Eggs-periment tricks crafty crows from plundering endangered species

Pied crows have long outsmarted other birds by raiding their nests. But they quickly changed their ways when scientists taught them a lesson by sabotaging their food supply with booby-trapped eggs.

The trick, in an experiment on the west coast, was an attempt to end a habit that is endangering some coastal bird species. And it worked, suggesting a method of protecting vulnerable birds from raiding crows and reducing the use of poison which often harms other species.

Scientists created 20 artificial nests and half-buried them in sand at six estuarine sites outside Velddrif. They monitored the nests with hidden cameras. While the Kittlitz’s Plover is not endangered, other plover species are, making them particularly vulnerable to nest-raiding pied crows.

Using quail eggs that resemble those of the Kittlitz’s plover, they first evaluated how often pied crows visited the nests. Then they injected the eggs with an egg mixture containing a non-lethal “aversion agent” that causes symptoms including diarrhoea and vomiting.

In these two phases, half of the sites received “control” eggs filled with water.

38: The number of crow egg snatches recorded on camera during the study.

55.8%: The increase in the nest survival rate after treatment with ‘aversion’ eggs.

—  Eggs in numbers

Finally, all nests were recharged with normal eggs to see if the crows had learnt their lesson or would resume their raids. 

Results published in the Journal for Nature Conservation showed that nests at sites with aversion eggs fared markedly better once they were recharged with normal eggs — a 45.9% increase in egg survival compared with the first phase of the experiment.

By contrast, the artificial nests with water-filled eggs recorded an 86.7% decrease in the survival rate in the same period.

This suggests “conditioned food aversion”, which has been used in multiple studies with other animals, modified the behaviour of the intelligent pied crows.

Study co-author Robert Thomson, from the University of Cape Town’s Fitzpatrick Institute of African Ornithology, said pied crows are notorious nest raiders, with some coastal birds particularly at risk because they built exposed nests in accessible wetlands.

The threat to wetland birds had increased due to habitat encroachment by other species, resulting in population declines, he said. But the pied crow habitat had expanded in recent years, possibly driven by more successful adaptation to climate change and human presence relative to other species. 

“We knew that these crows were hitting a huge number of (plover) nests,” Thomson said. “They have a very high nest predation risk.” The plover nests were therefore a useful decoy to experiment with food aversion. 

Thomson said the treated eggs could be a good alternative to the lethal poisons used by some farmers to eradicate pesky pied crows. 

However, the study also highlighted potential harmful impacts of food aversion intervention, evidenced by the significant decrease in the survival rate of the control nests. Provisioning nests could inadvertently attract more crows and therefore increase predation rather than limit it, particularly if the aversion agent did not work. 

“There’s a lot of potential here, but there are little things that people have to be careful about. The design of the project needs to be sorted out properly before you can use it in a real management set-up,” he said.