Deceptive smells

It’s not only humans who can be fooled with misinformation to change their choices. Animals can be tricked too, it seems – and when used with good intentions, fake cues might help conservation initiatives without resorting to lethal culling. 

By spraying pheromones at nesting sites in New Zealand weeks before native birds arrived there, scientists report in the journal Science Advances that they could discourage introduced mammals from preying on the chicks.

“A focus on the drivers and motivations of problem species will enable wildlife managers to manipulate them with counter tactics of deception and misinformation that are widespread in humans and primates and that currently challenge human society,” they write.

This works by making the deceptive cues worthwhile, they note. “Tactical misinformation, or ‘fake news’, can succeed if it diverts the selective attention of decision makers by changing the perceived value of information.” 

Mammalian carnivores use their noses to hunt for food. In a reversal of the classic Pavlov’s dog experiment that paired a bell to dinner, the predators lost interest in the bird smells after discovering they didn’t lead to the vulnerable prey.

“The expectation was that predators would sniff out that bird odour without getting a reward,” explains lead author Grant Norbury from Manaaki Whenua – Landcare Research. “So when the birds arrive for breeding, the predators wouldn’t pursue them.” 

The research was inspired by a pilot study run by co-authors Catherine Price and Peter Banks, from the University of Sydney, that successfully used fake odours in artificial nests to deter black rats (Rattus rattus) from preying on bird eggs.

Norbury and colleagues teamed up with them to test the method in a real-life scenario at a landscape scale in New Zealand, where concerted efforts – including culling of invasive predators – are being used to save the islands’ unique bird populations.

They chose four 1000-hectare sites on braided river ecosystems in Canterbury where three shorebird species – the double-banded plover (Charadrius bicinctus), wrybill (Anarhynchus frontalis) and South Island pied oystercatcher (Haematopus finschi) – nest on the ground and are under major threat from predators. 

The deceptive odour was made from a paste derived from chickens (Gallus gallus domesticus), quail (Coturnix japonica) and kelp gull (Larus dominicanus), mixed with petroleum jelly to prolong the scent. These birds were easiest to access and luckily the predators seemed to generalise across the different species’ smells. 

The team repeatedly applied the paste to two of the four sites for five weeks before the shorebirds arrived, and for eight weeks after a breeding season. They monitored the response of invasive ferrets (Mustela putorius furo), cats (Felis catis) and European hedgehogs (Erinaceus europaeus occidentalis) using camera traps.

They swapped the treatment and control sites for the following breeding season to strengthen the findings.

As predicted, the predators – especially cats and ferrets – were initially attracted to the odours but became less interested after 12 to 18 days. By the time nesting started, their interest had dropped to between 5% and 9% of initial levels.

At treated sites, chick production increased 1.7-fold over 25 to 35 days, doubling or tripling the chance of successful hatching. With modelling, the team found this could produce a 127% increase in bird populations over 25 years, presenting a viable, cost-effective and humane option that is comparable with other measures.

“[I]t wasn’t until we saw the work in Australia that we thought we could use this to help species that are affected by predation,” says Norbury. “Because in this case we’re not removing a single predator, we’re just fooling with their minds, trying to outsmart them – and it seems like we have.

“However, it only lasts for a short period – about a month. It’s not a technique that’s going to work in the vast backcountry of New Zealand; it’ll only work in small, specialised areas. But it’ll be useful during the start of breeding seasons, for short periods of time, and also perhaps during translocations.”