Ana Morales walked through the undergrowth, her radio receiver scanning the airways. The device had detected a signal from a transmitter previously attached to a Swainson's thrush, a little brown and white speckled bird native to the Americas. The same signal had shown on Morales' handheld device a few days before, coming from the same shrub in a park on the outskirts of Montreal, Canada.
This was a cause for concern. The transmitter remained linked to a live bird, which seemed unlikely - it had most likely fallen off and was hanging on a branch. Morales, a PhD student at McGill, gave the berry-laden shrub a gentle shake just to be sure – and then she observed a flutter of feathers among the branches and shadows. The thrush, to her astonishment, was still alive and well, hopping about the bush in anger of being disturbed. This small bird had lingered for far longer than she had intended.
Every autumn, Swainson's thrushes migrate from northern latitudes to Central and northern South America. Some, though, make ‘pit stops’ in and around towns like Montreal. According to a recent study published last month, Morales and her colleagues had been researching how Swainson's thrushes balance the need to migrate quickly – so as to maximise its benefits – with the need to refuel, such as by stopping over in places like Montreal. They caught and radio tagged a total of nearly 80 of the birds.
Countless migrating birds stop in cities all across the world on their incredible travels, which can cover thousands of kilometers. It is not always clear why people travel to cities. Some appear to be drawn to light. Others, like the Swainson's thrush in its berry bush, appear to enjoy the food on offer. However, cities are not always welcoming to newcomers.
Regrettably, the death toll is appalling. Domestic cats, for example, kill some migrating birds, while others collide with structures. Every year, hundreds of birds die in New York alone when they collide with the brightly lighted windows of skyscrapers, a well-known problem in megacities. A flock of blackbirds was recently spotted plummeting from the sky.
And yet, scientists are discovering that towns, while dangerous, can sometimes help to support migrating species. So how do we ensure that cities act more as travel lodges – and not death traps – for these species?
During their study, Morales and her colleagues found that Swainson's thrushes make surprisingly long stopovers in Montreal, where many of the birds moult – a process through which the birds shed and regrow some of their feathers. That helps to prepare them for the long migration. It's like putting a new set of tyres on your car.
