In this guest blog, Paul Cabrisy, research intern at RSPB Hope Farm, explains the importance of hedges and correctly managing them on a farm. Paul also tells us the outcome of a study carried by a masters student who looked at different preferences of farmland bird species to different frequencies of hedgerow management at Hope Farm.
The importance of hedgerows
Hedgerows are highly important to farm wildlife as, in the UK, over 600 plants, 1500 insects, 65 birds and 20 mammals are known to live or feed in hedges. Hedgerows provide nesting habitats for many species of farmland birds that nest in field boundaries and other species like owls, starlings, etc. Hedges are a source of valuable food throughout the seasons, particularly over winter by providing berry fruits and seeds. Hedges can act as a fundamental wildlife corridor, to facilitate the movement of less mobile species across farmed landscape, between woodland blocks. Not only essential for birds, hedgerows are good for so much other wildlife. Hedgerows on a farm can become foraging sites for bats. They also provide a cover for small mammals, reptiles and amphibians. Hedges produce pollen and nectar in late autumn as well as an abundance of invertebrates that feed chicks and other animals. In winter, hedgerows are important habitats to many invertebrates too. Another importance are the physical properties of the hedges as they can prevent soil erosion and store carbon. Having hedges on a farm can be good way to mitigate some of the greenhouse gases produced on farm.
A first key to a good hedge management is to allow natural variation and to let it get a bit messy! This is important in order to support greater diversity of wildlife and is mainly because animals like birds have different habitat preferences between species.
But why manage the field hedges in the first place? In fact, hedgerows need to be managed as the shrubs will eventually grow into trees and trimming is the easiest way to keep the structure of the hedge. The hedge’s response to being cut is to produce new shoots that will develop into branches and done correctly, can make the branches become denser overtime.
Whilst management is essential, it is worth noting that intensive management of hedgerows can be detrimental to wildlife by reducing their foraging and breeding success. Gappy or leggy base hedges, from repeated cutting to the same point in the hedge, or through neglect, will have less wildlife value as its base it will be more accessible to predators. You really want to cut to reduce gaps and have denser growth close to the ground.
Overall, to support more wildlife it is best to manage hedges for different sizes and shapes where you can. The diversity of birds using hedgerows will depend on the length, width and height, as well as plant species, because birds can have distinct preferences in terms of structure.
Having wider and taller hedges (about 4m tall and 4m wide) on a farm can provide rich habitats with food for many invertebrates and support nesting birds, such as bullfinches, turtle doves etc. Wider hedges offer better coverage for bad weather conditions and protection against natural predators. This kind of structure can be obtained by letting the plants grow for a longer time without trimming. Tall hedges are better located on wooded areas of the farmland and not in places used by ground nesting birds like lapwings.
Smaller hedges (around 2m tall) on the other hand have a thicker vegetation at the base than taller hedges and are better suited for ground nesting species like grey partridges, yellowhammers, song thrushes & whitethroats, to name a few. This type of hedge can be managed more often to keep suckers from spreading. However, before trimming it is important to considering the health of the plant with its long-term survival, as over-trimmed hedges can become gappy and die.
Lastly, it is best to not cut the hedges annually to increase the flowers and fruits for birds and pollinators. Trimming will remove the fruit-bearing stems of the plant and can also damage the flowers. When thinking about the best time of year for trimming, winter is better than autumn as it is less likely to cause stress to the plant. By cutting the leaves in autumn the hedge will lose the ability to regrow its leaves before the winter and will be deprived from feeding efficiently. Also, this will avoid the destruction of bird nests present from March to August.
Having a hedge plan
One of the best ways to manage hedges on a farmland is to make a plan of maintenance, creation and restoration. For this, it is ideal to map your farm’s hedges, noting where there are gaps, taller hedges (greater than 4m) and small smaller hedges (less than 4m).
Maintenance – Having a variety of hedge plants of different ages is important and better for the farms’ wildlife. Old hedges tend to have a better structure with more diverse plant life. Older hedges are also likely to hold more wildlife than recent plantings. Therefore, it is essential to preserve existing hedges on a farm and enhance their value as a priority over the creation of new hedges.
Creation – To support a wider range of wildlife a farmer can increase the type of hedges on its field boundaries, such as incorporating new hedge plants (preferably species native to the area) to existing hedgerows. New hedges can also be established to link up other existing ones to create more hedgerows and strengthen the connectivity of habitats on the farm.
Restoration – Young and developing hedges need to be protected from browsing and grazing animals e.g. rabbits. There are many ways to protect and restore hedges. Laying (cutting stems part way through and laying them along the hedge line) and coppicing (cutting the stems at ground level) are two techniques that can be used to restore hedgerows. However, depending the hedges’ state it is better to obtain advice from a specialist and old hedges may require specific management techniques. It is also possible to research hedge restoration methods that are available online and on Local Biodiversity Action Plans of your area.
At Hope Farm, our hedgerows are managed to provide essential habitats for many bird species. We have around 5 miles of hedgerows across 450 acres. Other features can be found along the hedgerows such as grass margins, flower margins, ditches and patches of woodland. We manage our hedgerows to improve their value for birds by providing suitable nesting and foraging habitats. Hedges are cut once every three years and on a rotation between fields. This allows the hedge plants to produce more food berries than those cut annually. Having this rotational cutting also avoids putting undue stress and keeps the hedges healthier for longer.
Thanks to the help from the local Cambridge Conservation Volunteers, we also lay about 50m of hedge each year which does a fantastic job of keeping vegetation thick at the bottom of the hedge. You can see where they have been hard at work all along our footpaths so keep an eye out.
With our hedgerow management and other conservation work on the farm, our farmland breeding bird index is now 185% above baseline with populations that have been stable over the past 10 years.
Credit: Flock of Grey Partridge coming out a hedgerow at RSPB Hope Farm. Photo by Paul Cabrisy.
Megan Tresise’s research
Megan is a masters student from the University of Leeds who carried a research at Hope Farm, looking at how our hedge management affected bird territories. She focuses her research on four bird species that use hedgerows on farmland, but which have all declined in the UK due to a reduction in available habitat: common whitethroats, yellowhammers, linnets and greenfinches. Megan found that the territory density of yellowhammers and linnets were higher when hedges were left unmanaged for one year whilst whitethroats held higher territory density with hedges left two years without managing (fig. 1).
When grouping bird species altogether, Megan found that the birds selected hedges for their territories when managed with a one year or two years between cutting rather than those left unmanaged for a greater length of time (fig. 2). Common whitethroats, yellowhammers and linnets also preferred to nest in shorter hedges with more scrub, rather than trees. Megan also found that the greenfinches at Hope Farm chose to hold territories where no management was undertaken (fig. 2). This could be explained as greenfinches prefer to nest in trees rather than shrubs, and so will benefit from leaving these habitats unmanaged to leave the hedges to grow into trees.
Figure 2: Mean territory density per 100m (±1SE) for yellowhammers, linnets and whitethroats (left) and greenfinches (right). No mgmt. = no management, Mgmt = managed that year, +1 year = 1 year unmanaged , +2 = 2 years unmanagement, +3 years = 3 years unmanaged, 4 years = 4 years unmanaged. Significant differences between groups (Dunn-Bonferroni; p<0.05) are indicated by different letters above each bar.
We would like to thank Megan Tresise for her time collecting and analysing important data gathered over the years at the farm and that was able to show, once again, the positive outcome of our wildlife friendly farming.