A trend for brightly coloured and inappropriate nest boxes endangers young birds, the RSPB warned on Friday.

The charity said it had received a rise in reports from the public over the past five years of wacky and ornamental boxes that put style over substance, such as ones in the shape of windmills and caravans.

Brightly coloured boxes could make baby birds more conspicuous to predators such as cats and squirrels, it said, urging shoppers to buy traditional wooden boxes instead.

Other boxes to avoid include those made from unusual materials such as ceramics, which might make boxes too cold for nesting birds, and metal ones which might overheat the birds inside on hot days or harm them on their way in and out.

“People tend to forget that a nest box will eventually contain tiny helpless, vulnerable baby birds so the appearance of the box should be the last thing on your mind,” said RSPB wildlife adviser, Ben Andrews.

“We have noticed in the last five or so years more and more people alerting us to places are selling unsuitable nest boxes. There’s been more of an emphasis on people’s gardens becoming more ornamental.”

He said people were unwittingly picking unsuitable materials, shapes and sizes. “It’s not to say a bird won’t try and use it. But the bird doesn’t really know it’s unsuitable until maybe it’s too late.”

For example, with a metal box, nesting birds could suffer extremes of heat and cold. “When it gets warm, that could incredibly heat up and the chicks inside could perish. You might also get opposite with box like that.”

He said that an increasing proportion of boxes being sold online, where distinctive designs could make them stand out in listings, was contributing to the problem.

Instead, people should opt for classic wooden boxes with a v-shaped roof that are waterproofed, perch-free and with the right size holes to keep predators out. If consumers did want to opt for an ornamental box then they should block up the holes to stop birds getting in, Andrews said.

Nest boxes play an essential role as a stand-in for holes in trees and hedgerows for garden birds such as blue tits, robins and house sparrows. The 14-21 February is ‘Nest Box Week’, which encourages people to put up the boxes in their gardens.

