Saturday, February 18, 2012

Egg Questions and Answers

We have the first Decorah Eagle egg of 2012! Mom laid it last night at 7:47 CST. Here is a video of the event (by razzle):

Egg Questions and Answers

How long does it take a bald eagle egg to hatch?
There are two ways to think about this: from egg laying to hatch, and from pip to hatch.
  • From egg-laying to hatch: Most experts say 35 days, give or take a few. It took the Decorah Eagles 37 days in 2009 and 2011, and 36 days in 2010. 35 to 37 days is a safe bet. 
  • From pip to hatch: Pipping occurs when the baby eagle's egg tooth first breaks through the eggshell. It can take as long as a day for the chick to fully hatch. 
How many eggs will the eagles lay? Will all of their eggs hatch?
Although the most common clutch size for eagles is two eggs, the Decorah Eagles have a history of laying three eggs. The breakdown among bald eagles in general is as follows:
  • 79% of clutches have two eggs
  • 17% of clutches have one egg
  • 4% of clutches have three eggs
The eagles hatched all of their eggs in 2009, 2010, and 2011; however, fertilized eggs can fail to develop due to extreme cold, soft shells, or microorganisms. We are hopeful they will have another great year, but we won't know until it happens.

When does incubation begin?
In northern climates, Bald eagles lay eggs when it is still quite cold. They begin incubating immediately after the first egg to prevent the egg from freezing. The eggs hatch the same order and roughly the same interval in which they were laid. While the eaglets are still young, it is possible to identify hatchling number one by its size: it is older and therefore bigger than its nest mates. As the eaglets grow, this difference disappears.

Does Dad help incubate the eggs?
Based on our observations of the Decorah eagles, yes! Before all of the eggs are laid, Mom spends most of the time incubating and laying eggs. Once she is done, Dad incubates about 50% of the time.
Having said that, other nests have reported different divisions of labor, with Mom spending much more time incubating than Dad. We don't know why the eagles act differently from nest to nest, but they do.

Oh no! The eagles aren't sitting on their eggs!
Caring for eggs is a serious business. Everyone knows that eggs shouldn't get too cold, but they also can't get too hot, or the embryos will die. The adults sit on the eggs when they need heat and get off them when they need to be cooled. In particular, the Decorah Bald eagles lay eggs and care for nestlings in late winter and early spring, so their challenges include snow storms, ice, and rain. These things can be frightening to us, but Bald eagles have been coping with them for tens of thousands of years. It is okay for the adults to spend some time off their eggs.

The adult eagle also turn their eggs regularly. Turning the eggs helps keep them all at the same temperature and prevents the babies from sticking to the insides of the eggshells.

What do Bald eagle eggs look like, and how large are they?
Bald eagles lay white, oval shaped eggs. The eggs weigh approximately 125 grams or 4.4 ounces, and are on average about 2.9 inches long and 2.2 inches wide. Size-wise, they are just a tad smaller than a tennis ball, although tennis balls are round, not oval.

What causes the egg to hatch?
Mom and Dad Bald eagle apply heat through incubation, which causes the embryo to grow. At a certain point, it penetrates the air cell - that white sack-like membrane that can also be seen in chicken eggs - and its egg tooth penetrates the shell, creating a hole or pip. It usually hatches 24 to 48 hours after first pip.
The yolk of the egg is absorbed into the baby's stomach at this point, which gives it the energy it needs to hatch.

What is a brood patch?
A brood patch is a bare spot where feathers are missing. Hormonal changes linked to reproduction cause feathers to fall out, which creates a bare patch of skin that allows the incubating bird to better transfer heat to the eggs. Bob estimates that a Bald eagle's skin temperature is about 101F.

Some Bald eagles develop brood patches and some don't. We have not seen a developed brood patch on any of the three adult eagles we have monitored in Decorah, and they all incubated their eggs successfully.

Do Mom and Dad mate for life?
Yes, but if one eagle dies, another eagle will replace it.

Decorah Eagle Egg Videos

We also have a third egg at Valmont! Snowflake laid her third egg early this morning or late last night. When we had chickens, I used to think of this time of year as the 'spring egg flood'. It looks like the egg flood has started! Here is a pic from the Valmont nest:
2/18/12: Three Eggs for Snowflake and Dan!

Happy birdwatching, everyone!