Cholao, the amazing Lesser Yellowlegs that gives wings to GPS monitoring across the Americas

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Cholao, a small Lesser Yellowlegs (Tringa flavipes) is becoming a well-known character among Audubon’s social media followers. Why? Because of its migratory journey, which started in the Cauca Valley, in southwestern Colombia,  and recently reached the boreal forests in Canada, was monitored with a GPS that tracks its whereabouts, its movement, its epic crossing over the Gulf of Mexico until touching down in Louisiana, and from there  to  Saskatchewan, Canada, where it is currently located and will surely remain.

Jorge Velásquez, Audubon’s Science Director for Latin America and the Caribbean, is perhaps the one who knows the most about the life and travels of Cholao —who is not just one, but several individuals— as he has spent some years tracking not only this specific bird, but the flights of 34 other “tringas”, as they are informally called. We asked him some questions received from our social media followers, and his answers are not only fascinating, but give an amazing picture of the epic travels migratory birds do.

How did the tracking of these migratory birds get started, and why focus on the Lesser Yellowlegs rather than another species?

We began tracking the movements of Tringas flavipes in the Cauca Valley, Colombia, in April 2022. The interest in this species is because it is one of the shorebirds with the greatest decline in its populations; in addition, it is a species associated with one of the emblematic crops —sugar cane— in one of the areas where we are working the most, such as the Cauca Valley.  

We are trying to promote greater sustainability of this crop through rotation with rice. And during that process, we noticed that this was a species that used them with a certain frequency. This sparked our interest in learning more about whether these populations stayed in this place throughout the winter, their densities and, above all, where they came from, what challenges they faced during their journey from their nesting areas to the Cauca Valley.

Have you always used the same tracking technology, or has it changed over the years?

They have  changed! We started with Motus stations and it’s important to mention that there weren’t any in the Cauca Valley. Tracking the movements of these tringas was not just a matter of fitting them with radio transmitters but also establishing the infrastructure in this region  to be able to monitor them. In fact, of the first 10 tringas we captured, during an exercise we did with Selva, a Colombian conservation organization, we never knew when they left the country, because when we tagged them, we didn’t have any Motus stations in the Cauca Valley.

Motus has been the technology in which we have invested the most time and resources to track the tringas. But this technology has a great limitation:  it only works where there is network coverage—which is very good in the United States, in certain countries in northern South America, such as Colombia, and in Central America. But once the birds cross the border into Canada, especially in the central part of the continent, Motus coverage is very poor.

Does that mean we don’t know what happens to them?

In all these years of tracking the birds that we have tagged —about 30— once they pass North Dakota we do not know where they go. That’s why, since last year, we started using satellite transmitters, which give us a better idea of local movements. Now, we are waiting to see if Cholao and his peers can finally show us where they are nesting.

Do these birds follow the same migration route each year, both on their way north and on their return to the Cauca Valley? How predictable are Lesser Yellowlegs?

I do think there are some patterns in their routes. We have tagged about thirty tringas with radio transmitters that are detected by the Motus network. And we have put satellite transmitters on five more. What we’ve seen, in general, is that during the spring migration, in the months of April and May, birds seem to be in a race to reach their nesting grounds in the boreal region of North America. So, on that journey, they use the most efficient route possible, the Central American migratory corridor or Midcontinent Flyway, which is almost a straight line through all the central states of the United States. But once they cross the northern border of the United States, we do not know what happens, whether they continue in a straight line or if some of them travel to Alaska, which is also part of their nesting areas.

In the fall migration, there is more variability: some go through the central states and make stops along the way, probably gathering food and energy. Others take some interesting detours: we have had tringas that enter through Minnesota and North Dakota in the U.S. Others go all the way to New York and come down the East Coast. There are also tringas that come down right through the middle of the country and end up in Kentucky, Tennessee, South Carolina, and leave through Florida. Still others come down the same route we’ve detected them using during the spring migration. It is highly variable.

Could it be related to group travel? Do they fly down as a family?

I’m not an expert, and I don’t know if it’s been established whether they travel in family groups or not, but I’d like to know. What we do know is that during both the spring and fall migration routes, before making those massive leaps, there are periods in which they try to store up as much energy as possible through a bit of a feeding frenzy, both before and after.

That is what we saw with Cholao, which is tagged with GPS: it made practically a two-and-a-half-day uninterrupted journey between Cali and the Louisiana coast. As soon as it arrived in Louisiana Cholao spent about a week in one place, I imagine recovering from the trip, feeding and gathering energy. Then it went to  Iowa, to a  large swamp that possibly has lots of food at this time of year. And then, up to Saskatchewan, Canada.

Could climate change be influencing their timing and routes?

Perhaps what they call “false springs”. March or April arrive, bringing a rise in temperatures, and many birds likely perceive them as the full arrival of spring, a signal to start the breeding cycle and other activities associated with that season. But it’s not, after those days, a frost hits, the birds suffer, and nests, along with the energy invested in breeding, are lost.

Migration requires a great amount of energy. Cholao’s followers ask if the Yellowlegs eat along the way or how they manage to withstand the effort.

Without being an expert in migration ecology, I can say that, in general terms, what happens with most migratory birds is that in the weeks before starting a migration, they feed frantically to increase their fat reserves as much as possible, which they rely on for the journey. There are some species, such as raptors, that can stop during their migratory route and occasionally feed in the areas they pass through. But many migratory birds simply depend on those energy reserves accumulated in their fat, especially if they are crossing the ocean and have nowhere to stop to eat. Of course, once they land, which is known as “stopover”, they feed once again. And they make their last leap to their breeding areas.

Another quick fact about Cholao: how fast do they fly?

Based on what we have recorded, at least in their crossing of the ocean and depending on factors such as wind and weather, they average 25 to 30 miles per hour.

How is this data useful to us, and how does it reflect in decision-making?

It provides us with the information needed to later define protected areas or to create campaigns in favor of conservation or simply to keep as a repository for study. 

For the Motus network, it is very important because it allows us to identify the stopover sites of migratory birds, what some call “restaurants” and “hotels”. When we have thirty, fifty, or a hundred tagged individuals, we start to see that certain areas are more heavily used, critical sites on the migration route of certain species. This, in turn, allows us to identify the threats these places face, enabling organizations like ours, dedicated to the conservation of birds and their environments, to manage actions to face those threats. 

It is equally useful on a more local scale, where data provided by the GPS is very useful. It goes into such detail that we can pinpoint exactly which lands, rivers, or specific crops the tringas use in the Cauca Valley, and how they spend their time  there. The amount of information we are receiving is overwhelming, impressive!

What is the equipment the bird carries like?

For both Motus transmitters and those that use satellite positioning, the type of transmitter is the same: a tiny backpack worn on the bird’s back, where only the shape and weight varies a little. The Motus transmitter is a bit flatter and lighter, weighing less than two grams, while satellite ones weigh two grams.

There is always a concern that they may suddenly impact bird mortality, not so much because they make them more attractive as prey, but because the harness can limit mobility, if it is not fitted correctly. That is why only trained personnel handle the installation. 

Another factor is that migration carries an energy cost and those two grams can make a big difference. In most species that are studied for migration, the aim is to keep the transmitters under 3% of the individual’s body weight. Until a few years ago, it was impossible to think of putting satellite transmitters on a bird like Cholao, on a tringa, because they were all too large, but that’s not the case anymore.

And what is the lifespan of these transmitters? How do they work?

Both types of transmitters, GPS and Motus, use sunlight to produce power. In the case of the Motus, they have a battery that recharges with sunlight, making them virtually eternal, just like the satellites. However, the longest we have managed to track a single individual is two years. It is believed that this happens due to wear and tear of the harness, which is made of nylon and falls off.

With the data collected so far, what has surprised you?

I was hoping for a stronger association between the tringas and natural habitats. That’s what you would imagine, that they would go to natural wetlands or, in the Cauca Valley,  concentrate around the Sonso lagoon or the Cauca River. But no! The association of the Lesser Yellowlegs with rice crops is incredible, and they are practically there, or will get there, wherever there is a rice crop. This means we are on the right track, and we must focus on implementing bird-friendly practices in the Cauca Valley, where these rice crops are a key piece of the migratory puzzle.