Early arrival dangerous for purple martins

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This article was published 17/12/2020 (1228 days ago), so information in it may no longer be current.

A bird familiar to Manitobans is being studied by the University of Manitoba with regards to how light pollution can kick off a too-early migration northward in the spring.

The purple martin, a songbird well known for nesting communally in martin houses, is the subject of a study to be published by the Avian Behaviour Consultation lab in the department of biological sciences at the U of M that shows artificial light at night has an impact on the timing of spring migration, as migratory birds use light cues to initiate their journey.

Former biological sciences honours student Reyd Smith led the project with assistance from honours student Maryse Gagné, and advised by associate professor Kevin Fraser, who runs the ABC lab.

They attached light-logging geolocators to the purple martins while the birds were nesting here in Manitoba, and then tracked the amount of artificial light – ALAN – the birds were exposed to at night while overwintering in South America.

“When I arrived here at U of M, I made purple martins one of the biggest part of my research lab, because there are lots of them here, and they can tell us about birds that we can’t work with so easily,” Fraser said.

“What makes them easy to work with is that they breed in those houses which can be cranked down the pole using a wire. That means you can get inside and monitor the nests and catch the adults with little trap doors you put on the entrances. You can catch large numbers of birds. You can measure them and put on the tracking tags,” he said, adding that’s something you can’t do with other songbirds that have nests spread out throughout a forest.

As they report in their paper, Pre-migration artificial light at night advances the spring migration timing of a trans-hemispheric migratory songbird, the purple martins that experienced the highest levels of ALAN left on average eight days earlier and arrived at their northern destinations eight days earlier.

“Migration is risky enough when you consider the amount of human-based hazards birds are now facing along the way,” Reyd, who is now a master’s student at the University of Windsor, said.

Eight days is a significant amount of time. These birds have timed their migration to coincide with the emergence of insects in spring so they can restore energy reserves after completing their migration, develop eggs, and feed their young, Fraser said.

“When the starting gun fires, that’s it. They’re off and they arrive earlier,” Fraser said, adding that for birds migrating from Brazil to Manitoba means a 9,000 km-long flight.

Such disruption — advancing the whole breeding cycle —  may lead to a mismatch with the availability of food and good weather, which would be particularly detrimental to species already exhibiting steep population declines, such as purple martins and other migratory aerial insectivores, Fraser said.

“We all know the early bird gets the worm. But in this case, it might not be. A week can make a big difference. At our latitude they often arrive in the last week of April. You can picture being a week early then, and the weather they could face. There’s evidence of mass mortality in other species of swallows, and early cold snaps that wiped them out. Being a small songbird needing aerial insects can be devastating for the purple martins,” he said.

The birds overwinter in South America, mostly in Brazil, where there can be upwards of 300,000 individuals roosting at night. The purple martins’ breeding area extends from Florida northward to central Alberta. They always return to the martin house they nested in the previous year.

“They would have originally been nesting in tree cavities in swamp land, but flooded forests were drained, and habitat was lost, while at the same time, people were building martin houses,” Fraser said.

In Manitoba, they prefer open areas over marshland and fields. “The colonies we work at are on campus, one at Oak Hammock Marsh, one at FortWhyte Alive, some at private residences,” Fraser said. “There will be a couple of new houses put up at Assiniboine Park, in a public area, possibly near the Duck Pond.”

In a separate project, Fraser has begun looking into if, and how, a field experiment that exposes chicks to alternative light cues — much like an artificial sun — can help purple martins adjust faster than evolution would allow.

For more, see news.umanitoba.ca/light-pollution-altering-avian-migration-um-study-finds/