In addition to cooler weather and golden aspen leaves, another sign of fall is the gathering of migratory birds into flocks as they prepare to head south for the winter. Canada geese and other waterfowl may be some of the most visible examples to those of us on the ground, as they tend to fly low and make frequents stops in ponds and reservoirs. As any avid bird-watcher will tell you though, raptor migrations may be some of the most exciting to observe.
In autumn, we tend to think all our migratory birds leave us behind for warmer climates. While some summer populations of northern harriers and Swainson's hawks may leave the state in autumn, many birds from farther north make Colorado their winter home. Golden and bald eagles from as far as Alaska are known to winter in Colorado. The state's populations of North America's two largest hawk species, red-tailed and ferruginous hawks, actually peak in the months of January and February. Rough-legged hawks are normally uncommon in Colorado and can almost only be found here in winter months.
Part of what makes raptor migrations so exciting to bird-watchers is that this is one of the best times to observe birds of prey in high concentrations. While you won't find hawks flying in the "V" shaped flocks typical of migrating geese, many raptors follow the same flight paths. Soaring birds, like ravens and golden eagles, take advantage of a phenomenon called ridge lift at all times of year. Ridge lift is an updraft of air that occurs when wind close to the earth's surface hits a mountain range or ridge and is directed upwards. The energy saving advantages of ridge lift for migrating raptors are immense and many species of birds of prey can be observed flying along ridges that run north-south in spring and fall.
If you want to see migrating raptors this fall, take a hike or have a picnic on a hill or ridgeline. Keep your eyes on the sky and watch for the silhouettes of large birds that soar and flap their wings infrequently. If you want to better identify the birds you see, take along a pair of binoculars and a field guide for birds. Pay special attention to the shape of the bird's wings and tail. Does it have a long, straight tail and slightly pointed wings? If so, it is likely a member of the genus Accipiter, like a Cooper's hawk or goshawk. If it has a relatively short tail and broad wings it is probably a Buteo, like a red-tailed or ferruginous hawk, or maybe an eagle if you're lucky.
If you are feeling down about summer coming to a close, or if are wondering what to do with yourself until the snow starts to fly, remember that fall offers some unique opportunities when it comes to observing wildlife. Animals are preparing for winter in various ways, but the soaring, sailing raptors are some of the most visible and inspiring to watch.
Pete Wadden is an educator at Walking Mountains Science Center. Watch out for him in roundabouts in Avon as he tends to get distracted by birds overhead and may swerve into your lane.