What should I eat today? Where will I get it? How will I catch it? These are the questions that a Tropical Kingbird asks itself everyday.
Tropical Kingbirds are small, flying omnivores. From Santa Cruz, California (occasionally) to Argentina, Tropical Kingbirds are a common sight in open habitats like marshes, savannas, and parking lots. Tropical Kingbirds have low wing loading and a low aspect ratio, so they are quite maneuverable in the air. They use this morphology to their advantage to capture insects in mid-air or glean them from vegetation.
This week I helped students on an undergraduate study abroad course design and implement a two-day study to determine the factors that predict foraging success for Tropical Kingbirds. To do so, we observed kingbirds in Palo Verde National Park in Guanacaste, Costa Rica. For each time a kingbird attacked an insect, we recorded the type of perch it was on, the height of the perch, the habitat the bird was in, the type of foraging behavior it employed (i.e., aerial hawk, sally glean, perch-to-ground), and the direction that the bird swooped to attack its prey (up, down, or even with the height of the perch). We also recorded whether or not the attack was successful.
We found that the type of foraging maneuver is an important predictor of foraging success for Tropical Kingbirds. Kingbirds were much more successful when they gleaned insects from vegetation than when they hawked insects out of mid-air. This makes sense – insects in the air are moving targets, and many of them have a wide range of vision allowing them to sense kingbirds approaching from a distance (dragonflies, for example, only have a 1-degree blind spot directly behind them). Additionally, the initiation of escape behavior is easier for an insect already moving.
So why do kingbirds go for insects in the air at all? We suspect that kingbirds are primarily searching for prey in foliage and on the ground – higher odds – and only taking flying prey opportunistically when a better option doesn’t present itself right away.
We also noted that individual birds varied considerably in their ability to consistently capture insects. Whether this is due to the random chance associated with short observations or due to real differences in the foraging abilities of different kingbird individuals is up for debate.
As always, working with the OTS USAP course was a tremendous pleasure. The students were highly engaged (I think each of the 25 students asked me a question during one of my two lectures), and Palo Verde is a beautiful place to visit.