As the last of our autumn leaves fall to the ground and winter approaches, I am beginning to notice more and more robins sitting quietly on the branches of chokecherry, hawthorn, and mountain ash. It seems odd that they aren’t flying frantically from branch to branch in search of food to fatten up before continuing with their southward migration. Why are they just loafing about?
They may actually be busy digesting their food. If so, it would be odd because researchers generally believe that feeding rates are limited by the availability of food; that is, the more food available, the faster a bird will feed. There are periods, however, during which food is so readily available that birds can fill their digestive tracts faster than food can pass through! At those times, birds must stop collecting food and wait for their digestive system to catch up.
This is apparently what happens to robins, which encounter a superabundance of food when they switch from about 90% insects in summer to about 90% fruit in the fall and winter. This diet switch occurs at a time of year when they need to gain weight rapidly in order to fuel their journey south. In fact, some migratory birds gain weight at the rate of 10% of their body mass per day, which is equivalent to a 200-pound man gaining 20 lbs a day! Given this need to gain weight, it would be advantageous to digest as much food as fast as possible. So how do birds like the robin deal with a digestive system bottleneck?
One way is to digest food faster. When a robin switches from eating insects to fruit, it actually speeds up its digestive process by about two times. The process doesn’t appear to be very efficient because a lot of the fruit passes through more or less undigested, but there are plenty of berries to make up for the waste.
A second way to deal with a relatively slow digestive process is to grow more intestine. William Karasov, a wildlife ecology professor at the University of Wisconsin, once reported that House Wrens can actually grow up to 22% more intestine when given unlimited food. Karasov believes this may be a fairly common trait in other migratory bird species as well. Even humans can grow more intestine to replace a section lost during surgery!
So, these two tricks (speeding up the passage rate of food and increasing the length of the digestive system) allow birds such as robins to process more food per unit time, and to really put on the weight when it’s needed for migration and over-winter survival.