Breeding season can be a dangerous time of year for birds. Parent birds are concentrating on building nests, incubating eggs, and rearing their chicks, which means they are vulnerable to predation.
Egg and nestlings are also easy pickings for predators who may be on the hunt for food for their own young. Birds that nest in the trees, shrubs, and bushes in your garden will do their best to tuck their nests away from the prying eyes of predators.
However, any nest boxes you put up in your garden are much more visible, and you should follow a few simple steps to minimise the risk of predation and provide the safest space possible in which wild birds can bring up their offspring.
Common mammalian predators of nests in nest boxes include cats, squirrels, mice, and rats, as well as stoats and weasels who are all excellent climbers.
Larger birds will also take the eggs and chicks from nest boxes including magpies, jays and other members of the crow family, and birds of prey, such as sparrowhawks. Great spotted woodpeckers are another culprit and their ability to chisel and excavate holes means they can drill through wooden nest boxes with relative ease. And starlings have been known to remove nests and birds from nest boxes so they can take over the space for themselves.
Most predators hunt early in the morning so you may not be aware that a predation has happened for a while, but if you do see a predator approach a nest box, the best thing to do is leave well alone and let nature take its course. Although this may be difficult, your actions could frighten the parent birds who may abandon the nest. And the predator still must eat which means it will go searching for another nest to raid.
The easiest way for a predator to gain access to a nest box is via the entrance hole, so choose a bird box that has a small hole which makes it more difficult for predators to enter and get at the birds inside.
However, a determined predator will often chew around the hole to make it larger. Metal nest box plates fitted around the outside of the hole will help prevent this from happening and will also stop larger birds from entering the nest box. You can also use nest box plates to modify a bird box if you want to attract different species of birds.
Another option is to fit a tube to the entrance of your nest box. Small birds won’t mind using the tube to access the box, but predators will find it more difficult to stretch through the tube and reach the birds inside. A piece of PVC or rubber pipe will suffice as a DIY solution. Alternatively, you can buy special nest box protectors that are designed to be screwed to the front of the bird box. Covering a bird box with a wire mesh will also help prevent predators from accessing it.
Some predators, such as squirrels and cats, will sit on top of a bird box and use their front paws to dig into the hole below. Choose a bird box with a roof that extends about 12 cm in front of the box so that predators will find it more difficult to lean over and reach inside. A roof with a long overhang will also provide better shelter from the wind and rain as well as providing protection from the sun on hot days.
Many bird boxes are supplied with perches. Although a perch may appear to be a useful design feature, it can actually be a dangerous addition. Birds don’t need a perch to access a bird house, but a predator can use it as somewhere to hang on to as they attempt to reach inside the box. If you’ve bought a nest box with a perch, remove it before you put it up.
Make sure you place your bird box somewhere where predators cannot easily get to it. Mount it at least 2 metres from the ground and away from overhanging branches from which a predator could jump. Avoid sites where foliage obscures the entrance hole, so birds have a clear flight path in and out of the box and can be aware of any nearby threats.
Locating a nest box in or over spiky or thorny bushes may help deter cats from climbing up to nest boxes, or you could try installing some cat deterrents to keep them out of your garden. If you have squirrels visiting you may want to stop feeding nuts during breeding season. And make sure you practice good hygiene around your bird tables and feeders to discourage rats and other vermin.
Although it can be heart-breaking to find a family of baby birds destroyed by a predator, remember that birds, their chicks, and their eggs are an important part of the food chain and they lay a lot of eggs to cope with losses. For example, it’s estimated that less than half of blue tits eggs fledge and outside gardens the most common cause of death is predation.
For larger birds and animals to survive the pillage of nests is necessary but they also play their part in controlling the number of pests and other unwelcome visitors.