Blog # 436 Hatching Starts Days Before

May 16, 2023

The hatching of an Osprey egg is miraculous process. In just five weeks, two cells divide and transform into a graceful hunter and ruler of the sky. The climax of this transformation, the hatching of the baby Osprey, usually occurs in the morning. The average incubation period is 39 days, which makes Ursula’s first egg due to hatch around May 11-13—with the others following a few days apart in the order they were laid. Unfortunately, the Salt Point Nest Cam is not operating because an animal gnawed the electrical connectors leaving the date up to guesswork until the camera is repaired.

Osprey embryo cracking (external pipping) the shell to crack it open, courtesy of Dfyi.

As incubation draws to an end, the growing embryo fills nearly the entire egg. As incubation draws to a close, the embryo starts to retract the residual yolk into its body cavity. Until that point, the embryo has used the chorio-allantoic membrane (CAM) for respiration, diffusing CO2 and O2 through the eggshell and the CAM. (The CAM is represented by the chorion membrane in the illustration below.) The embryo’s oxygen demand increases along with its growth and activity while oxygen supplies from the CAM run low.

The shortage of oxygen triggers the embryo to start lung respiration. The embryo instinctively puts its head under its right wing with the beak pointing toward the membrane separating the egg contents from the air pocket. (I think that is so cool.)

A few days before hatching, the embryo pierces the inner membrane and starts lung ventilation in the air pocket.

Cross section of an bird egg showing the air pocket, courtesy of Brainly.in.

This is called internal pipping. Even though lung respiration has started, the CAM remains important for respiration for a few more days.

A few days before hatching, an embryo will start peeping, communicating with its sibling embryos and its parents, from inside the egg. These faint “care-soliciting” peeps prompt the adults to turn the eggs more often and spend more time incubating. They also build up the sides of the nest to protect and contain the hatchlings.

After 12 hours of pipping the embryo pierces the inner membrane and begins tapping on the eggshell with its egg tooth, a sharp and a strong temporary structure on the top of its beak. After hours of repeatedly tapping the same spot on the eggshell it eventually weakens and breaks. This is called external pipping. Pipping demands great effort. The pipping muscle, a neck muscle used to make this specific movement, is bulging by the time the embryo has finished hatching. From the outside, a small star-shaped crack or hole in the eggshell appears 2–3 days before hatch day, sometimes with the point of a beak sticking out.

After external pipping, the exhausted embryo rests. It has one last challenge: breaking free from the eggshell, which normally takes another 12 hours after external pipping. To get out, the embryo starts cutting the eggshell with its egg tooth. While cutting, the embryo turns around inside the egg, using its wing for direction and legs to apply force. Once it has cut a near circle at the blunt end of the egg, the embryo tries to push itself out of the egg stretching its legs. This causes the last bit of the shell, the cap to break off.

Osprey nestling (top) and hatching in progress below, courtesy of Alan F. Poole.

cap to break loose, allowing the embryo to push itself free. New hatchlings are wet, tired, and vulnerable. Hours after hatching their down feathers dry, and they become fluffy. After they’ve recovered from the hatching process they become more active.

The hitchling used its “egg tooth” (black growth on upper beak) to crack the egg shell, courtesy of OspreyWatch.

Note the size difference between two nest mates born 2 days apart, courtesy of CU Maurice River.org.

Altricial bird eggs are usually smaller than the eggs of precocial species. That is because precocial species are completely developed when they hatch where as the altricial birds have more developing to complete. Osprey hatchlings are semi-altricial birds meaning their young are born nearly helpless, but not completely. They are unable to thermo-regulate their body temperature but are capable of limited motion. Covered with down, the hatchlings are born with their eyes open.

Since the eggs were laid serially about 2–3 days apart, the hatchlings are of graduated sizes with slightly less nutrion as the eggs get smaller. If fish suddenly become scarce, brood reduction, the sacrificing of the youngest hatchling through sibling rivalry, is employed to ensure the more robust nestlings survive. Fortunately, Cayuga Lake offers an ample supply of fish throughout the Osprey season making brood reduction unnecessary.

Eyes to the sky!

Candace

Candace E. Cornell

Cayuga Lake Osprey Network

Friends of Salt Point

Lansing, NY