Planet Earth Online

Scientists shed light on mob rules, in reed warblers at least

2 February 2009

Cuckoos are well-known brood parasites, using other birds' nests to lay their eggs. The hatched chick then tricks its host parents into feeding it until it leaves the nest.

Reed warblers evolved the ability to recognise and get rid of eggs different from their own. Cuckoos responded by laying eggs that look similar to the hosts' eggs. 'This is evidence that reed warblers and cuckoos have been locked in a co-evolutionary arms race,' says Dr Justin Welbergen from the University of Cambridge and one of the authors of the study.

Since it's in a reed warbler's interest to bring up its own chicks, not those of a sly cuckoo, a host bird guards its nest from cuckoos at the outset.

Mobbing tactics

Dr Welbergen and Professor Nicholas Davies, also from the University of Cambridge, decided to test whether or not mobbing tactics used by reed warblers are an effective defence against cuckoos. Reed warblers use threatening postures, swoops and direct attacks to defend their nests. While it's beneficial to have another bird invest in your offspring, parasitism is risky for cuckoos - they can lose feathers or get injured by the mobbing hosts.

Welbergen and Davies looked at what made some nests more likely to be parasitized than others by analysing 697 reed warbler nests in a site around the Cambridge Fens. Cuckoos figure out which nests to use by watching reed warblers from concealed perches in trees. If a perch was close to a nest, the researchers labelled the nest as more risky than a nest further away from a cuckoo perch. If there were lots of both parasites and hosts close to the nest and reed warblers were laying eggs, the nest site was considered risky.

To see whether or not birds fine-tuned their mobbing behaviour according to the risk of parasitism, the pair put a model of a cuckoo at 191 reed warbler nests with complete clutches of eggs and recorded mobbing behaviour for five minutes.

The researchers found that at 52 per cent of the nests, reed warblers didn't mob at all. But at 31 per cent of the nests, the birds mobbed the cuckoo model aggressively. 'Interestingly, birds with riskier nests were more likely to defend themselves by mobbing. In addition, birds that nested in riskier locations were less likely to be parasitized if they mobbed. If you have a risky nest, it pays to be aggressive to cuckoos, because it reduces the probability that you'll be parasitized,' says Welbergen.

But they found reed warblers will only mob if they have to, because mobbing is risky. Mobbing can attract predators or other cuckoos. Not only this, but cuckoos are similar in appearance to sparrow-hawks, which kill reed warblers, so they have to be sure they're mobbing a cuckoo and not a sparrow-hawk. Mobbing from a less risky nest site also led to a slight increase in parasitism. 'Staying quiet and remaining concealed is a good idea if you don't want to draw attention to yourself and you're not even in a site cuckoos frequently use,' adds Welbergen.

'Mobbing is the front line of defence against cuckoos. But this behaviour is part of an entire defence portfolio that reed warblers use to guard against parasitism,' adds Welbergen. But as you'd expect in a co-evolutionary arms race, the cuckoo has evolved the ability to lay its eggs in a matter of seconds - which is often fast enough to avoid detection.

The mobbing behaviour described by Welbergen and Davies is perfect example of so-called phenotypic plasticity - the reed warbler adjusts its behaviour according to changes in its environment.

Keywords: Biodiversity, Europe,

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