Katrine Turner, MSc, PhD

By Katrine Turner.

Where would starlings build their nests if we didn't put up nesting boxes for them? Before humans came along and took control of the landscape, birds built their nests in old trees, in places hidden by tall grass, in bushes or on rocks, but many of these options have gradually disappeared as we have expanded, and birds often lack opportunities to breed. Most urban spaces would benefit greatly from putting up nest boxes for birds as a replacement for their lack of natural nesting places.

Different bird species have different preferences as regards where, when and how they want to build their nests. Swallows and swifts build under canopies, eaves and in old barns, where they are a pleasure to watch as they elegantly dash in and out—a sure sign that summer is on its way. Swallows mix mud with their saliva, and they are good at repairing old nests to keep using them. Blackbirds weave their nests out of twigs and moss and hide them in bushes, hedges or treetops. However, many of the birds we see in urban areas prefer to use spaces they could previously find in old trees. These birds would be happy to use nesting boxes. The size of both the nesting box and the entrance hole can be adapted to attract specific bird species. And maybe you didn’t know that it’s not only the classic garden bird species that benefit from man-made houses?

It's important to think about the size of the entrance hole and location when setting up bird boxes, as some species can be quite picky. Thought it’s always better to have a random or incorrect hole size than not put up a bird box at all. It’s a good idea to adapt the configuration to the species of bird you want to inhabit the box. Some species such as sparrows or starlings like to live close together in colonies, while others such as great tits and blue tits are territorial, so the boxes must be put up at a good distance from each other, or neighbourly disputes will occur. Similarly, some birds want to breed in the shelter of bushes, while others would prefer to be up high and have a good view, e.g. kestrels. We have gathered this information together in the table below.

You won’t necessarily have birds moving in right away, but they will eventually. It may be a good idea to put up the boxes in the autumn or winter, so that the birds can get used to them and the human smell can wear off. When the nest boxes are set up and birds have moved in, they should not be disturbed. Although the adult birds might not be home, they will probably be nearby and watching as you look for the small chirping chicks. This makes the birds stressed and in the worst-case scenario might even force them to find a new place to breed and leave the chicks in the box. You don’t need to clean bird boxes, the birds can do this for themselves, and if you don’t clean them, you might get lucky and have a colony of bumblebees move into the box the next year. This won't happen if you clean the box because they reuse the birds’ nesting material. As well as the nesting boxes themselves, it’s always a good idea to have a lot of scrub and brushwood and preferably native flowers nearby, where the birds can find shelter and peace, and insects they can eat, as not all species of birds eat seeds.

Gnarled old trees and dead wood, i.e. dead branches, cuttings and tall tree stumps, are important for encouraging more wild nature into your garden. Dead trees and brushwood are traditionally seen as litter which must be taken straight to the recycling centre, but disorder makes for thriving life. They form fantastic habitats and excellent sources of food for many insects, fungi, birds and mammals.

Gnarled old trees
Thousands of insect species thrive in old trees. Yet some of us humans are not very keen on them. As soon as an old tree gets a little fungus on it, puts the garden in shade or looks like it might fall over in the next storm, we start thinking about it as a danger and fell it. We have actually got to the situation where we simply do not have enough old trees across the entire country, especially those which have been allowed to grow in the sun and become beautiful and gnarled. Of course, it takes a ridiculously long time to produce an old tree and they are now in short supply. As a result, the many species that are linked to habitats in old trees are now also in decline. If you have an old tree in your garden or a communal area, take really good care of it. Instead of felling old trees, pollard and trim them, so that there is no risk of them falling over or dropping branches onto the heads of people below.

You can also help by creating good habitats in old trees by “veteranising” them, i.e. damaging them without destroying them, thereby creating cavities for use as natural bird’s nests and places where bats, ladybirds and small tortoiseshell butterflies can overwinter, and tree-dwelling beetles, fungi and bees can live.

However, if you do have to fell a tree, leave a 2 to 4 metre high stump and leave it to decay very slowly. Tree stumps form valuable habitats, especially in sunny spots. Climbing plants such as honeysuckle and ivy can be planted up against the stump, or bird boxes can be put up if you don’t think the stump is sufficiently attractive in itself.

Brushwood and firewood stacks
Piles of brushwood, leaves, garden waste and hedge clippings or a stack of old firewood logs can also be used to create important habitats around the garden. These are the most effective bug hotels around. They have many nooks and crannies which the animals can use to hide in, eat off and decompose. Piles of brushwood are a hedgehog's winter paradise and also act as nests for their young over the summer. These piles are easy to make, and the bigger the pile and the more piles you can create, the better. So, keep all your garden waste and leave it lying around the garden, both in sunny spots and under a hedge or bush, for example. This will maximise the variation in conditions in the stacks, enabling as many different species to live there as possible.

Once you have created your brushwood stack, it can just be left to look after itself. Do not disturb the stacks, but add new material from time to time. Then the animals will not have to move to a new stack every year. You can look inside the piles and admire the life in them if you do it carefully, just remember to put the branches and leaves back in place again.

Stacks of brushwood and firewood can be arranged in interesting or artistic piles, like attractive brushwood hedges, or placed in large piles. Small animals and fungi will not care what you do, so let your imagination run riot. If you want a stack of brushwood on your balcony, it is a good idea to create a bug hotel, so that it all fits together. You can also use some hollow pipes for bees to lay their eggs in. Just make sure they are at least 15 cm deep.

What you can do:

• Don’t fell old trees. Pollard or trim them
• If an old tree has to be felled, leave a 2 to 4 m stump in situ
• Veteranise trees by drilling holes in them
• Remember that stacks of brushwood and firewood form effective bug hotels

Here at VILDSKAB, we have put together a little self-help guide to biodiversity on commercial land. It explains five easy steps you can follow on your land to encourage a little more biodiversity, which will produce results almost immediately.

1. Let the grass grow and cut it less often. An immaculate, carefully tended lawn is something many people dream of, yet for animals and plants, it is nothing but a desert. It also costs more to maintain and has a higher carbon footprint. Instead of a lawn, you could create a “grassland”, which could be home to many hundreds of species and provide food and hiding places for many animals. It’s incredibly easy to do, simply let everything grow a little, especially over the summer. If you’re afraid of complaints from customers or neighbours, put up a sign to explain what you are doing. It will help if other people know you are “cultivating a wilderness”. Cut the grass in the early spring and again in September or October, but remove the grass clippings each time!

2. An insect bank, ideally on a south-facing slope, is an excellent place to create better conditions for many wild species. South-facing slopes are particularly favourable, as they are really good places for many wild bee species to make their nests in. A south-facing bank, perhaps with a stone or two, will also trap the warmth of the spring or morning sun, which is perfect for many thermophilic (warmth-loving) animals, such as butterflies and lizards, which need to soak up the heat in order to move around. If the bank is created from a gravelly, nutrient-poor soil, it can also provide favourable conditions for many wild plant species which otherwise find it difficult to establish themselves in nutrient-rich topsoil and lawns. This includes bird’s-foot trefoil, thyme, field scabious and lady's bedstraw.

3. Gnarled old trees and dead wood, i.e. dead branches, cuttings and tall tree stumps, are important for creating more wild natural habitats. In fact, it is both easier and cheaper to preserve what is already of value! If there is an old tree on the plot, take really good care of it. Instead of felling old trees, they can be pollarded and cut back, so that there is no risk of them falling over or dropping branches onto the heads of people below. However, if you do have to fell a tree, leave a 2 to 4 metre high stump in situ to decay very slowly. Tree stumps form a valuable habitat, especially in sunny spots. Climbing plants such as honeysuckle and ivy can be planted up against the stump, or bird boxes can be put up if you don’t think the stump is sufficiently attractive in itself. Branches and cuttings can be stacked beneath trees and bushes and will become habitats for all manner of species. These are the most effective bug hotels around. They have many small nooks and crannies which the animals can use to hide in, eat off and decompose.

4. Flowering trees and bushes are the wildest biodiversity installations. If you plant willow, mirabelle plum, common hawthorn, bird cherry, wild apple, elderberry and guelder-rose, they will flower throughout the spring from March to June and provide food and shelter for insects and other animals. Most of these trees and bushes also produce berries and nuts, which provide food for birds and red squirrels in the autumn and long into the winter, enabling them to survive through to the spring. A tree is much easier to maintain than a flower meadow, and it produces many, many more flowers per square metre than a flower seed mix!

5. But, yes, there is no getting away from it. Everyone do want a flower meadow, even if they are difficult to manage successfully, particularly in the good quality loam soils used for lawns. If you want to use a seed mix, look for those with native species. The grass must be removed—completely—otherwise it won’t work properly. Look for mixtures with devil’s-bit scabious, field scabious, bird’s-foot trefoil, lady’s bedstraw, small scabious*, broad-leaved thyme, oregano, red clover, hemp agrimony and valerian. These species can compete with other species in powdery, nutrient-rich soils and produce beautiful flowers!

Remember to enjoy all the wonderful species you attract to the area and explain to your customers what you are trying to do. If you are looking for other simple ways to improve biodiversity, contact VILDSKAB at info@vildskab.nu and find out how we can help you. We design, hold courses, draw up biodiversity strategies and management plans, give talks and teach about Danish biodiversity and urban nature.

* Denmark