Bird Eggs Part 2, The Clutch
by Stan Tekiela
© NatureSmart
April 14, 2006
Maintaining our spring
theme, this week we will continue our look at
bird nests, eggs and incubation (see the last
Nature Smart column for part one of this series).
This time we are going to look at clutches.
No, not the thing that helps your car’s
transmission change gears. A clutch is defined
as the number of eggs produced or incubated
at one time by a bird.
In general, some species of birds will lay
only one egg such as the California Condor,
while many birds of prey such as the Bald Eagle
will usually lay two eggs. Many song birds such
as cardinals and jays will have 4 to 6 eggs
while some large birds such as ducks and turkeys
will lay as many as 15. The species that get
the award for laying the most eggs in one season
is the Brown-headed Cowbird. The adult female
will produce between 30 and 40 eggs before the
season is through. What is even more interesting
is, she doesn’t build her own nest to
deposit all of these eggs, rather she searches
out other bird’s nests and lays an egg
in the “host” bird’s nest.
Many factors dictate the number of eggs a bird
will lay. It’s a combination of factors
that determine the number of eggs an individual
bird will lay. How long a species lives is one
determination. The shorter the birds life, such
as a goldfinch or chickadee which on average
live about 5 years, the more eggs the female
will lay. In addition these birds will often
have two or even three clutches of eggs per
season. Eastern Bluebirds often have two and
some even have three clutches per season. The
longer a species lives such as the California
Condor which lives to 70 or 80 years lays only
one egg every other year. The Common Loon almost
always lays two eggs. These birds can live to
about 30 years.
Food is another clutch size factor. For instance,
birds that eat and feed its chicks easily obtained
food such as insects will lay more eggs then
a bird that specialized in food that it requires
skilled hunting. Hawks and owls are good examples
of this. A hawk or owl may only catch one mouse
or other prey item for every 5 or 6 attempts.
The harder it is to catch the food the fewer
the chicks.
Birds that nest closer to the poles (higher
latitudes) have larger clutch sizes due to the
high concentrations of insects. If you have
ever been to the arctic or even northern Canada
or Alaska in summer you know what I mean.
Clutch sizes also increase with the type of
nest. Cavity nesters such as woodpeckers have
increased clutch sizes as compared to open nesters.
This speaks to the fact that nest cavities are
safer places to incubate and the mother can
“invest” all of the seasons’
eggs at one time. Open nesting birds often nest
several times a season just in case a predator
gets their eggs.
Most birds will replace their clutch if it
is destroyed by a predator or severe weather.
However many single-brood species will not re-lay
if their clutch of eggs is lost. In this instance
if the clutch is lost early in the nesting season
they may be able to replace the eggs providing
the female’s hormonal system to recycle
into laying mode isn’t shut off for the
season. Birds such as Common Loons will not
relay eggs if the first clutch is lost while
other birds such as the Northern Cardinal will
automatically start a new clutch of eggs.
Some birds will respond to the removal of
an egg. Birds classified as determinate layers
will not lay another egg if one is removed whereas
indeterminate layers will lay a replacement
egg in the absence of one.
Younger females tend to lay fewer eggs than
older females. Why this is remains a mystery
to ornithologists. One idea is, younger birds
nest later in the season when clutch sizes tend
to decline even in older females. The second
clutch of the season (later in the season) is
usually smaller then the first based on a strategic
adjustment by the laying female to the prospects
of finding enough food to feed the growing chicks
later in season when fewer food resources are
available.
As the nesting season progresses we will look
at more aspects of this interesting part of
a bird’s life. Until next time…
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