Over the last few weeks, I have written several columns on topics related to migration. However, it has occurred to me that I haven't really addressed the subject of why birds migrate in the fall.
Most people would probably assume that birds leave the northern latitudes due to day length and colder temperatures. Those answers are true - in part. But the bigger reason for migration is related to the availability of food.
Most of our migratory birds do not migrate because they cannot withstand cold. They migrate because the food sources they rely upon during the spring and summer months are no longer available to them once we begin to experience freezing temperatures.
For example, some of our more common summer breeding residents are hummingbirds and orioles. The majority of their food intake is linked to flowers - either in the form of harvesting nectar, or insects found visiting flowers. What happens to flowers once we start getting freezing temperatures?
Very few native plants bloom in the fall and winter months. When this food source 'goes away,' the birds that rely on this food source also go away, moving south to climates where a constant source of flowering plants and abundant insects are available to them.
In addition to hummingbirds and orioles, there are many other insect-eating varieties of birds that summer here, including flycatchers (which includes kingbirds, phoebes and pewees), vireos, warblers, and tanagers, where their diet is almost exclusively insects. Like flowers, what happens to insects when we start to experience freezing temperatures?
The majority of insects die once we begin to experience a hard frost night after night. When this source of food 'goes away,' the birds that rely on this food source go away as well - moving south to more temperate climates and habitats where their preferred food source is sufficiently abundant to sustain them until it is time to migrate north again in the spring.
It is true that there are some insect-eaters that winter over, such as Bewick's wrens or bushtits, and certainly woodpeckers and nuthatches. These species are better adapted to find insects, insect eggs, and larvae in winter so they can eek out an existence even though there are fewer insects available during the winter months.
Interestingly, there are some insect-eating bird species that summer north of the Arizona Central Highlands - often at much higher elevations, such as in the Rockies. These species migrate south, and choose to winter over in the Prescott area where they are proficient in finding sufficient food to sustain them. Examples of this behavior include yellow-rumped warblers and ruby-crowned kinglets.
Many individuals who feed birds choose to provide suet in the winter months for the insect-eating varieties of birds that winter over. Suet is a high-fat, high-protein food source for birds in winter.
There are certainly a lot of species in the Arizona Central Highlands that are year-round residents and don't need to migrate, such as Gambel's quail, mourning doves, crissal thrashers, spotted towhees, and house finches. While some of these species take advantage of insects seasonally, they are well adapted to survive on a diet of seeds and can winter over without any difficulty.
A quick reminder - the entire month of September is the submission period for the Jay's Bird Barn (now The Lookout) Annual Wild Bird Photography Contest. The event is free and open to the public and I invite you to participate. For submission guidelines, check out the Jay's Bird Barn (now The Lookout) website. The exhibit opens to the public on Tuesday, October 1st, as part of our Annual Fall Seed Sale and Anniversary Celebration.
Until next week, Happy Birding!