Blog #398 All in the Month of June


Four-and-a-half-week-old chicks with rusty-golden feathers around their neck, courtesy of Andy Morffew.


Ursula’s and Orpheus’s nestlings are in their fourth week of life and are looking more and more like their parents as their feathers grow in. At 2 weeks of age, their first feathers start replacing the down with golden-rusty feathers around the head and neck. Darker body feathers follow. By 20–25 days, the nestlings’ primaries, secondaries, and rectrices (outer feathers of the wing and tail) have grown in—and they itch. Finally, the flight feathers grow in on the wings and tail. These are the longest feathers and take time to grow. Once these are formed, the fledgling is ready to fly!


During the period of rapid growth between 15–30 days, chicks were gaining up to 0.1 pounds a day (about 2–3% of their final weight). After 30 days, they are 70–80% of their adult size, and their period of rapid growth has slowed. The males' growth is almost complete, but from day 30–35 females will continue to put on weight and grow. Therefore, weight is a reliable tool for sexing nestlings older than 30–35 days. Adult male Ospreys weigh around 2 pounds whereas adult females can weigh 4–4.4 pounds. Interestingly, males have less weight to gain after 35 days and therefore mature sooner and sometimes fledge before older females. Nestlings in areas with limited food supplies grow more slowly than their well-fed equivalents and usually fledge later.

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


Ospreys in areas with limited fish have smaller broods. Well-fed nestlings rarely fight, but when food is scarce, the oldest, largest, dominant chick will push its way to its mother for food, blocking the younger smaller chicks' access to food. When the largest chick is full, the younger ones eat the remainder of the fish. Sometimes food is so scarce that there is only enough food for one or two chicks and the younger ones perish. Parents do not interfere because it’s in their best interest to raise fewer well-fed healthy offspring instead of three or four weak ones. Siblicide (the death of a chick resulting from aggression by one or more siblings) is common in raptors, but only in Ospreys suffering from food insecurity. Fish availability is not an issue for the Ospreys living in the Greater Cayuga Lake Basin.


Offspring get all the water and nutrients needed for growth from the fish they eat. During the breeding season, researchers measured fish consumption and found that most fish were caught during the rapid growth period and the least was taken, as expected, during incubation. Even though nestlings continue to need ample food to grow, Orpheus and the other fathers level off the number of food deliveries to the nest when the nestlings are 30 days old. Researchers have found that a week before the chicks fledge (at about 33–48 days old) parents decrease the amount of fish they feed the chicks. Some think this encourages the young to fly sooner.



Ursula feeding Moana (L) and Makani (R), courtesy of Karel and Cynthia Sedlacek.



Eyes to the sky!

Candace


Candace E. Cornell 

Friends of Salt Point, Lansing, NY

Cayuga Lake Osprey Network

cec22@cornell.edu

  


EYES TO THE SKY!


WATCH!

Salt Point Osprey Nest Cam


READ!

On Osprey Time: Ospreys of the Finger Lakes


VISIT!

Cayuga Lake Osprey Trail


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