Omnivore dilemmas in Guanacaste

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.

Tropical Kingbird (Tyrannus melancholicus) – pondering its next meal

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.

USAP students on the Palo Verde airstrip watching a Tropical Kingbird (the tiny point at the top of the tree). Photo courtesy of Erika Deinert.

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.

The marsh in front of the Palo Verde Biological Station. Kingbirds in the open marsh seemed to have much higher attack rates than birds in other habitats like Typha-dominated marshes, yards, dry forest, and rice fields.

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.

Green-breasted Mango (Anthracothorax prevostii) – a hummingbird that would certainly make a fine tattoo.

Aside from Tropical Kingbirds, Groove-billed Anis (Crotophaga sulcirostris) were one of the most common open country birds around Palo Verde.

Limpkins (Aramus guarauna) were common in the Palo Verde marsh. I watched them peck into the mud like feathery sewing machines, but I never saw one capture an apple snail.

Northern Jacanas (Jacana spinosa) wander through the marsh calling loudly and flashing yellow feathers on the underside of their wings. For what purpose, I do not know. There were lots of juveniles present during my visit.

Roseate Spoonbills (Platalea ajaja) and Wood Storks (Mycteria americana) are two of the most charismatic waders at Palo Verde. Here we found some in a slough adjacent to some rice fields just outside of the park entrance. (The most charismatic wader is the Jabiru (Jabiru mycteria), which we did not see this week).

We found a Double-striped Thick-knee (Burhinus bistriatus) incubating a big, cream-colored egg with brown splotches in front of the entrance station at El Negrito. A nearby mate attempted to run a distraction, but it didn’t do a broken wing display.

Scissor-tailed Flycatchers (Tyrannus forficatus) outnumbered Tropical Kingbirds at El Negrito. Perhaps these two species competitively exclude one another?




About Leighton Reid

Restoration Ecologist
This entry was posted in Behavior, Birds, Teaching and tagged , , , , , , , , , , , , , , . Bookmark the permalink.

2 Responses to Omnivore dilemmas in Guanacaste

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