Spring bird migration has run its course. Birds are busy attracting mates, establishing territories and building nests. Before I put migration to rest, I want to add a bit on the remarkable physiological demands of bird migration and the adaptations birds have to meet those demands.
In the last column, we explored the engineering problem migrating birds have of how much fuel (fat) it bird should carry. Carrying extra fat for insurance means a bird’s mileage will be reduced because of the excess weight. Cutting the fat stores too close might mean running out of fuel and perishing.
Thanks to the work of Dr. Scott McWilliams and his students at the University of Rhode Island, we know many migrating birds have a trick up their wing to improve their flight performance.
In preparation for migration, a bird enlarges and lengthens its gut to allow it to feed more rapidly and put on weight. It’s an adaptation for gluttony. The cells of the intestines become larger and new cells are formed.
However, the physiological demands of the gut are high. So during migration, once fat stores have been loaded, a bird essentially shuts down its gut and the gut decreases in size and weight. The diversion of energy from the gut can be used to fuel the flapping of the wings for a migrating bird.
Once a bird has stopped after completing a leg of its journey, it is unable to feed efficiently because its gut has shut down. It must rebuild the gut to allow food to be digested properly.
McWilliams showed that proteins are essential to get the digestive tract functioning well. Birds that only have access to fruit at a stopover will refuel more slowly, often entailing a delay in their migration.
Let’s revisit the Semipalmated Sandpipers discussed in the last column. To fuel their four-day migration over the Atlantic Ocean from the Bay of Fundy to Suriname, the birds pig out on small crustaceans called Corophium in the upper Bay of Fundy mudflats. Dr. Jean-Michel Weber of the University of Ottawa noted that the Corophium are high in omega-3 fatty acids. He found that the efficiency of the sandpipers’ muscles increased over the two weeks or so that birds spent fattening on the mudflats. Weber suspected that the omega-3 fatty acids might be the reason for that increase in efficiency. However, he could not rule out other reasons (hormonal changes, exercise) to explain the muscle improvement.
He resorted to some lab experiments with Bobwhite quail. These birds rarely fly and do not migrate, eliminating exercise and migration-related hormonal changes as possible factors. By supplementing the diet of the Bobwhite with omega-3 fatty acids, he found a direct increase in muscle efficiency between 58% and 90%. These changes are similar to ones noted in Semipalmated Sandpipers shortly before they embarked on their 2,400-mile jaunt to South America. Remarkable!
Switching gears, we know that migrating birds often overshoot their intended breeding destinations. Summer Tanagers, Hooded Warblers and Kentucky Warblers occasionally appear in Maine in the spring but presumably withdraw to their more southerly breeding grounds. These birds likely made a navigation error.
A different explanation may explain the appearance of some out-of-range birds. Recently in southeastern Florida, a number of Caribbean birds appeared to the delights of Florida birders. These birds included Bahama Mockingbirds, a LaSagra’s Flycatcher, a Fork-tailed Flycatcher, a Thick-billed Vireo, two Cuban Vireos and many Bananaquits and Western Spindalis (a type of tanager).
Why this influx of rarities? Some ornithologists believe that these appearances were driven by the devastation wrought by Hurricane Matthew last fall that hit eastern Cuba and the Bahamas with its full fury. Lots of bird habitat was destroyed. The hypothesis is that some Caribbean birds returned to their normal breeding grounds, found it to be destroyed, and kept on trucking to Florida.
[First published on June 11, 2017]