Is the spectacle of birds bustling around a nest box just a selfish pleasure? The question arises, reading a study published in May in the journal of ornithology Avian Research. This compilation of more than 300 research papers published since 2003 highlights the negative effects of these artificial shelters when they are poorly designed or installed. But "the building materials, color, smell and structural design" of many nest boxes are unsuitable, the authors of the study observe, while their location is often inappropriate. Defects that can lead to "a reduction in reproductive success" - the opposite of the desired effect.
Well placed, these roosts nevertheless promote the growth of passerine populations that nest in cavities increasingly rare in the wild. The first advice of the League for the Protection of Birds (LPO) to garden owners is to plant trees, conserve dead wood and preserve hedges to provide shelter for small wildlife. Failing this, the association also recommends the installation of nesting boxes. "But there are rules to follow to prevent the birds welcomed from diseases and predation," says Clémence Lerondeau, head of the LPO's Refuges program.
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The first advice is to buy a good quality birdhouse, not a small 8 mm wooden box, with a hole and a nice perch, like most of those sold in supermarkets, says Grégoire Loïs, ornithologist at the National Museum of Natural History. It takes a thick wood to protect the chicks from rain and heat, but also a certain depth so that the squirrels cannot catch them. " The LPO recommends giving preference to raw wood (not stained or treated) to get as close as possible to the appearance of natural cavities. The authors of the study note that blue and green are an effective camouflage.
Identify the bird you want to welcome. "If the installation rate is very high in chickadees, rarer species, such as the woodcreeper or the gray flycatcher, are more demanding and the nest box can remain empty," notes Grégoire Loïs. It is also interesting to target swallows with artificial nests or bats. " Each species corresponds to a nest box, with a flight hole of different sizes and shapes. Be careful, however, not to install too many nest boxes for the same species, which could lead to competition between individuals.
When to put your nest box? As soon as possible, to allow the birds to spot it upstream but also to take refuge there during the nights of extreme cold in winter. But a nesting box laid late in the spring can be invested in a few hours.
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When installing, choose a quiet place, away from artificial light. Avoid placing the shelter on a branch. Hang it directly with a wire on a trunk, interspersing small pieces of wood to protect the tree, at a height of between two and five meters. "Orient it to the east or southeast, sheltered from the wind," adds Clémence Lerondeau.
To avoid parasites, ornithologists recommend cleaning the nest box by ridding it on All Saints' Day of the remains of the previous nest with a metal brush. Linseed oil can be applied to ensure good sealing and preservation of the wood. Grégoire Loïs finally calls to check that the nest box is not occupied by other visitors, such as bats or hornets. Which, even if it was not the expected result, "is also a success" from the point of view of biodiversity, notes the researcher.