: it must beg to get fed but it risks attracting a predator in the process. Are there ways a nestling could beg but yet minimise the risk of attracting unwanted attention?

The physics of sound transmission suggests the answer is yes. Not all sounds travel equally well through the environment and so some types of calls are easier for predators to locate. Low pitch sounds travel further than high pitch sounds (as anyone with a neighbour that plays a bass guitar can attest). This is because high pitch sounds are more quickly absorbed. Loud sounds have more energy and travel further than quiet sounds. For species subject to a high risk of predation, selection should favour the evolution of begging calls that minimise their locatability. A nestling could do this simply by begging more quietly and at a higher pitch.

To test this, Tom Martin and Paul Martin of the University of Montana and James Briskie of Canterbury University recorded the begging calls of 24 species of birds nesting in central Arizona. They measured the amplitude (loudness) and frequency (pitch) of calls for each species and compared this to the rate of predation. The range of species used comprised almost the entire local community of birds.

The rate of predation varied from less than 5% to over 90%, and thus included birds which faced both a high risk and a low risk of attracting predators. If predation risk has influenced the evolution of begging behaviour, then you would expect to find differences in the loudness and pitch of calls between these species.

Results showed this to be the case. Loudness varied over five orders of magnitude, and pitch varied almost three-fold, but much of this variation was explained by the risk of predation. Those species subject to high predation had significantly quieter and higher-pitched calls than species less at risk. This pattern held even when differences in body size, brood size and phylogenetic relationships were controlled

Unlike in North America, birds in New Zealand evolved with few predators. The vulnerability of many native birds to introduced predators indicates just how poorly some species cope. Are the begging calls of our native birds loud and low pitched, as found in American birds with low rates of predation?

To answer this, the researchers have now extended their study to this country. Preliminary results suggest that some native birds do indeed beg loudly -- so much so that nests can be found simply by following the noise of the chicks.

Before the arrival of introduced predators, such extravagant displays were safe; today they may invite danger. Although selection should favour more subdued calls over time, it is doubtful whether this will happen fast enough to reduce the risk of attracting predators. Knowledge of which birds beg conspicuously and whether this increases their susceptibility to predation may help in managing some threatened species -- perhaps the provision of "sound-proof" nest boxes might help particularly vulnerable species.

James Briskie is in the Zoology Department at the University of Canterbury.