Bonaparte's gulls and tree swallows often migrate north to their nesting areas at the same time in March and April. Both kinds move swiftly along the Susquehanna River here in southeastern Pennsylvania, as they do across much of North America. And I have also seen both kinds of migrant birds flying low over inland, human-made impoundments in this area in spring. Both species, as they push north, catch and eat flying insects over those waters. Though these species are from different bird families, they obviously have characteristics in common because they share similar habitats and lifestyles, including when on migration, that shape their bodily structures and their habits. By fitting into a shared habitat, they have converged with each other, including grabbing insects while on the wing in migration.
Bonaparte's gulls, which are a species of small gulls, about the size of mourning doves, and tree swallows are dainty and graceful in flight. In fact, "bonies" appear tern-like, with a bouyant, bounding flight. Bonies and swallows both have small beaks they use to seize insects on the wing and invertebrates off the surfaces of bodies of water.
Bonies winter in flocks along ocean shorelines where the water doesn't freeze. But in March and April, gatherings of them commonly migrate together up rivers and across farmlands and forests to their nesting territories in northern Alaska and western Canada. I've even seen a few Bonaparte's at a time flitting and hovering gracefully and entertainingly over local farm ponds and flooded pastures, while catching flying insects.
Anyone can easily identify these small gulls, whether in winter or summer plumage, by the long, white patch of feathers on each wing that flutters like a banner in the wind as the birds fly. Many bonies on the wing, particularly in sunlight, resemble several white butterflies on the wing.
Thousands of tree swallows, in flocks large and smaller, dash north across southeastern Pennsylvania during March and April, and some of them stay here to nest in abandoned woodpecker holes, other tree cavities and bird boxes erected for them and eastern bluebirds. These pretty, insect-eating swallows are iridescent blue on top and white below, alternately flashing dark, then white, then dark, as they sweep and turn across fields, meadows and impoundments after flying insects to eat. Parent tree swallows also feed insects to their young.
Bonaparte's gulls are also unusual in that they hatch young in twig and grass nurseries they saddle on horizontal limbs on spruce and fir trees about fifteen to twenty feet above the ground near lakes and marshes in spruce/fir forests. They feed their fuzzy chicks, and themselves, insects they catch in mid-air over nearby lakes, ponds and wetlands and off water surfaces. When picking insects off water, they fly with a few deep wing beats into the wind, glide with their red legs dangling toward the water, hover briefly to pick up a morsel in their bills, then bound away in flight low over the water and repeat that process.
Since they hatch youngsters in nests in trees, bonies also converged with solitary sandpipers. Each female solitary lays four eggs in a deserted American robin, rusty blackbird or jay cradle about four to twenty feet high in a spruce or fir tree near ponds and bogs in coniferous forests in Alaska and Canada. Solitaries and bonies' nest sites have converged for the safety of incubating parents and their eggs and chicks away from ground predators.
Solitary sandpipers are unique because they migrate and feed alone for the most part, rather than in flocks like their relatives. Solitaries pass through southeastern Pennsylvania by late April and into early to mid-May, as do bonies and tree swallows in March and April. Solitaries also fly buoyantly and daintily, like bonies and swallows. But those sandpipers feed on aquatic insects and small crustaceans along shallow shorelines.
It's interesting how creatures living in a habitat are unwittingly molded by it to be similar in appearance and habits to fit into that habitat well so those critters can live and reproduce successfully in it for indefinite periods of time. Those animals, and plants, converge with each other in a particular habitat because of natural selection weeding out those forms of life that don't fit in. Fish and whales are streamlined in water and swallows and bonies are streamlined in the air and able to catch flying insects. And bonies and solitaries nest in trees, though the relatives of each species does not, to protect themselves and their eggs and young from ground predators. Nature is so grandly beautiful and intriguing.
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