One of the first signs that spring has finally arrived is the return of summer songbirds to our feeders and the sound of their singing. Compared to the slow process of fall migration, when birds gather and leave at different times depending on available food supplies, the arrival of our spring migrants is unmistakably sudden. Birds that summer in North America are on a very strict schedule. There’s not much time to stake out territories, mate, build nests and raise young before cold weather begins again and food sources dwindle. Birds arriving in their breeding range on time have a better chance of claiming the best nesting sites and attracting mates.
While shorter day lengths and the impending loss of food sources for insect and fruit eating birds seem to be the main triggers for fall migration south, what compels the birds to return north to their summer breeding grounds?
Researchers have few solid answers for this question. Day length is considered one factor, but near the equator day lengths are almost always the same. Hormones and the bird’s reproductive cycle as well as temperature must certainly play a part in a bird’s readiness to migrate north, but just how all these factors all come together is not quite known.
North American birds that migrate do so by means of a complicated network of flyways and routes. The largest geographical areas of travel in North America are known as the Atlantic, Pacific, Central and Mississippi Flyways. Within these flyways are countless routes which have been established by groups of birds coming and going from various locations.
Most birds time their arrival in North America based on an abundance of a certain food. In Delaware, shorebirds arrive in late May, just when horseshoe crabs are laying eggs. This feast allows the birds to fatten up and continue their journey to their arctic breeding grounds. Hummingbirds follow a nectar trail of flowers while other birds arrive at their destinations just when there’s an outbreak of caterpillars or other insects or when leaves are beginning to bud.
Just as in fall, spring migrants frequent stopovers where they refuel for their remaining journey. Although these sites are declining due to the fragmentation of wetlands and forests, there is a new awareness of the importance of these feeding grounds and efforts are being made to preserve them. Should you be lucky enough to be located along a migratory route, your yard may be host to an astonishing variety of birds as they stop to rest before proceeding to their final destinations. Providing food, water and shelter for these traveling birds will strengthen them for the remaining journey and the stresses they face when they finally return home.
— Written by Roxanne Brune