Cuckoo Stopover durations

The term stopover duration refers to how long a migratory bird spends at any given site during migration. Birds pause their migration at certain sites for different reasons, sometimes its to rest between successive nocturnal flights, sometimes it's to avoid unfavorable weather conditions and sometimes it's to gain fuel for the next stage of the journey. Stopover sites used for fueling are especially important, as they determine how far and how quickly a bird can migrate.

Generally, the longer a bird's stopover duration is, the more important a site is for determining the outcome of migration. Sites where birds spend up to 2 weeks fueling tend to have a disproportionate effect on the outcome of migration, compared to sites used for one or two days. For this reason being able to measure how long a bird stays at a stopover site is a key step towards understanding a site's importance.

For this reason we fitted Yellow-billed Cuckoos with radiotransmitters and recorded how long they remained at our study site in northern Colombia through an automated telemetry receiver (see photo below). If birds made multi-day stopovers then we would have evidence for the importance of the Colombia's Caribbean tropical dry forest in sustaining the Cuckoo's long-distance migration.

 

Two 9-element Yagi antennas linked to an automatic Motus receiver at the study site.

We fitted eight birds with radio-transmitters, each of which was named by one of our donors. Of the eight transmitters one failed but the other 7 gave us our first insight into how long Cuckoos stay to fuel in the dry forest (see figure below). Stopover durations were variable but all but one bird stayed more than one day, with the longest duration being 15 days. On average Cuckoos stayed 6.4 days providing concrete evidence for a multi-day stopover in the dry forest for the first time. If birds increased their fuel reserves or body mass by around 3-5% per day, as is typical in many migratory birds, then they probably left with fuel reserves equivalent to 30-50% of their normal weight. That's a lot of fuel and when converted into wing beats, estimated flight ranges will be between 1000 to 2500 km!

 

The story so far of our named Cuckoos, all of which had departed the study site by the 12 May. Each green or orange circle represents a radio-transmitter being detected by the automated receiver.

So all this means that after leaving Colombia's dry forest, Cuckoos may well have enough fuel to fly directly to North America! We are still waiting to hear where our Cuckoo's got detected having left Colombia and with any luck we will get confirmation of long-distance non-stop flights. Watch this space........