International detections of tagged Cuckoos

A total of eight Yellow-billed Cuckoo were fitted with tiny radiotransmitters as part of this project. One of the main objectives of deploying the transmitters was to determine how long the birds remained at our study site, which in turn would tell us about the importance of the site and the wider region for the migration strategy of this enigmatic bird.

The transmitters revealed that on average birds stopped for just over 6 days but that some individuals remained in the area for as much as two weeks, presumably gorging themselves on juicy caterpillars. However, the tagged birds had other surprises in store, with two of the named birds, Fast Bertha and Barbara Delia, being detected beyond Colombia by automated telemetry receivers. These receivers consist of two or three long antennas and have been set up as part of the Motus wildlife tracking system. This exciting network of researchers maintains over 400 automated receivers across the Americas, making it possible to follow the migration of individual birds carrying transmitters.

The first of our cuckoos was detected flying past a receiver on the Caribbean coast of Panama just before midnight on the 26th April. The graph in the figure below shows a perfect detection with signal strength from the transmitter starting off low and then increasing as it approaches the receiver and then tailing off as the bird continues northwards. It also reminds us that cuckoos, like many other migratory birds, are nocturnal migrants, covering 100s of kilometers under the cover of darkness.

 

Two of the named cuckoos were detected beyond Colombia

The second cuckoo flew over 3800 km before being detected over the Salt Plains Wildlife Refuge, Oklahoma, roughly in the middle of the North American continent (this cuckoo was very aptly named Fast Bertha!). Again this was a perfect flyby, with the bird passing through in the dead of the night (12:45 am). Unfortunately, neither of these detections tell us much about the speed of migration as both occurred some days after the birds were last detected in Colombia. Hopefully with 20 tags available for spring 2018, we will get more detections and have the chance to learn more about the migration of this fascinating bird.

Finally, the graph below shows the wonderful detail that we can gain from radiotransmitters. This plot shows the signal strength from the transmitter fitted to Pearl over the course of several days. Where signal strength is highly variable, we can deduce that the bird was actively moving, flying up and down, hence the considerable variation in signal strength. We can also see periods where the detections form a thick continuous line, indicating that the bird was hardly moving. These periods correspond to when the bird was sleeping at night, hence the lack of movement!