One of the questions we are frequently asked about the White-tailed Eagle project, which we run in partnership with Forestry England, is what do the birds eat?
The White-tailed Eagle is a generalist predator, typically favouring the most seasonally abundant prey. Various diet studies across Europe have shown that fish is favoured when it is available, but that a variety of birds and smaller mammals are also taken according to their availability. Most of these diet studies are based on prey remains collected from occupied nests at the end of the breeding season, including this excellent recent one in Scotland, but this methodology typically underestimates how much fish is caught, and does not provide evidence of how prey was acquired.
We have been keen to monitor the diet of the eagles released on the Isle of Wight from the outset, and with the first pair not breeding until 2023, we have based our work on field observations. This not only provides evidence of what the birds are eating, but also how prey is acquired. The fact that every bird in the population is satellite tracked is extremely helpful in this regard, because it enables us to identify the areas favoured by the birds – including the individual trees and perches they use – and then to base our field studies at these sites. Nevertheless, it requires a huge amount of time and patience. White-tailed Eagles favour the sit-and-wait strategy and often perch for hours at a time between hunting attempts
Our research, involving more than 5000 hours of observation, has been led by Steve Egerton-Read, the White-tailed Eagle project officer who is based on the Isle of Wight. We have now amassed more than 600 observations, 83% of which are in Isle of Wight, Hampshire, Forest and West Sussex southern England, where the first territorial pairs have become established.
We are in the process of writing this work up in a scientific paper, but in the meantime, our key findings to date are summarised below. It is also important to emphasises that there have been no cases of livestock predation since the project began.
What they eat
As expected, the eagles have a broad diet, which is illustrated in the pie chart below. Birds constitute 36%, with Canada Goose the most frequently recorded species; fish make up 25%, with both marine and freshwater species readily caught; and mammals, primarily rabbits and brown hares, account for 24%. Cuttlefish, a marine mollusc, which are abundant in the seagrass beds of the South Coast during spring and early summer when they enter the warm water to spawn, have become another key item. Cuttlefish are typically found low in the water, out of reach of White-tailed-Eagles. However, after spawning, these individuals die at the surface and then become available to foraging eagles. As a result, molluscs account for 5% of diet.
Diet composition of White-tailed Eagles released on the Isle of Wight to date
G274 with a cuttlefish (photo by Ainsley Bennett)
How they acquire food
Overall, the eagles prefer live caught prey, and items attained through direct predation account for 45% of the observations in our study. Carrion constitutes 21%, while piracy from other predatory birds make up 9%. If the observer did not see how an eagle attained its food, it is listed as unknown.
Changes with age
Perhaps the most interesting element of our study is that diet changes with age. Carrion is very important for young birds in their first and second years, and they also favour mammals – particularly rabbits, when they are young. However, as they become older, they tend to gravitate towards wetland sites, where fish becomes the preferred prey item, as illustrated in the graphs below. Fish are either live caught or pirated from other piscivorous birds. Last year, when the first pair of White-tailed Eagles bred successfully, more than 50% of the prey items brought to the nest by the adults birds, G405 and G471, were fish.
Diet compassion by age class (1CY = first calendar year, and so on)
Feeding strategy by age class (1CY = first calendar year, and so on)
G818 with a mullet at Christchurch Harbour in Dorset (photo by Peter Twamley)
G625, the first English wild-fledged chick for 240 years, with a pike (photo by Trevor Goodfellow)
The post White-tailed Eagle diet appeared first on Roy Dennis Wildlife Foundation.
The White-tailed Eagle is a generalist predator, typically favouring the most seasonally abundant prey. Various diet studies across Europe have shown that fish is favoured when it is available, but that a variety of birds and smaller mammals are also taken according to their availability. Most of these diet studies are based on prey remains collected from occupied nests at the end of the breeding season, including this excellent recent one in Scotland, but this methodology typically underestimates how much fish is caught, and does not provide evidence of how prey was acquired.
We have been keen to monitor the diet of the eagles released on the Isle of Wight from the outset, and with the first pair not breeding until 2023, we have based our work on field observations. This not only provides evidence of what the birds are eating, but also how prey is acquired. The fact that every bird in the population is satellite tracked is extremely helpful in this regard, because it enables us to identify the areas favoured by the birds – including the individual trees and perches they use – and then to base our field studies at these sites. Nevertheless, it requires a huge amount of time and patience. White-tailed Eagles favour the sit-and-wait strategy and often perch for hours at a time between hunting attempts
Our research, involving more than 5000 hours of observation, has been led by Steve Egerton-Read, the White-tailed Eagle project officer who is based on the Isle of Wight. We have now amassed more than 600 observations, 83% of which are in Isle of Wight, Hampshire, Forest and West Sussex southern England, where the first territorial pairs have become established.
We are in the process of writing this work up in a scientific paper, but in the meantime, our key findings to date are summarised below. It is also important to emphasises that there have been no cases of livestock predation since the project began.
What they eat
As expected, the eagles have a broad diet, which is illustrated in the pie chart below. Birds constitute 36%, with Canada Goose the most frequently recorded species; fish make up 25%, with both marine and freshwater species readily caught; and mammals, primarily rabbits and brown hares, account for 24%. Cuttlefish, a marine mollusc, which are abundant in the seagrass beds of the South Coast during spring and early summer when they enter the warm water to spawn, have become another key item. Cuttlefish are typically found low in the water, out of reach of White-tailed-Eagles. However, after spawning, these individuals die at the surface and then become available to foraging eagles. As a result, molluscs account for 5% of diet.
Diet composition of White-tailed Eagles released on the Isle of Wight to date
G274 with a cuttlefish (photo by Ainsley Bennett)
How they acquire food
Overall, the eagles prefer live caught prey, and items attained through direct predation account for 45% of the observations in our study. Carrion constitutes 21%, while piracy from other predatory birds make up 9%. If the observer did not see how an eagle attained its food, it is listed as unknown.
Changes with age
Perhaps the most interesting element of our study is that diet changes with age. Carrion is very important for young birds in their first and second years, and they also favour mammals – particularly rabbits, when they are young. However, as they become older, they tend to gravitate towards wetland sites, where fish becomes the preferred prey item, as illustrated in the graphs below. Fish are either live caught or pirated from other piscivorous birds. Last year, when the first pair of White-tailed Eagles bred successfully, more than 50% of the prey items brought to the nest by the adults birds, G405 and G471, were fish.
Diet compassion by age class (1CY = first calendar year, and so on)
Feeding strategy by age class (1CY = first calendar year, and so on)
G818 with a mullet at Christchurch Harbour in Dorset (photo by Peter Twamley)
G625, the first English wild-fledged chick for 240 years, with a pike (photo by Trevor Goodfellow)
The post White-tailed Eagle diet appeared first on Roy Dennis Wildlife Foundation.