Blog #468 What’s for Dinner?

Looks like yellow perch is on the menu. Good job, Orpheus! courtesy of Cindy Sedlacek.

The nestlings are thriving, growing, and changing daily fueled only by Cayuga Lake fish and the care of their parents. Ospreys are like engines that run on fish. The bird extracts needed energy to form high-energy ATP (adenosine triphosphate) from its food by breaking down its tissues into its molecular components. They must eat enough fish to maintain their body’s high metabolism and internal temperature of 101ºF.

Ospreys are opportunistic hunters taking whatever fish they find provided they are not below 3 feet deep—the Osprey’s diving limit. Fish species vary in their nutrient and energetic values as well as their percentage of edible parts requiring the Osprey to consume several small fish or a few larger ones daily to meet its energy requirements. This variability makes it difficult to provide the exact number of fish an Osprey eats daily.

Numerous studies estimate that an adult Osprey needs slightly less than a pound (400 g) of fish meat per day to satisfy its own energy needs. Although some catches weigh as much as 2 1/2 lb. the average fish captured is only 1/3 lb. to 2/3 lb. Only the meat of the fish is edible, and the amount available to the Osprey varies with the species and size. Charts at the end of this blog show the average weights of fish Orpheus frequently captures.

The number of fish a single Osprey must eat per day can be approximated using a 1 lb. benchmark as follows:

• 4 fish of about 1/4 lb.

• 2 fish of about 1/2 lb.

• 1 fish of 1lb. or larger

Osprey may catch a variable number of fish depending on the species and whether the bird consumes the entire fish or eats less, before discarding it. Bulky fish such as suckers and bullheads have more edible parts than sunfish providing a larger meal.

These doppelgängers for the Salt Point’s crew are nearly identical to O’Leary (second born), 3-week-old O’Connor (eldest), and the youngest, O’Shea, still with down, courtesy of Flicker.

During the breeding season, Orpheus must double his hunting efforts to feed his mate Ursula. The sight of newly hatched chicks prompts Orpheus to immediately double or triple his catch to meet the growing needs of his family. Studies indicate that adult males with dependents average a total of 1 1/2 hours per day in flight actively hunting although they will extend hunting sessions if necessary. Aside from time spent eating, preening, and aiding in nest defense, the males spend approximately 13 of the remaining 16 hours of daylight in inactivity. The extreme is a male observed hunting for a total of 4.7 hours or 31 % of the active day.

On average, an Osprey can catch a fish on 70% of his dives, largely depending on the weather. Wind disturbs the surface of the lake, impacting Orpheus’ ability to hunt his prey. Any amount of wind makes foraging more difficult, requiring more fishing trips to feed the family. Once wind speeds reach above 6-7 m/s, Ospreys will stop trying and take cover in their preferred trees. For Orpheus, this is a stately cottonwood just north of his nest.

Ospreys typically catch and carry fish between 1/3–1 lb. although very large fish weighing over 4 lb. could be carried by a large (>4 lb.) female. Catching a heavy fish can be dangerous, especially if the bird’s talons get stuck in the fish’s meat or bones, making it difficult to release if necessary.

The additional hunting trips in the first 6 weeks take extra energy. During this time a male Osprey will typically consume the head of the fish before taking the rest to his begging mate, but he does not increase his caloric intake. He reserves any additional fish caught for his three offspring. As a result, Orpheus loses weight over the breeding season but gains it back quickly before his fall migration.

Brown bullhead, a favorite meal of the Cayuga Lake Ospreys, by N. Amer. Native Fishtanks.

Extended periods of cold, wet weather combined with the affiliated difficulties of fishing can be fatal to the youngest chick and sometimes the entire brood. Ursula, like many female Ospreys, is an extraordinarily tough and dedicated mother able to keep her brood warm and dry despite driving rains and biting-cold winds.

Fish are not formally stored or cached, but, depending on the weather, partially eaten fish may be kept in the nest for a few hours for later consumption. In warm weather, Ospreys discard whatever fish remains after all family members have eaten. After all their efforts hunting, Ospreys occasionally drop a fish. Most will leave it and return to the hunt, but a few will attempt a recovery.

Orpheus splashing down as he grabs a fish, courtesy of Cindy Sedlacek.

Few bird species have diets as specialized as the piscivorous Osprey. Over 99% of their diet is comprised of fish. Most Ospreys eat only fish throughout their lives. However, periods of bad weather, large fish die-offs, or freezing temperatures can cause sudden, severe fish shortages that force starving Ospreys to seek non-fish prey. Such food stress will cause Ospreys, especially younger, less experienced ones, to catch non-fish aquatic prey such as snakes, muskrats, small rodents, aquatic baby birds, mollusks, and very rarely carrion. There is also a legendary Osprey eating a small alligator. Curiously, there is a male Osprey currently nesting at a campground at the north end of Cayuga Lake that routinely catches water snakes for his family despite the lake’s abundant fish. The Osprey gets quite a welcome when he flies over the campground with the snake dangling from his talons.

Interestingly, Ospreys commonly employ the same high-diving technique used to catch fish when they hunt non-fish prey. The rare sightings of Ospreys stalking mice, rats, and squirrels report the birds did so walking awkwardly on the ground. Under normal seasonal conditions, these aberrant non-prey feeding behaviors are the exception and unusual in habitats, such as around Cayuga Lake, where fish are readily available.

Eyes to the sky!

Candace

Candace E. Cornell

Salt Point Natural Area

Cayuga Lake Osprey Network

cec222@gmail.com

Guest photographer:

Ithaca’s Cindy Sedlacek brilliantly depicts the Ospreys of Cayuga Lake with the intimacy of a true bird lover. Enamored by the animal’s gorgeous wings and graceful flight, her work is a tribute to the power and grace of these stunning raptors.

A black and white drawing of an eagle

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