The importance of grass field margins for foraging yellowhammers, as well as other passerines, is well known (Brickle et al. 2000; Wilson 2001; Perkins et al. 2002). This has been attributed to higher arthropod abundance than in adjacent cereal crops (Thomas & Marshall 1999; Hart et al. 2006), a finding supported by the present study. In our initial observations, yellowhammers showed a clear preference for margins in early summer, with little use of cereals. In late summer, habitat use appeared more varied, with a marked decline in margin use and greater use of cereals, despite margins supporting significantly higher abundance of invertebrate prey.
Two plausible mechanisms could explain the seasonal shift in habitat selection. Firstly, a reduction in insect accessibility in margins associated with seasonal vegetation growth, which was most pronounced within margins, a mechanism that has been previously suggested (Hart et al. 2006) but not quantified. Secondly, a progressive increase in arthropod abundance in cereal crops during the summer, as they migrate into fields from overwinter refuges in boundaries (Coombes & Sotherton 1986). Although chick-food abundance was significantly higher in margins than either cereal crop across the whole season, in July this difference in relative abundance was small. These two mechanisms are not necessarily competing hypotheses, indeed they may operate synergistically to influence seasonal patterns of habitat selection by altering the relative availability of arthropod prey between habitats. Although yellowhammers may also visit cereal fields to collect semi-ripe grain, nestling diet comprises mainly of arthropods (Stoate et al. 1998; Wilson et al. 1999), suggesting that cereal use was driven primarily by arthropod availability. The hypothesis that birds responded to declining accessibility of prey in margins is strongly supported by our experimental margin manipulations. These demonstrated a significant increase in the use of cut patches between early and late summer, and greater use of cut areas in taller uncut margins. The low rate of use of cut patches in early summer suggests that when the height of uncut margins is low (e.g. < 60 cm), food accessibility for foraging birds is less constrained by vegetation height. In late summer, however, when uncut margins are tall, birds are likely to become more dependent on open patches to access food, and hence, the observed increase in the use of cut patches. A previous study of yellowhammers foraging in experimentally manipulated field margins found no difference in use of cut and uncut areas; however, small sample sizes may have reduced the power to detect a difference (Perkins et al. 2002).
Short, sparse swards have been shown to enhance foraging efficiency for birds, through increasing prey accessibility and detectability (Butler & Gillings 2004; Stillman & Simmons 2006), facilitating forager mobility (Devereux et al. 2004) and reducing perceived predation risk (Whittingham & Evans 2004). Numerous studies of passerines in arable and grassland habitats have demonstrated foraging preferences for shorter swards consistent with the results of the present study (see Wilson et al. 2005 and references therein). Short swards are, however, generally associated with lower abundance and diversity of invertebrates (Vickery et al. 2001; McCracken & Tallowin 2004). Open patches may increase food availability at the interface between long vegetation, which serves as a reservoir of invertebrates, and short vegetation, where prey becomes more accessible. In the present experimental study, the majority of the sward (and hence foliar arthropods) was removed from within the cut patches but limited visibility of birds within these patches meant it was not possible to determine whether they foraged along the uncut edges or the soil surface.
The vegetation structure of margins, and their height relative to cereal crops, could vary across sites for a number of reasons, potentially influencing foraging patterns: firstly, through the method of margin establishment (natural regeneration or sown grass mixes), and secondly, the ratio of spring- to winter-sown crops (and hence, the relative timing of crop and margin development). However, as a general rule, margin vegetation will undoubtedly continue to become taller and denser throughout the summer, unless management to improve accessibility is implemented. We suggest that seasonal changes in food accessibility across a range of habitats may be a more important driver of foraging patterns than previously thought. Current margin management practices may provide sub-optimal foraging habitats for birds in late summer, reducing their value in agri-environment schemes (see Management Recommendations).
We have used vegetation height as a surrogate for sward accessibility, an approach used successfully in previous studies of habitat selection (e.g. Butler & Gillings 2004; Stillman & Simmons 2006). Whilst accessibility for foraging birds may be determined by a complex range of factors such as sward height, density and structural complexity, many of these measures are confounded (Devereux et al. 2006), and further work is required to disentangle their relative influence on avian foraging behaviour. Although yellowhammers are multi-brooded (Bradbury et al. 2000), brood sizes during the foraging watches differed little between early and late summer (3·04 ± 0·15 and 2·95 ± 0·14, respectively), suggesting that seasonal variation in foraging patterns was unlikely to have been driven by varying nestling demand. In addition, most breeding pairs (c. 78%) remained in the same locality all season (judged by individually identifying pairs using colour ringing or plumage characteristics), suggesting little effect of within-season territory shifts on foraging preferences. Hence, any seasonal variations in the availability of habitats within the foraging radius of nests reflected the fact that watches were conducted at a small number of different territories in early and late summer, respectively.
The use of vacuum sampling to measure invertebrate abundance could potentially bias comparisons between habitats, as sampling efficiency may be influenced by vegetation structure (Thomas & Marshall 1999). However, if sampling efficiency is expected to be lower in the taller, denser margin vegetation, the greater abundance of invertebrates recorded in margins relative to cereals could potentially represent an underestimate of the true difference.
The importance of invertebrate-rich field margins as foraging habitat for farmland birds is well documented and field margin options are central to many Agri-Environment Schemes (AES) (Vickery et al. 2002, 2004). Current guidelines for margin management usually allow cutting at intervals to control woody growth (Anon 2005), but these are often infrequent and give no consideration to the requirements of foraging birds. The evidence from this study suggests more effective management is required if these habitats are to deliver improved foraging resources in late summer, a crucial period in determining population trends for many multi-brooded farmland species (Wilson et al. 1997; Siriwardena et al. 2000; Brickle & Harper 2002).
A detailed consideration of the optimal margin management techniques was outside the scope of this study. However, approaches that promote a more heterogeneous sward structure, such as a mosaic of short and long vegetation in close proximity, are likely to provide the greatest benefits for foraging birds like the yellowhammer. AES are designed to enhance a range of wildlife and more active margin management should also consider the effects on wider margin biodiversity such as insects, forbes and nesting birds. Management during the breeding season could minimize disturbance of nests and dependent young by cutting only outer portions of margins and avoiding the inner margin (e.g. near the base of hedgerows) that tends to be favoured for nesting (Rands 1986; Bradbury et al. 2000). The use of scarification or graminicides to open up grass margins may provide alternative benefits to cutting (Potts et al. 2007). Future work should investigate the biodiversity impacts and cost-effectiveness of creating more open margin swards through a range of methods. The availability of short, sparse vegetation for foraging has been shown to enhance productivity and local population trends on farmland for a number of passerine species, for example skylark (Morris et al. 2004; Donald & Morris 2005) and northern wheatear Oenanthe oenanthe L. (Pärt 2001; Arlt et al. 2008). Researching the effectiveness of margin manipulations in enhancing productivity, as well as foraging, would therefore also be desirable.
A recent study suggests that targeting AES options at the cropped area of farmland rather than field margins may deliver greater benefits for farmland birds (Butler, Vickery & Norris 2007). However more effective management of grass margins (and other margin types) would undoubtedly increase their value for birds by extending their ‘useful lifetime’ into late summer, and may reduce the reliance on cropped areas by some species. Although more active management is likely to require higher AES payments, encouraging such management has several key benefits. Firstly, it will increase the overall cost-effectiveness of margins as an AES option, particularly important given their high uptake rates (e.g. over 16 000 ha of margin options within ELS; Grice et al. 2007). Secondly, enhancing the quality of margins will increase the biodiversity benefits without increasing the area under AES options, particularly important given rising commodity prices and increasing pressure on land for other ‘products’ such as biofuels, ecosystem services and climate change mitigation measures (Sutherland et al. 2008). Thirdly, enhancing margin management may also increase the value of adjacent hedgerow options (e.g. by providing nearby foraging habitat for hedgerow nesting species) and in-field options (e.g. skylark plots) (Morris et al. 2004; Cook et al. 2007).