I sent this to Bob yesterday, but I'll send it out to the rest of the
list as well if it helps.  Bob had a great question and others may be
wondering the same thing.

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As birds return from over-wintering grounds they're typically pretty
hungry and looking for a quick and easy source of food, usually with
energy that's easy to get.  That's where berries come in.  The sugar
content of many winter berries increases, and sugar is a very quick
source of energy that's easy to absorb and utilize by birds.  Some of
these berries also have some fat content to them (less than 5%), and fat
has even more energy than sugar (there are some physiological and
metabolic differences in how fat & sugar are utilized by birds, and not
every bird can digest fat).  Additionally, berries require very little
effort to find and eat compared to foraging for prey, so many birds will
preferentially choose berries over other food types if berries are
available and/or abundant.  As berries dwindle, and invertebrates become
more available and abundant in the warmer weather of spring and summer
birds will begin to consume them more frequently as well as feed them to
their young.

 

Many kinds of earthworms are relatively new to Ohio and areas further
north, where they've been introduced as non-native species (see:
http://audubonmagazine.org/exotics/exotics0403.html).  While robins do
eat earthworms, much of their winter diet is typically comprised of
fruit (~90%).  In spring, March and April, when days are fairly warm to
mild but evenings are cooler, robins will eat earthworms and other
invertebrates, but still depend on fruit as a source of energy to help
them survive the night temperatures.

 

Many thrushes tend to be frugivorous throughout the year, as well as
mimic thrushes and a number of other birds.  Some birds that we
traditionally think of as solely insectivorous, like warblers, will
switch to a fruit diet during the fall as they're migrating.

 

Some birds start out eating fruit as nestlings.  Northern Mockingbirds,
for example, feed fruit to their young while they're still in the nest.
During the first few days of an altricial chick's life it is incapable
of maintaining its own body temperature, which is why the parents sit on
the nest for much of the time.  However, about the time the young birds'
metabolism kicks in to maintain their own body temperature the
Mockingbird parents begin feeding them fruit.  The fruit is a source of
energy for their metabolism, and the invertebrates are a source of
protein and minerals for growing muscles and bones.

 

I hope this helps.

 

Thanks,

 

Casey

 

 

Casey Tucker

Education Specialist

Audubon Ohio

692 N. High St., Suite 303

Columbus, OH 43215

614-224-3303 (phone)

614-224-3305 (fax)

[log in to unmask]

www.audubonohio.org

 

 


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