The impressive coordinated migrations
of wildlife like monarch butterflies and salmon have always made me curious. How
do they know where they’re going, when to start, what route to take, and when
they have arrived at their destination? Those are questions I think about every
spring, too, as millions of birds migrate to our area, many traveling great
distances under treacherous conditions. And some of them still have a long way
to go once they get here.
The tiny blackpoll warbler, for
instance, undertakes a two- or three-day flight across open water each autumn
from southern New England to the Caribbean with nowhere to land if it gets
tired. And the even tinier ruby-throated hummingbird crosses the entire Gulf of
Mexico in 24 hours without stopping, their buzzing wings flapping at about 50
beats per second the whole way.
But how are they able to keep going for so long without refueling?
The answer, according to Scott McWilliams, has everything to do
with the digestive physiology of the birds. Or, as the University of Rhode
Island ornithologist likes to say, “migration takes guts.”
McWilliams studies the physiology of migrating birds. He captures
migrant yellow-rumped warblers, hermit thrushes, white-throated sparrows and
other species on Block Island or at the Kingston Wildlife Research Station near
URI. For most of the birds, Rhode Island is only a rest stop at the halfway
point of their seasonal migration. He then evaluates the physical condition of
the birds, the food they eat and other factors – sometimes conducting short
dietary experiments with them before releasing them again – all to understand
the physiological changes that take place when the birds migrate.
What he has found continues to astound me every time I think about
it.
The URI professor says it takes a great deal more than an innate
knowledge of what route to travel and strong flight muscles to successfully
complete their twice-a-year migration. It takes a digestive system that is
highly adaptable to different activities and conditions, what scientists call
phenotypic plasticity. This physiological flexibility enables some species to
double their weight, support long-distance flight, and withstand changing
temperatures.
In the days preceding migration,
songbirds increase the size of their gut by producing new and larger cells.
This makes it possible for the birds to dramatically increase how much food
they can consume so they can store up extra energy.
The digestive system, however, uses massive
amounts of energy to operate. So once the birds have packed on the calories,
they can shut down their digestive tract so the energy that would have been
used to process food can be diverted to fuel flight.
McWilliams said that the birds’ guts actually begin to atrophy
during migration. Unfortunately, that means that when the birds stop along
their migratory route or reach their ultimate destination – a time when they
are surely hankering for a meal – they are unable to eat for a day or two until
their gut resumes operating normally again.
This discovery explains why birds do
not immediately gain weight when they complete their migration. It also provides
insight into their dietary requirements prior to departing. Because rather than
exclusively consuming high energy foods to sustain their flight, they also must
eat proteins to help rebuild their digestive tract at the end of the trip.
Which reminds me that I should
probably go for a bike ride. It’s not only birds that can modify their bodies
to accomplish formidable feats of endurance. With a little exercise, people can
too.
This article first appeared in the Independent on April 20, 2017.
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