Bald Eagle Eggs in the Nest
From the MN DNR
Our EagleCam nest has its first and second eggs! On Monday, February 19, during a cozy ice storm here in Minnesota, the beloved mother at our EagleCam laid her first egg. The excitement came at about 1:15 pm. The second egg came on Thursday, February 22nd. This female has been known to lay three eggs every year since 2013. Based on her past egg laying schedule, another egg arrive should this weekend.
Snow and cold temperatures since Monday have caused concern among viewers. Let us alleviate your fears with some facts:
Embryos are less sensitive to cold than to heat, particularly before incubation has started. Mallard eggs have been known to crack by freezing and still hatch successfully. Eggs cool when incubation is interrupted, but this is not usually harmful, and few birds incubate continuously. Instead, egg temperature is regulated in response to changes in the temperature of the environment by varying the length of time that a parent bird sits on them or the tightness of the “sit.”
Many birds apparently sense the egg temperature with receptors in their brood patches, which helps them to regulate their attentiveness (time spent incubating) more accurately. Since the embryo itself increasingly generates heat as it develops, periods of attentiveness should generally decline as incubation progresses. Attentiveness is also influenced by the insulating properties of a particular nest.
Copulation and egg fertilization
Contrary to some stories you may have heard, eagles do not mate in the air. We have observed mating in the nest this year, but most often it will be on a high tree branch near the nest site. The female will bow her head and raise her tail, and the male will clench his talons as he gently mounts and balances on top of the female. She will then move her tail aside to expose her cloaca to his reach, and he will arch his body so his cloaca can touch hers while he totters and flaps his wings to stay balanced. The brief rubbing of cloacas as they touch face to face may last only seconds, but the sperm is transferred quickly during this exchange and it travels up the oviduct to meet her ova for fertilization. Copulations occur often during the breeding season but slow down once the eggs are laid, and stop after the eggs hatch.
How do eagle eggs develop? It is believed that sperm is viable in the female oviduct or infundibulum for about 10 days. One mating may fertilize all eggs laid in a clutch, or successive copulations after an egg lay could produce a fertilized egg.
Inside the female, sperm swim up a tube called the oviduct, at the end of which there is an ovum. A mature ovum is already equipped with yolk — the yellow part of the future egg. The sperm may now fertilize the ovum by penetrating it and uniting the two cells’ genetic material. Once the yolk is fertilized, the outer egg membrane seals off, preventing further sperm entry and the fertilized egg now begins the 3-day journey down the oviduct and the chick embryo will develop. The yolk will serve as food for the developing chick, and the white will mainly keep the yolk from drying out while giving it physical support. Membranes are added around the yolk and egg white, The shell is mostly calcium carbonate – the same chemical formula as limestone – making it hard yet brittle. The process continues for each new egg that is laid every three to five days until the
How big are the 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. , or about
How many eggs have been laid by the Minnesota eagles? Three eggs laid in each season since 2013; the first year, 2013, the nest failed and the eggs did not hatch. Since then, 10 eaglets have been successfully raised and fledged from this nest . While it is rare for bald eagles to lay three eggs, this female has a history of doing just that! Incubation Period: 35-37 days Both parents take part in incubation, sitting on the eggs to keep them at 99.5-100 degrees. The female, being larger, takes the longer incubation periods overnight. It is believed that her larger body weight gives her more endurance. Both parents have a brood patch, a natural thinning of the abdomen feathers caused by hormonal changes, where their skin is in direct contact with the eggs to transfer warmth. Since incubation starts from the time the first egg is laid, eggs will hatch about 3 days apart in the order they were laid.
How do eagle parents care for eggs? Eagle parents ensure optimal temperature and humidity of the eggs by rolling or turning them every 1-2 hours while brooding. Getting off the nest bowl at times allows fresh air to circulate over the eggs, dropping humidity level and allows cooling when needed or to slightly delay incubation. While turning eggs might be a matter of instinct, it also prevents the embryos delicate blood vessels from sticking to the insides of their eggshells, and it optimizes membrane growth. Both parents move very carefully around the eggs, often balling or partially closing their long, sharp talons to keep from stepping on or puncturing the eggs. A constant supply of soft grasses is brought in to line the egg cup of the bowl, keeping the eggs both warm and dry.
Keep watching this weekend – there should be a third egg by Sunday. At the nest sight, we are expecting another few inches of snow on Saturday. Watch as these amazing birds protect their eggs from predators, intruders and the weather. It truly is a natural wonder to experience first-hand how eagles live in the wild!