To train them to perform tricks for entertainment, food is withheld from the birds, such as falcons, hawks and eagles, which are naturally frightened of people, until their hunger outweighs their fear, a report says.
It’s claimed the tactic – together with forcing them to perform in displays – makes such zoos “a circus by any other name”.
The government is in the process of banning wild animals in circuses, with a bill currently going through parliament.
Freedom for Animals, a charity that opposes animal captivity, says avian veterinary experts believe going without food for three days may cause dehydration and kidney disease.
Depriving wild birds of food, known as “manning”, can take several days.
The group investigated 25 zoos around the UK, filming in 24 of them, finding one in three enclosures did not have clean drinking water.
Birds of prey spent an average of just 11 minutes 18 seconds flying during a public display, the investigation found.
At one zoo, a staff member told visitors how “naturally wild birds are terrified of humans” so to get one to sit on his glove, food is withheld and “then it is a matter of waiting until appetite outweighs fear”.
This “appears to be an extreme approach to training, especially when keepers have a legal obligation to provide food for animals ... we see this carried out widely across UK zoos,” the charity said.
“In captivity there is no excuse for an animal being deprived of food other than for medical or natural biological reasons. When this is being carried out to control an individual for human purposes, this is contrary to welfare.”
Government guidelines on zoos state that food and water are basic needs.
According to the report, in the wild most birds, especially smaller ones, eat daily. A 100g bird will need a quarter of its bodyweight in food and a 1,200g one needs 10.7 per cent of its bodyweight.
Once a young bird or one new to a zoo has been trained, food is still restricted for life. The birds are weighed daily and their food judged accordingly.
Freedom for Animals also noted some shows had loud theatrical music playing, pointing out that this would be stressful for birds of prey with very acute hearing.
“For example, a great grey owl can hear a beetle running through grass 100ft away or a mouse squeaking at half a mile. When a bird has such highly tuned senses we can only imagine the experience of loud music and crowds of spectators,” says the report, entitled Tether and Torment.
Cases where visitors were allowed to walk up to and stroke owls caused “noticeable stress” such as pacing in owls.
According to the charity, a zookeeper said of one owl: “He doesn’t like running children and has freaked out before.”
Some zoos put birds’ wings at risk of damage by making birds of prey fly through gaps in people’s looped arms, which their wings hit.
Witnesses also saw trainers encouraging birds to land on visitors’ heads and move from one person to another.
The birds showed signs of stress such as panting, beak clapping and hissing.
A spokesperson for the British and Irish Association of Zoos and Aquariums said: “The tethering of some birds of prey and the careful management of their diets are both practices recognised within the Secretary of State’s Standards of Modern Zoo Practice – the professional guidance document pertaining to UK zoo licensing.
“Both of these practices enable the birds to exhibit one of their natural behaviours, namely flight.
“The calls for a ban by animal rights organisations to these practices are based solely on their ideology and do not consider rigorous global animal welfare research nor do they reflect the high standards of welfare in our zoos.
“Our 121 zoo and aquarium members are centres of excellence where evidence-based animal welfare, education and conservation is at the forefront at all times.
“As a welfare-centric community, we condemn poor practice, particularly as there are clear guidelines and protocols that prevent it from happening.”
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