By Susan Benson,
CNHM Director of Education
What causes the push for bird migration? With an early spring, do the birds return early? Food and reproduction are the two most critical factors in migration. The changing seasons cause decreases in a bird’s food supply, causing birds to move to an area with more plentiful food availability. With a greater density of birds in the southern areas during our winters, food supply gets shorter, causing a push to move to another area where greater food supplies exist. A second factor is that birds also migrate to an area to raise their young. What are the internal and external cues that lead to this bi-annual migration for so many birds?
A major cue for bird migration is the change in day length, or photoperiod. These changes cause different hormone levels in birds that affect metabolism and the drive to reproduce. As daylight lengthens, there is a change in the lower part of the brain, the hypothalamus, that controls hunger and causes birds to gain weight through overeating, leading to as much as a forty percent weight gain. These fat deposits are stored in the bird’s flight muscles, under their skin, and in their abdominal area. Birds maintain this weight gain metabolism throughout their migratory activity. As daylight changes, other hormonal changes in birds lead to a display of higher activity, or restlessness in birds, especially at night.
Hormonal changes also bring a drive for reproduction. Spring migration becomes an urgent time for birds, as there is an optimal time for birds to arrive in their breeding areas. Stronger males arrive first to stake out the best nesting habitats. When females arrive, they select the prime areas in which to raise their young. For example, the house wren males arrive first into the northwoods habitats. They will begin building up to twelve different nest sites. Although they weight the equivalent of two quarters, the males will harass other larger birds, even taking eggs or young out of a nest site they want. When the female arrives, she chooses what she believes will be the most successful nest, and that male becomes her mate. Younger males will learn from their elders, usually choosing to nest near the older males, gaining knowledge of the best nesting habitats.
It is believed that day length is not the only factor that stimulates the drive to migrate. Temperature also plays a part. When spring is late, birds do not arrive early, and when spring is early, the birds can move in earlier to take advantage of the best natural resources. Scientists also believe that vegetative cover can influence light levels to impact reproduction.
Although there can be variability from year to year, scientists do believe that long-term averages show a trend that, in the last 20 years, birds are arriving earlier, and a larger number of species are shifting farther north in search of food. The theory is that climate change could be disrupting some birds’ migratory patterns.
Is there any time better than spring (besides, summer, autumn, and winter, of course?) Right now, spring rules our world, bringing us opportunities for observing birds in their bright plumage, or outdoor concerts as the males sing, declaring their territory. Spring is so sweet!
For over 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility and new exhibit, Our Shared Planet, in Cable at 13470 County Highway M. Also find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Post your own stories on the Nature Watch blog at cablemuseumnaturewatch.blogspot.com.
What dates have the American robins return? The following dates were reported by Museum members: