Guest blog by Megan Tresise, Faculty of Biological Sciences, University of Leeds, on her new paper with RSPB Hope Farm, and the Centre for Conservation Science.
Using thirteen years of songbird territory and farm management records from RSPB Hope Farm, research has shown that hedgerow specialists had significantly greater territory densities in hedges adjacent to oilseed rape crop, but lower territory densities with tree presence. Conversely, greenfinch (farmland generalist) territories were significantly higher in the presence of trees and when hedgerows were adjacent to roads or fields without a main crop sown.
The post-war intensification of agriculture led to losses of farmland biodiversity, although local scale reversal of these trends has been observed at RSPB Hope Farm, where Agri-environment scheme (AES) implementation has led to over a 200% increase in bird territories since 2000.
Yellowhammer returning to hedgerow after feeding on the ground at RSPB Hope Farm © Ben Andrew (rspb-images.com)
For my Masters winter research project, I modelled the boundary-level drivers of songbird territories at Hope Farm by using breeding bird territory maps for yellowhammer, linnet, common whitethroat (Sylvia communis) and greenfinch (Carduelis chloris), and hedgerow management, in-field cropping and hedgerow habitat records.
Territory density and habitat variables
Digitalised territory maps were used to estimate the number of territories per section of hedgerow, as well as calculate territory density (no. territories/100m). A hedgerow management variable was created by categorising management into six timings (no management [since 2000], management that year, 1 year since, 2 years since, 3 years since and 4 or more years since last management). In-field crop records, hedgerow location (farm edge or road) and boundary tree and AES trial presence were also modelled.
Modelling of territories with hedgerow variables revealed differences between species, with possible drivers including adjacent crop type, hedgerow spatial location and boundary habitat features.
The three hedgerow specialist species (yellowhammer, linnet and whitethroat) responded positively to oilseed rape presence, but negatively to boundary tree presence. Yellowhammer territories also increased in the present of AES trials at the boundary, and linnets responded negatively to fields without a main crop sown, as well as hedges close to the road or farm edge. Whitethroat territory density was higher when hedgerows were not managed for at least 2 years, although the model was weak.
Linnet responded well to presence of oilseed rape, but negatively to presence of boundary trees © Andy Hay (rspb-images.com)
On the other hand, the hedgerow generalist greenfinch showed significant increases in territory density where hedgerows were close to the road, and when hedges were adjacent to fields without a main crop sown and those with trees. This species responded negatively to AES trial presence.
Implications for AES and future policy
Farmland birds rely on hedgerows and in-field crops for both invertebrate and seed food provision and nesting territory, so the research findings from this paper on territory drivers could have potential impacts on future AES options and policy.
Surprisingly, hedgerow management was not a significant driver of territory density for most of the species studied, although the graphical trends demonstrate a slight preference for infrequent management for greenfinch and more frequent (1 or 2 year cycles) management for yellowhammers and linnets. These results indicate that medium-term hedgerow management is still beneficial for territory settlement in farmland birds but can also increase management flexibility for farmers.
Greenfinch showed increases in territory density where hedgerows were close to the road © Ben Andrew (rspb-images.com)
The strongest in-field drivers were oilseed rape, or no main crop sown near hedgerows, lending support to the need for increased cropping diversity in the farmed landscape, for example by including break crops, areas of set-aside or grassland.
Tree and AES trial effects were opposing for the hedgerow specialists and generalist species but present themselves as strong drivers for territory location. AES trials (e.g. floral margins and nectar mixes in the grass strips next to hedgerows) increased yellowhammer territory density by over 40%, yet greenfinches showed significant declines in territories near these features.
However, this could be a spatial effect of greenfinch territory clustering in hedgerows bordering the smaller fields at Hope Farm where fewer AES trials were ongoing. Strong territory density declines were found for specialist species in hedgerows with trees, although opposing significant increases for generalist greenfinches demonstrates generalist behaviour in utilising woodland copses and boundary trees on-farm.
Greenfinches also utilised hedgerows close to the road running through Knapwell village, which is likely to forage in nearby residential gardens with bird feeders. Yellowhammers, linnets and whitethroats held fewer territories in farm perimeter hedgerows, which could be due to disturbance from the road and better food resources in and around more open farmland hedgerows.
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