Ospreys feed almost exclusively on fish, and they have a unique hunting strategy. While hunting for fish, Ospreys typically catch the type of fish that is more abundant in the area. But, studies have shown that some individuals show a preference for certain fish types and sizes. This page discusses the most relevant aspects of Osprey feeding ecology.
- How does an osprey catch a fish?
- How do ospreys see fish and are so good at catching them underwater?
- What time of day do osprey hunt?
- When Ospreys dive and plunge, do they catch a fish every time?
- What type of fish do Osprey eat?
- How many fish does an Osprey eat in a day?
- What size fish can an Osprey carry?
- Do Ospreys eat the whole fish?
- It is rare for ospreys to take in non-fish prey items in addition to fish
How does an osprey catch a fish?
Ospreys catch fish by spotting them from the air or while perched over the water, diving and plunging into the water to catch a single fish, and taking off from the water.
If the attempt is successful, the bird proceeds to take the fish to perch for consumption. If not, it searches for another fish.
Osprey hunting behavior can break down into the following sequence of events:
- Spotting a fish: The bird flies over the water at a height of 30 – 130 feet. During its scan of the water below, an Osprey uses wing strokes interspersed with gliding, depending on the wind speed. Osprey biologists suggest that foraging birds use search images to identify and quickly react to a potential prey.
- Dive: Once a potential prey has been spotted the Osprey hovers briefly and quickly decides whether to dive or move on for a better opportunity. It is possible for the Osprey to dive without hovering when the opportunity arises and has potential to be a successful catch. Likewise, diving without hovering may occur when the prey is moving and there is no time to hover before diving for the fish.
- Plunge: Osprey dive on a vertical or diagonal trajectory depending on the location of the fish. In order to gain speed or cover some distance, the bird may take some wing beats. Before plunging into the water, the Osprey extends its legs forward.
- Take off from the water: If the bird catches a fish, it may take some time to accommodate and grasp the struggling fish before it takes off. Larger fish take longer to handle. If the attempt fails, the bird takes off quickly.
Observe how this Osprey moves its legs forward just before plunging into the water.
How do ospreys see fish and are so good at catching them underwater?
Ospreys are good at catching fish underwater because they can see them very well. Birds, including ospreys, have evolved physiological and behavioral adaptations to have the best eyesight.
The vision of birds of prey is exceptional. As a comparison, a standard definition video would be what human vision is, but raptors have 4K high-definition vision.
Ospreys have exceptional vision due to three main adaptations.
- Humans have only one fovea, which is a depression in the retina densely filled with color-detecting cone cells. Our fovea is responsible for our sharp vision and detailed focus. Birds of prey often have two foveas, which creates a larger field of sharp focus.
- Birds also have a structure called “pecten”, which contains photoreceptor cells responsible for vision resolution. Humans have photoreceptors in the pecten at a density of 200,000/square mm and birds of prey have an approximate density of 1,000,000/square mm.
- Birds of prey have Crampton’s muscles, which aid ocular lenses to see prey and objects at great distances.
In addition to physiological vision attributes, Osprey uses behavioral adaptations that make them effective at catching fish underwater. Ospreys position themselves at steeper angles relative to the potential prey, which is crucial to reduce the effect of the refraction caused by the water.
A steep angle of attack may cause a magnified impact with potential consequences on the attacking bird. Birds of prey reduce the impact of their attacks by attacking their prey from a diagonal angle. Because the water has a cushioning effect, ospreys can attack their prey at steep angles.
Ospreys use an “image search” approach to find and identify their favorite fish. Studies have shown that some Ospreys prefer some species of fish over others as preferred fish are caught in numbers that are not consistent with the amount available in nature.
What time of day do osprey hunt?
Osprey hunting times vary with locality and are influenced by weather conditions, particularly wind speed and tide schedules, where tides occur.
A study of foraging Ospreys found that birds tend to hunt more often at dawn and dusk. However, Ospreys concentrated their hunting efforts and were more successful during mid-tide regardless of the direction to low or high tide.
An Osprey’s hunting schedule and success are affected by wind speed. According to field observations, ospreys were more successful in catching fish during calm wind hours. As the wind speed increases, the water becomes choppier, and Ospreys have less success catching fish. Consequently, Ospreys tend to avoid hunting during high winds.
The hunting success rate of Ospreys is not affected by overcast or bright days or even rain.
When Ospreys dive and plunge, do they catch a fish every time?
The rate at which Ospreys catch fish depends on whether birds forage in deep or shallow waters, water visibility, and prevailing wind speed.
Ornithologists Machmer and Ydenberg (1990) reviewed 13 studies that analyzed Osprey foraging success and found that the lowest success rate was 24%, meaning that of 100 dives and plunges, only 24 resulted in Ospreys catching a fish. This is to say that of every 10 dives only 2 were successful.
In other studies, the success rate was up to 82%, meaning that of every 10 dives more than 8 were successful. In this study, Ospreys caught a fish every 11.8 minutes on average.
The success rate varies throughout the breeding season. It can be affected by changes in the water depth of the hunting areas, changes in fish availability, and weather conditions, namely wind speed.
Another study found that dive success rates did not vary throughout the day, but the success rate was highest during mid-tide.
What type of fish do Osprey eat?
Ospreys eat different kinds of fish, depending on the habitat type they forage. Collectively, studies on the Osprey diet have identified over 80 species of fish throughout the species’ range.
A general rule is that Ospreys will eat the type of fish that is most readily available to them, and only two or three species of fish dominate the diet of Ospreys in a locality.
Ospreys foraging in freshwater take essentially the same type of fish throughout the season.
Ospreys foraging in estuaries and saltwater consume a wider variety of fish types. A broad fish diet is because these habitats are more susceptible to changes in visibility through the water and fish availability due to fish migration, spawning periods, or regular seasonal movements, which impacts what type of fish is available Ospreys.
The table below shows that two to three types of fish dominate 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 |
| Eagle Lake, ne. Califormia | Tui chub (Gila bicolor) (48%); rainbow trout (Salmo gairdneri) (34), tahoe suckers (Catostomus tahoensis) (18%) |
| Cheppewa Nat’l. Forest, MN | Bluegills (Lepomis macrochirus) (35%); black crappie (31%); unidentified/other (34%) |
| Paynes Prairie, FL | Sunfishes (Centrarchidae) (95%) |
| Chesapeake Bay | Menhaden (Brevoortia tyrannus) (75%); unidentified/other (14%); white perch (Morone americana) (7%) |
| Creston, British Columbia | Tui chub (48%); rainbow trout (34%); tahoe suckers (18%) |
How many fish does an Osprey eat in a day?
An Osprey eats a certain number of fish each day based on the size of the fish available and its energy needs. An Osprey consumes several small fish or a few larger ones daily to meet its energy requirements.
Fish have varying energetic value and edible content, and Ospreys do not always eat every edible part of the fish they catch. Therefore, it is difficult to provide the number of fish an Osprey eats daily.
Three separate studies estimated that an adult Osprey needs approximately 400 g (0.88 lb) of fish meat per day to meet its energy needs. These studies also found that Osprey caught fish between 50 to 1200 g, (0.1-2.6 lb) but most were between 150-300 g (0.33-0.66 lb).
The number of fish an Osprey eat per day can be approximated using the 400 g (0.88 lb) daily energetic needs as follows:
- 8 fish of about 50 g (0.1 lb)
- 4 fish of about 100 g (0.22 lb)
- 2 fish of about 200 g (0.44 lb)
- 1 fish of 400 g or larger (0.88 lb or larger)
It is important to note that an Osprey may catch more fish than those calculated here. Field observations indicate that an adult Osprey consumes approximately 300 g (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 east the full 300 g from it or less, before discarding it.
What size fish can an Osprey carry?
Multiple field studies of foraging Ospreys found that most fish caught and successfully carried away for consumption ranged between 136 g to 408 g or 0.3 lb to 1 lb.
Although rare, large female Ospreys weighing 4.4 lb have been observed lifting fish close to the bird’s weight.
Large fish can pose a danger, though. Ospreys have drowned after catching fish so large that they get dragged to the bottom.
Ospreys have specialized talons adapted to catch and firmly grip a fish. The talons are hooked for a better grip, but they can also get stuck in the fish’s meat and bones, making it difficult to release it underwater.
Ospreys are often seen flying around with a fish in their talons. They 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.
Do Ospreys eat the whole fish?
An Osprey can swallow a whole fish at once only when it is very small. Most fish are taken to a perch where the bird tears small chunks to ingest. Regardless of the size of fish, field observations indicate that an adult Osprey consumes approximately 300 g of fish meat in a meal; then, it proceeds to discard the rest regardless of the size of the fish.
Ospreys do not store or cache fish for consumption days after they catch one. They may keep fish for consumption hours later but this depends on weather conditions. In cold weather where fish preserves better, Ospreys have been observed carrying partially eaten fish for later consumption. In warm weather where fish do not preserve well, Ospreys tend to discard partially eaten fish soon after they have enough for a meal.
It is rare for ospreys to take in non-fish prey items in addition to fish
Ospreys differ from other raptors in their diet and foraging strategies. Other raptors and even other birds do not come close to the level of specialization on only one type of prey hunted with a distinct strategy.
Ospreys primarily eat fish. Studies have shown that over 99% of their diet consists of fish. Most Ospreys studied have never been seen eating anything other than fish. They usually eat only fish throughout their lives.
Observations of Ospreys hunting and consuming other kinds of prey have been linked with periods of hunger due to the scarcity of fish. Periods of bad weather, partially frozen bodies of water due to an early arrival to the nesting grounds, fish die-offs, or other circumstances cause fish to become scarce.
Interestingly, non-fish prey was mainly caught in the water using the same hunting technique that Ospreys used to catch fish. Ospreys have been observed catching and eating snakes, muskrats, small rodents, aquatic baby birds, mollusks, and even a small alligator. Young Ospreys are perhaps more likely to consume non-fish prey than experienced adults.
On 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 while walking on the ground. The feeding behaviors described here are rare and are not expected in conditions where fish is readily available.
References:
- Poole, A. (1989a). Ospreys: a natural and unnatural history. Cambridge, U.K: Cambridge Univ. Press.
- Mclean, P. K. and M. A. Byrd. (1991a). Diet of Chesapeake Bay Ospreys and their impact on the local fishery. Journal of Raptor Research 25:109-112.
- Machmer, M. M. and R. C. Ydenberg. (1990). Weather and Osprey foraging energetics. Canadian Journal of Zoology 68:40-43.