Our first lab exercise in Vertebrate Biology (Biol 220 at Austin College, spring semester 2017) was an observation and comparison of foraging in ring-billed gulls,
These three species were all foraging simultaneously along the Red River (the one that separates Texas and Oklahoma) just below Denison Dam (the one that impounds Lake Texoma). We do a comparison of foraging behavior, microhabitat, and morphological features related to foraging. These three species differ in many ways with respect to these adaptations.
Ring-billed gulls fly in circles above the water, preferentially above the turbulent water as it spills out of the powerhouse, and they occasionally drop down to the river’s surface to catch a fish that has been stunned by the passage through the turbines. They mostly flap rather than glide on days with little wind (which ours was), but on days with wind, they flap upwind and glide downwind. Their long, tapered, swept-back wings are designed for mostly flying and not much gliding or soaring. Their short, straight, pointed beak is designed for plucking fish (mostly small ones) out of the water.
The American white pelicans were doing something unusual on the day of our observation. They were mostly flying above the water (gliding as well as flapping), and they would occasionally flare the wings and stick out the feet, dropping to the water’s surface, then scooping up a stunned fish. They were obviously being successful at this foraging strategy, because they almost always could be seen swallowing a fish. American white pelicans usually forage by swimming along the water’s surface, occasionally dipping the beak in to catch fish. Sometimes they do this in a coordinated fashion – several individuals swim along together, shoulder to shoulder, driving schooling fish ahead of them, then all scooping them up simultaneously.
The double crested cormorants were foraging in their normal fashion. They float low in the water, with head, neck, and part of their back above, and all the rest below. They periodically dive under the surface to catch fish. They have a straight beak with a hooked end to facilitate catching fish. Their bodies are hydrodynamic, and their webbed feet are placed posteriorly to act as propellers.