Blog # 467 Growth Spurts

May 30, 2024

Orpheus brought a yellow perch, which Ursula feeds to the chicks.

The Salt Point nestlings are about to pass another milestone as they approach two weeks old. The 11-day old chick, still covered with the camouflaging striped gray and buff-colored down, looks completely different than its older siblings. The eldest is two weeks old and entering the Reptilian Phase, a transitional phase where the buff down is quickly being replaced with a dense, wooly down, which will last another two weeks. In the Reptilian Phase the chicks resemble prehistoric raptors with oversized bluish-grey feet with long, black talons.

A representation of Bambiraptor, courtesy of Fossil Wiki.

The unearthing of Bambiraptor, a dinosaur thought to have feathers, led to intriguing new ideas about the strong links between modern birds and dinosaurs. The modern roadrunner’s anatomy is similar to Bambiraptor: it has an S-shaped neck, a pubis bone in the hip that points backward, a V-shaped wishbone, folding arms, and three primary toes on the feet.

Modern day roadrunner has similar anatomy to Bambiraptor, courtesy of Jennifer Greenwald.

All three nestlings are feeding well and growing like lightening. Instead of having teeth to grind food, Ospreys digest their food using a two-part stomach. The first chamber is known as the crop where saliva-like chemicals begin to break down the food. Once the food is coated in these chemicals it is moved to the second chamber called the gizzard. Here the food is ground using sand and grain ingested by the bird and stored until digestion can be completed. As the young become satiated on fish, their crops are noticeably distended with food. A conspicuous light-tan streak runs down its back for added camouflage against the background of the nest.

R: 10 day-old (left) and 14 day-old (right) chicks, L: Full, bulging crops on the chicks

courtesy of Harry Collins

Salt Point’s siblings are imprinting and forming critical attachments with their parents, especially their mothers, and learning to be Ospreys. The nestlings are well camouflaged, their defense against aerial predators, with a light spinal stripe and dark sides that resemble twigs in an empty nest. Ursula has taught her brood to lay flat immediately and remain still in the nest when she gives a sharp warning call—a harsh Cheerp!

The highly mobile chicks have no trouble backing up to the nest rim to defecate although they sometimes miss and hit their mom. The eldest and biggest nestling easily pushes aside its siblings to be the first to feed with its large orange eyes and enormous beak, but all the siblings are fed. Their enlarged crops tell the story.

Osprey chicks grow slowly during their first days after hatching with their weights doubling every week in these early stages. At about ten days old, they enter an accelerated growth spurt for 20 more days until approaching their adult size by day 30. During this accelerated growth phase, represented in the graph below, the appearance and size of the chicks seems to change daily as they experience this massive growth. By day 35, the growth of the male chicks levels off while the females continue to grow, becoming much bigger than the males. In lieu of taking leg bone measurements, the size differences of the sexes can be used to distinguished them.

GROWTH RATE OF OSPREY CHICKS

Growth rate of Ospreys, courtesy of Montana Osprey Project

Osprey chicks grow fast—very fast—but the RATE of the growth is not a constant. They start to develop slowly but, accelerates this growth until it hits a maximum at about 20 days–at which point, something interesting happens. Sexual dimorphism takes over; the female Ospreys keep growing while the growth of the males plateaus. Females need to be large to defend the nest, but the males are hunters and need to be fast and agile.

It is a race against time: the sooner the Osprey chicks grow, the more time available to learn flying and fishing, the life skills they must learn before migrating in September.

Eyes to the sky!

Candace

Candace E. Cornell

Cayuga Lake Osprey Network

Salt Point Natural Area

cec222@gmail.com

A black and white drawing of an eagle

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