Blog #386 What is for dinner?
Orpheus carrying a white sucker, courtesy of Cynthia Sedlacek.
The elegant 100-foot dive of the Osprey displaying its remarkable athletic ability was celebrated in Blog #385. In this blog, we’ll look at the object of their efforts—fish. Ospreys are excellent anglers. Most sources claim that Ospreys can catch fish at least once in every 4 dives, with success rates sometimes as high as 70–80%. The average time they spend hunting before making a catch is about 12 minutes— something to give fishermen pause. The success rate varies throughout the breeding season and can be affected by changes in the water depth, time of day, fish availability, and weather conditions, especially the wind speed.
Ospreys eat different species of fish, depending on the habitat type they forage. Studies in the US on the Osprey diet have identified at least 80 species of fish. Ospreys are opportunists and will eat whatever fish is readily available to them, with only a few species of fish dominating their diets in a locality. Although rare, large female Ospreys weighing 4.4 lb. have been observed lifting fish close to the bird’s weight.
Ospreys foraging in freshwater tend to take the same type of fish throughout the season. In contrast, Ospreys foraging in estuaries and saltwater consume a wider variety of fish as these habitats are more susceptible to fluctuating fish availability due to fish migration, spawning periods, or regular seasonal movements.
The Brown Bullhead is one of Orpheus’s favorite fish, courtesy of Flicker.
The table below shows the dominant fish in an Osprey’s diet. In some cases, such as in Paines Prairie, Florida, the diet of an Osprey included only one type of fish.
Locality Type and proportion of fish eaten by Ospreys
It is difficult to determine the number of fish Ospreys eat daily. The species and size of the fish Ospreys eat vary along with their energetic value and edible content. Besides, Ospreys do not always eat every edible part of the fish they catch. It is estimated that an adult Osprey needs approximately 0.88 lb. of fish meat per day to satisfy its energy needs. These studies also found that Ospreys caught fish between 0.1 to 2.6 lb but most were between 0.33–0.66 lb. An adult Osprey consumes approximately 0.66 lb. of fish meat in a meal and then proceeds to discard the fish regardless of its size. Given this observation, an Osprey may catch a variable amount of fish depending on whether the bird eats the full 300 g from it or less, before discarding it. Ospreys flying with a fish in their talons may appear to be wandering, but in reality, they are either heading to a perch to consume it or to the nest site to deliver the fish to the calling female and chicks.
Males take their catch to a perch where the bird tears small chunks to ingest. They begin with the lips and consume the entire head. Once satisfied, he brings the remainder to the nest for the female and her brood. Ospreys may cache fish for consumption for several hours but this depends on weather conditions. In cold weather where fish preserves better, Ospreys have been observed carrying partially eaten fish. In warm weather, Ospreys tend to discard partially eaten fish.
Yellow perch is one of Orpheus’ favorite meals, courtesy of Wikipedia.
Ospreys differ from other raptors in their diet and foraging strategies. Over 99% of an Osprey's diet consists of fish. Observations of Ospreys hunting and consuming other kinds of prey have been linked to the scarcity of fish arising from bad weather, partially frozen bodies of water due to an early arrival to the nesting grounds, or fish die-offs.
Most people have heard of owl pellets–lumps of material consisting of the indigestible parts of a bird’s diet—but they are not the only bird that regurgitates such pellets. Ospreys and other raptors along with
Orpheus casts a pellet of indigestible fish parts, courtesy of Cynthia Sedlacek.
grebes, herons, cormorants, gulls, terns, crows, jays, dippers, shrikes, swallows, and most shorebirds. cast pellets a few hours after they’ve eaten their prey. Ospreys often consume their prey whole, including parts that are not easily digestible such as scales and bones. These parts get as far as the proventriculus, an organ located between the esophagus and the gizzard, where they are packed into a pellet. A pellet’s size correlates to the size of the bird. Osprey pellets are usually a few inches long.
Interestingly, when non-fish prey is taken, Ospreys use the same hunting technique that they use to catch fish. On rare occasions, Ospreys have been observed catching and eating snakes, muskrats, aquatic baby birds, mollusks, and even a small alligator. A male Osprey at the River Crossing Campground Marina off Route 89 in Tyre frequently takes water snakes to his nest. It appears that young Ospreys are more likely to consume these non-fish prey than experienced adults. On a few rare occasions, Ospreys have been observed feeding on carrion. Other unusual observations report Ospreys attempting to capture small ground mammals such as mice, rats, and squirrels walking on the ground. These feeding behaviors are rare and are not expected in conditions where fish is readily available.
Eyes to the sky! Candace
Candace E. Cornell
Friends of Salt Point, Lansing, NY Cayuga Lake Osprey Network
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