Woodland edges: why their structure is important for birds

Blog post by Paul Bellamy, Senior Conservation Scientist, RSPB Centre for Conservation Science

The patchwork of woods and fields seen across much of lowland Britain contain extensive lengths of woodland edge that could have a big impact on our woodland bird populations.

A new study between Bournemouth University, Centre for Ecology and Hydrology and the RSPB looked at how the structure of trees and shrubs in Cambridgeshire woods affect bird populations.

We compared woodland edges with the interiors as well as seeing how the structure of vegetation along woodland edges affects the numbers and diversity of birds.

This is important information that can inform our reserve management and the advice we give other landowners.

The issue: why the study was needed

Historical forest clearance for agriculture left remnant woods used for fuel and building material, along with later planting of woods for game hunting. The resulting landscape is a patchwork of woodland fragments surrounded by other land-uses, mainly agriculture.

An example of a scrubby “soft” woodland edge, where most of the vegetation is less than four metres tall. This section of woodland edge had an average of 10.2 species recorded each year.

Most native woodland in the Britain occurs as relatively small woods surrounded by open land, 50% of all broadleaved woodland occurs as blocks of less than 20 ha. These small woods have relatively more edge and so the influence of the woodland edge structure on bird populations can have a major impact.

Using Lidar - lasers for conservation

We studied the impact of woodland structure and edges on bird populations in ancient woodland fragments in Cambridgeshire’s intensive arable landscape. We mapped the locations of all birds during the breeding season each year from 2012-2015, recording 28 regular breeding species across all four woods.

We then measured the woodland structure using Lidar - using a sensor fitted to the underside of a plane that records the pattern of reflections from a laser as it is flown over the wood.

The relative numbers of laser reflections from the ground and different heights from the ground allow researchers to estimate the density and height of woodland vegetation at a fine spatial scale. We were then able to link where the birds were found to the structure of the wood in that location.

The woods were split up into areas of approximately 200m² to compare the number and diversity of birds in each area with the variety of different vegetation layers, density of vegetation in each vegetation layer and distance from the edge.

Fig. 1: The study area and the four target woods displayed as Canopy Height Models, which show the top surface of the vegetation and its height (lighter shading indicates taller vegetation). From Melin et al. Landscape Ecology https://doi.org/10.1007/s10980-018-0639-7

Comparing areas across the whole wood, we found that after taking account of differences in woodland size and vegetation structure between woods, there were more birds of more species nearer to the woodland edge. There were also more birds in areas with a greater volume of vegetation but only if it included shrub as well as canopy layers.

What makes a good edge for birds?

To find out we looked at how the number of birds and species varies with woodland structure along the edge. This showed that most birds were found on edges with a high proportion of shrub cover below four metres.

This highlights the importance of soft woodland edges where woods with shrubby edges between the field and mature woodland trees have more birds than edges where there is a sharp contrast between tall woodland trees and open fields with no intervening shrub layer. An example from one wood showed this contrast.

The edge on one side was dominated by tall elm trees with very little shrub layer below four metres high and had an average of only 1.5 species per section, whereas an average of 10.2 species per section was found using a shrubby edge of blackthorn and bramble on the other side of the wood where most of the vegetation was below four metres.

An example of a "hard" woodland edge, with tall trees adjacent to the field with little scrub present. This section of edge had an average of 1.5 species recorded each year.

Earlier work in the same study area has shown that woods with a high proportion of woodland edge had more birds of the wider countryside as well as general woodland species.

Species such as blackbirds, carrion crows and yellowhammers use woodland edges to access resources in neighbouring fields and hedges. Scrubby edges can therefore provide resources for scrub specialists like warblers and bullfinches, as well as woodland specialists and farmland birds.

Putting the science into practice

This study provides evidence that supports the work we do on reserves to create soft woodland edges to benefit plants, insects and birds.

It also supports the woodland management advice we provide to other landowners, advocating scrubby edges along open space within woodland as well as designing soft edges in new woodland plantings.

The paper: 'Living on the edge: utilising lidar data to assess the importance of vegetation structure for avian diversity in fragmented woodlands and their edges' has just been published in Landscape Ecology.