University of Havana professor Daniela Ventura has intercepted a migrant.

Her net, stretched taut on a forest trail in Havana's Botanical Garden, trembles. A catbird – a discretely clad gray bird with a black cap – thrashes then succumbs as she plucks it from the net's near invisible webbing.

Ventura calls out observations to her students who take notes nearby – species, weight, an estimate of the bird's body fat. They are fresh data points in a project that aims to unravel mysteries into how and where migratory birds from the United States and Canada spend their winters in Cuba.

"We know a lot about their ecology in the breeding zone (in North America) but very little about what happens in their wintering zone," Ventura said in an interview.

Part of the problem is politics, said Lourdes Mugica, an ornithologist who helped to organize the research.

A U.S. Cold War-era embargo, which restricts trade and financial transactions on the Caribbean island, has long complicated cooperation – even in science – between the U.S. and Cuba. The birds, though, are indifferent, Mugica said.

"Birds don't understand embargoes or geographic borders, they don't need a visa to enter our country," she said. "I hope there comes a time when relations are normal and that we can have joint projects between (the U.S. and Cuba)."

Mugica and Martin Acosta, pioneering ornithologists in Cuba, say this project – with partnership from Environment and Climate Change Canada – hints at what is possible.

Together, the Canadian and Cuban partners have installed a radio telemetry antenna – Cuba's first under an international tracking program called MOTUS – which follows birds radio-tagged in other parts of North America.

The antenna recently detected in Cuba a tiny Swainson's thrush, first tagged in British Columbia, Canada, a voyage of 5,000 km.

"We never thought we'd reach the level of sophistication we now enjoy," said Acosta.

Mugica and Acosta recall hard days during their careers - Mugica said she lost 32 kg doing research decades ago, at a time when food in Cuba was scarce.

Logistics remain an issue even today. The scientists' 20-year-old pick-up truck needed a push start on a recent morning.

The catbird ensnared in the mist net had been captured and affixed with an identifying leg band back in November at the same site, Ventura's data showed. It had gained body fat in the three months since, she said, ahead of its coming migration north across the Gulf of Mexico.

"To think these little birds, which weigh less than 10 grams, cross the sea and come back and survive is spectacular," she said. "It's humbling ...to think that other living beings can perform these feats."