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In the dappled light beneath the forest canopy, ornithologist pauses for a moment to listen. It is just after 6am on a humid July morning, and through the chorus of birdsong, she has detected the distinct chatter of an eastern towhee 鈥� the species we鈥檙e looking for.

As we move through the understory at the outside of Philadelphia, Pennsylvania, I dodge a set of mist nets that Eshleman and her team set out that morning: 12-metre-long nets made out of super-fine thread designed to safely entangle birds. She is hoping to lure one of the black-and-orange birds into the netting so that she can attach a jelly-bean-sized device to its back.

That little bean is an ultra-lightweight radio 鈥渘anotag鈥�, and Eshleman is one of a handful of scientists pioneering their use to map birds鈥� locations. The towhees (Pipilo erythrophthalmus) she has started tagging this summer are some of the first of their species to ever be tracked this way. Unlike traditional radio trackers, which are too heavy for starling-sized birds like towhees, the tags used in what鈥檚 known as the can weigh as little as a few raindrops.

鈥淲here Motus excels is being able to tag and track the smallest animals over the largest distances,鈥� says Stu Mackenzie at Birds Canada in Ontario, which pioneered the use of the technology.

Motus tags use radio waves to track thousands of animals, pinging receiver towers as far as 15 kilometres away. The technology was first introduced in 2012 by Birds Canada, but it is only with the recent expansion of the tower network that it is starting to revolutionise how we track migratory species 鈥� and the level of detail we can gather about them. Today, there are more than .

We check the nets every 40 minutes for the next 6 hours but find no towhees this morning. So Eshleman demonstrates the process of attaching a Motus tag to a similarly sized species called a Grey catbird (Dumetella carolinensis). She carefully removes the dark grey bird from the fine netting and slides a loop of clear stretch cord around each of the bird鈥檚 thighs, as if it were stepping into a tiny pair of pants. Eshleman positions the attached tag between the bird鈥檚 wings in the centre of its back. The entire process takes around 2 minutes.

鈥淚t鈥檚 like a fanny pack meets a backpack,鈥� says Eshleman.

Before the advent of these new devices, researchers had to rely on coded aluminium leg bands to track small birds, which required recapturing the birds every time they wanted to log their band code and current location. With a Motus tag, the bird needs to be captured just once.

The lifespan of each device depends on how frequently researchers program it to ping towers on the ground. The more detail scientists want, the more frequent the pings and the faster the tag鈥檚 battery drains. Hardier species like towhees can be outfitted with larger Motus tags with solar panels that recharge the device endlessly, while the smaller version of the tags used on hummingbirds and insects lasts just weeks.

When the , which runs the Rushton Woods Preserve and others, first began working with Motus in 2017, there were only a handful of receiver towers in the north-eastern US. Since then, the trust and other groups that together form the have installed more than 115 receiver stations from Maryland to Maine. So far, those receivers have picked up more than 25 million data points contributing to Motus鈥� more than 1 billion total observations of .

鈥淭he number of stations is increasing almost weekly,鈥� says , who has been using the tags to study Northern Saw-whet owls (Aegolius acadicus) at the Rushton preserve. 鈥淲e鈥檙e finally at the point now where we can start doing the kind of research that we were hoping to do seven or eight years ago, but the infrastructure was not there.鈥�

The nanotags are already turning up surprises. A federally protected shorebird species called red knots (Calidris canutus) is known to relish coastal areas, but their path between resting places was less clear. Motus nanotags showed that many of on their journey from the coasts of New Jersey and Delaware each spring, a discovery that could help guide the placement of wind farms in the area as birds can collide with the structures and die.

These kinds of detailed insights come at a crucial time, given the significant threats to wildlife posed by climate change and habitat loss. The US and Canada have lost more than one-quarter of their birds 鈥� an estimated 3 billion animals 鈥� due to human activity in the last 50 years. According to the latest report, around 48 per cent of birds worldwide are known or suspected to be undergoing population declines.

鈥淚t鈥檚 about knowing where to focus our resources to try to reverse these declines that are occurring,鈥� says Lisa Kiziuk, director of bird conservation at the Willistown Conservation Trust. 鈥淪o many things are being learned that we never even thought to ask before.鈥�

Researchers have also used the tags to track several species of ground-loving birds called thrushes from their wintering location in South America to far-north regions of Canada. One Swainson鈥檚 thrush (Catharus ustulatus) tagged in Colombia was detected in northern Saskatchewan, Canada, a month later. It had journeyed far faster and further than researchers would have predicted based on existing knowledge of the species: in just 34 days, the bird flew an impressive 6000 kilometres, averaging more than 175 kilometres per day.

The nanotags not only make it easier than ever to track animals on the move, but the information they collect is also publicly . Kiziuk says that is one of her favourite things about Motus. When people can learn about the movement of birds in their area, they are in the species in their backyard, which in turn makes them more likely to contribute to ever more crucial conservation efforts, she says.

鈥淵ou could work your whole life telling people what you鈥檙e seeing in the field and showing them data that you collect鈥� and it means nothing to them,鈥� says Kiziuk. 鈥淏ut you show them a map with a track of one bird and they鈥檙e like 鈥極h!鈥� It鈥檚 like a light bulb. That鈥檚 what it takes.鈥�