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Image courtesy Gerrit Vyn

SLIPPERY ROCK, Pa.—As the Tweetspeak team geared up for round two at Jennings Environmental Education Center on April 25th, we heard different bird calls all around us—a lot more activity was going on this time around.  We headed off onto a different trail than our previous visit, and it didn’t take long for us to start identifying various birds. Goldfinches here, various sparrows there, mixed with plenty of robins in between.  We stopped in a field, observing several Tree Swallows.


Our team hadn’t had the opportunity to observe this species before, since they hadn’t migrated here until recently, so we took notes on their behavior.  Most of them were either defending their nest boxes or gathering nesting materials.  According to the Audubon Society’s webpage on Tree Swallows, we saw the males call to females, showing off potential nesting sites.  For this bird, that’s usually a cavity in a tree, which is where they got their name.  However, they will take just as quickly to nest boxes, as we saw that day.  The females tend to build most of the nest, which is a cup shape made of grass and other plant material.  They will add feathers a bit later to cover the eggs for warmth.



There are several steps to this process, which is pretty energetically expensive (much in the same way that pregnancy is energetically expensive for humans).  The yolk part of the egg, which


contains the female’s genes along with food for the embryo, is released from the ovary.    If fertilization occurs, it happens here.  The yolk then moves through the oviduct to gain layers of albumen, the part we’d call the egg white.  The egg collects more albumen in other parts of the oviduct, as well, along with the shell membrane.  Once in the uterus, the shell gland adds crystals of calcium to the shell membrane, which grow together to form the shell.  At this point, some species of birds add spots of pigment to the egg shell with a different gland for camouflage.  Since Tree Swallows nest in cavities and nest boxes, this pigment isn’t really necessary.
These eggs are usually laid a pinkish color, which fades to solid white.




If you haven’t noticed, bird eggs aren’t just the same shape of the chicken egg we’re used to.  Some are rounder, some are pointier on one end, some are more of an oval shape, and so on.  What bird eggs have in common, however, is the general spherical shape.  This provides the strength the egg needs while also conserving materials ( The eggshell must be strong, yet also able to be broken by the chick from the inside.

As for the different shapes in eggs across the board, it’s thought that different nest habitats will affect this.  In other words, birds who nest high up in trees or on cliffs seem to lay eggs that have pointier ends. This causes the egg to roll in tighter circles and therefore, not out of the nest and onto the ground.
On the opposite end of the spectrum, birds who nest on
the ground tend to lay rounder eggs.



Female Tree Swallows generally lay 4-7 eggs, which is called their clutch size. Clutch size varies within and between species.  Reasons include the energy needed to lay and then raise the chicks, their expected survival rate, and other variables.  She, like most females of other bird species, will lay no more than one egg per day until her clutch is complete (  Each egg weighs about one tenth of her body weight.  That’s like a 140-pound woman giving birth to a 14-pound baby!  Imagine doing that up to seven times in a matter of a few weeks.  Talk about exhausting!

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