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BOU Register of Avian Thesis Abstracts

Simon Butler

Stubble field prescriptions for farmland birds - the role of sward structure in mediating food availability

Institution: EGI, University of Oxford, U.K.
Supervisors: J Krebs, M Whittingham, R Bradbury (RSPB)
Details: D. Phil. 2004 (Completed)

Current Address: RSPB, The Lodge, Sandy, Beds, SG19 2DL (Oct 2005)
Email: simon.butler @ rspb.org.uk (remove spaces)

Subject Keywords: Predation risk, starvation risk, stubble height, food availability, habitat selection, habitat management

Species Keywords: Farmland passerines

Abstract:

Many animals trade-off energetic gain with predation risk when selecting where to forage. Individuals are expected to feed in the patch with the lowest predation risk, all other things being equal, or need to be rewarded with increased energetic gains to accept a greater risk of predation. The value of a patch will depend on its characteristics and how these suit an individual’s foraging requirements and predator escape strategy. This thesis concentrates on the role of sward structure in determining foraging site selection, since this can influence both potential energetic gain and predation risk. In particular, I investigate how stubble height manipulation can be used to optimise this important habitat for overwintering farmland birds, many of which are of current conservation concern.
Results from aviary based experiments showed that chaffinches Fringilla coelebs altered the time allocated to foraging and vigilance behaviours in response to changes in visual obstruction (stubble height). Vigilance period duration was greater, and peck rates consequently slower, in more visually obstructed patches. Despite this apparent compensation for reduced visibility, birds in long stubble were slower to respond to predator attack. Forager mobility was also reduced in the long stubble but this effect is likely to be of only limited importance in my experiments since food abundance was high. When individuals were free to choose between two patches which varied in both food abundance and habitat structure, there needed to be nearly three times more food in the ‘risky’ long stubble before they demonstrated parity of use between it and the ‘safe’ short stubble. Data collection efficiency was maximised by commencing experiments on individuals shortly after their capture rather than allowing an initial settling-in period.
The final experiment demonstrated the value of using individual-based models to predict population scale responses to habitat manipulation. As expected, within-field vegetation height manipulation led to differential spatial use of stubble fields by a range of farmland bird species. Granivorous passerines and invertebrate feeders were more abundant in treatment plots, which had undergone stubble height reduction, whilst skylark and partridge abundances were higher in control plots.
Increasing structural heterogeneity on stubble fields is likely to optimise its value as a foraging habitat, making it better suited to the foraging requirements and predator escape strategies of a greater diversity of species. Further research would usefully trial methods of achieving such heterogeneity. Targeted management options in agri-environment schemes are likely to be the most cost-effective strategy to achieve this.

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