Written by Sarahi Farias
Migration is the seasonal movement of animals from one region to another and it is an important aspect of well-being for birds. When birds migrate, there are many different things to take into consideration. What does a bird prioritize? Are they trying to get to their destination before any other birds (fly the fastest)? Are they trying to be as efficient as possible so as to not deplete their fat reserves? Or are they just trying to get there safely and avoid predators? We will go through these migration motivators and migration facts for some of our raptor ambassadors at The Urban Interface.
Red-tailed hawks (Buteo jamaicensis), like our ambassador Freyja, are called partial migrants. A large part of the northern population will migrate south for the winter (usually between October and December), but a few birds will stay behind to maintain their breeding territories. Even more individuals in the mid-latitude regions opt out of migrating and choose to brave the cold. Red-tails in the south are not known for migrating since the weather rarely gets harsh enough for them to have a reason to leave. When they do migrate, their primary migration motivator seems to be speed as well as energy efficiency. Juveniles are the first to start migrating south and they tend to travel longer distances than the adults, although most of their migration is done within North America. Their goal is to make it to the southern breeding grounds before the adults.
To conserve energy, most hawks choose to travel during favorable weather conditions, so as to not have to exert as much effort during their flight. This can include avoiding large bodies of water, which can onset unfavorable weather conditions. An additional means to conserve energy is to fly at higher altitudes. Many have been observed flying over long ridges where there is a push of air coming from underneath. This updraft allows them to glide and therefore conserve energy. Red-tails even utilize two types of soaring to use less energy: thermal soaring and slope soaring. Thermal soaring is the use of convection currents, referred to as thermals, which can help sustain flight without any additional power source (wing flapping), and this is utilized by red-tails in cities. The other form of soaring is called slope soaring. This occurs when the hawk glides with the air that is naturally moving up as it moves parallel to the face of a slope, which can be seen in the picture above. The longest distances covered by red tailed hawks are done by using slope soaring since this form of flight allows them to fly at 30-40 mph.
Our next migratory ambassador is Circe, our Merlin (Falco columbarius). Merlins, in general, have very similar migration patterns to red-tailed hawks, although they tend to do the majority of their flying early or late in the day, helping infer that their main migration motivator is safety. A merlin can use the cover of night to avoid predation by other birds, yet can take advantage of the available food during the day to re-energize. They also take advantage of thermal soaring and updrafts. There are many different subspecies of merlins and their migration patterns will depend on this. Similarly to the red-tailed hawk, the northern individuals are the most likely to migrate in the winter, whereas the southern individuals tend to remain in their territories year-round. This is due largely to the influx of songbirds available as prey during the winter months. Merlins tend to live near the edges of forests, but during migration they can be found in a more diverse range of habitats, including coasts and agricultural lands. They can migrate as far south as northern South America, and their peak migration time occurs in mid-October.
Peregrine falcons (Falco peregrinus) such as Valkyrie have a more widespread migration than merlins, although they tend to follow more established routes. These established routes are called leading lines and they follow an elliptical path. The path stretches out to the East Coast on the way south and along the Gulf Coast on the way back up north. Peregrine falcons cover very long distances and some may cover as many as 13,000 km (8,078 miles) one-way. They also exhibit a “leap-frog” pattern in which the southern peregrines will ‘leap’ over where the northern peregrines have decided to overwinter and fly even farther south. Unlike the red-tailed hawk, peregrines fly lower to the ground and do not avoid large bodies of water, which requires more flight energy due to unfavorable weather conditions. Peregrines actually prefer to travel over water so that they have a large hunting ground, meaning that their main migration motivator isn’t to be efficient. This, in fact, suggest that the peregrine falcon’s migration motivator is maintaining fat reserves. They are not concerned with speed since there is no time constraint, and they only migrate at speeds of about 30 mph. They are also not concerned with safety because they are apex predators.
Our final migratory ambassador is Pandora’s species, the Swainson’s Hawk (Buteo swainsoni). These hawks have the longest migration route of all the North American raptors. Their route may start as far north as Canada and can finish in Argentina. Swainson’s travel in migratory flocks called kettles in which numbers can reach the hundreds-of-thousands. As many as 845,000 have been counted migrating through Veracruz, Mexico. The main migratory motivator for the Swainson’s hawk is speed. They cover more distance than any other raptor, yet winter doesn’t slow down for them. In order to make it to their wintering grounds on time, they often go several days without eating.
Now that we have gone over the migration patterns for our ambassadors, make sure to keep an eye out for these birds when they are migrating through your area. The majority of their fall migrations take place in the month of October, while spring migrations typically occur from March to May. Also be sure to look around our website (www.theurbaninterface.org) to familiarize yourselves with our ambassadors. Learning the stories about ambassadors or even meeting them in person at events may make seeing in them in nature even more special!