Blog #397 Halfway Mark: Three Weeks

On Osprey Time just moved to blogger.com. Pardon the delay in posting.

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A mother feeding her 3.5-week-old offspring, courtesy of Cynthia Sedlacek.

Three and a half weeks into the nestling period, the three chicks have grown halfway to their adult size with Moana, the eldest, weighing just shy of a pound. Full-grown adult males usually weigh 2 pounds and females can be up to 4.4 pounds. Sleeping through most of the day and all night, the pile of offspring springs to life whenever their father, Orpheus, brings a fish to the nest. When awake, the nestlings waddle around the nest with their enormous feet, defecating over the nest edge, and peering over the rim at the people below. They are starting to use their wings for balance and rock forward on them to get a good stretch. When it’s mealtime, the eager chicks make their way across the nest to get fed by their mother Ursula.

Once their crops (storage organs; see second photo below) are full of food, they find a place to snooze. This is Ursula’s first brood and she is being an attentive mother, staying close to her chicks during this time because they are vulnerable to predators, and they can’t thermoregulate their temperature yet. Ursula spends up to 9 hours a day, every day, creating shade for her offspring by holding her wings partly open or covering the chicks from the rain. Osprey observers call this remarkable behavior the “mombrella” pose.

Shading her chicks by holding her wings partially out (“Mombrella” pose), courtesy of Cynthia Sedlacek.

Moana with a bulging crop of food.

Orpheus, the father, is in charge of hunting to feed the family from the beginning of the breeding season until the fledglings leave on migration. After catching a fish, Orpheus either delivers it whole to the nest or takes his share—usually the head. Ursula takes the fish, rips off pieces of fish flesh, and gently feeds it to her offspring starting the moment the young can hold their heads up. For the next five weeks, the parents feed the chicks directly—Ursula and sometimes Orpheus ripping morsels from the fish and giving them individually one chick at a time. Usually by five weeks old, the nestlings learn to peck at the fish and feed themselves.

Now that the chicks are three weeks old mealtimes can be a bit of a frenzy. However, Ursula will maintain order. Typically the largest, most aggressive nestling gapes first and wide so it gets fed first. When (s)he’s full with a bulging crop—the smaller siblings get to eat. Ursula usually eats after the chicks, consuming whatever is left such as the fins, spines, scales, and other indigestible materials. Like owls, other hawks, and eagles, Ospreys periodically expel or “cast” pellets of these inedible parts.

Sigmoidal Growth

The gist of the last blog #396 The Race Against Time and this blog is that Osprey chicks grow fast—extremely fast—going from newborn to being able to fly in six short weeks. Most bird offspring grow rapidly in an "S” or “sigmoidal growth curve." The graph below is used ubiquitously in ornithological literature to show schematically how baby birds grow. Osprey hatchlings grow slowly during their first two weeks. At three weeks old, our nestlings Moana, Makani, and Lani are now in the accelerated growth phase and observers claim you can see a size difference between morning and evening “like watching grass grow.” Their growth will slow down again when the nestlings approach 28 days old and their adult size. Female chicks will continue to grow but the male chicks will taper off. When they are full-grown, the females will be twice the size of the males.

S-shaped growth curve showing the rapid growth period, modified from Dyfi.

Adult male bringing a fish to the nest for his family’s lunch, courtesy of Cynthia Sedlacek.

Raising baby birds from egg to migration is hard work for both Osprey mates. It is especially energetically

stressful for Orpheus who has to catch enough fish for five hungry mouths. During the hotter summer

months, Orpheus tends to fish from daybreak until noon. His frequency lets up a bit until late afternoon

when dinner begins. Today for example was 84º and humid. Orpheus caught and delivered nine 9-12 inch

fish to the nest from 9-noon and it took an impressive average of 5 minutes to catch each fish. Ursula will

probably lose about 10% of her body mass from the start to the end of the breeding season. Orpheus will

lose less body mass since he can eat whenever he wants.

Preening as Feathers Emerge

Preen gland next to the arrow.

All birds spend a great deal of time preening their feathers, which keeps the feathers in good shape and

spreads protective uropygial oils on them to maintain them. At three weeks, the nestlings begin to get their first set of adult feathers unfurling from the pin (developing) feather tracks. The hatchlings start preening as their feathers begin to push out of their sheaths, which is presumably a very itchy process. Once the feathers are out of their sheaths and open up, the hatchlings spend a lot of time pushing, brushing, and running their beaks through them to keep them in good shape in preparation for flight.