What initiates migration?
The same factors stimulate migration and reproduction. Before migration, metabolic changes occur. The thyroid gland controls these changes. Food consumption increases and fat accumulates under the skin tissues. This will provide the energy for long flights. The Ruby-throated Hummingbird stores enough fat to fly 26 hours non-stop at 25 miles an hour. This is enough to span the Gulf of Mexico. Variations in metabolism and related phenomena are controlled by another endocrine gland, the pituitary, located in the lower part of the brain. It sends out instructions by way of hormones.
