FARMLAND ECOLOGY - BARN SWALLOWS | 30 Figure 2 25
Relationship between swallow foraging activity and the log transformed mean adjacent boundary height (p<0.05)
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15 KEY FINDINGS 10 5 0 0 0.5 1.0 1.5 2.0 Log (mean boundary height) 2.5 3.0
been directed at these species guilds and, although it has been suggested that field margins and hedgerows managed for birds and invertebrates under AES may have wider benefits, their potential benefit to aerial feeders remains unquantified. To address this, we compared swallow foraging activity on arable farmland involved
in an AES during the summer of 2016. Preliminary visits were made to seven arable farms in May to ensure breeding colonies were present for foraging surveys. When breeding, swallows are central-place foragers, meaning that all foraging watches needed to take place within their usual foraging range of 600 metres from the nesting colony. We selected all six-metre grass margin buffer strips (EE3, HE3) and floristi- cally enhanced arable field margins, sown with wildflowers and legumes (HE10, HF4 or EF4), present within this zone for our surveys (n=56). Surveys were 20 minutes in duration, taking place twice (once in June and once in July) along field boundaries over a 200x35 metre transect belt, encompassing an arable margin and a cropped habitat. The average height of the boundary habitat present along transects was recorded to the nearest 0.5 metres. Surveys started no earlier than 6am and finished at 11am, taking place only when weather conditions were calm. We recorded the number of flies present above transect belts by taking four samples directly after foraging watches. Each sample consisted of 30 sweeps using a lightweight kite net. Cloud cover was recorded as a nuisance variable owing to the impact of air pressure on aerial inver- tebrate movements. Two measures of swallow activity were recorded: 1) swallow presence/absence on the transect belt over the survey period; and 2) an index of foraging activity calculated as the total of each maximum swallow count per minute. The probability of observing swallows along belt transects that included a floristi-
cally enhanced margin was 25% higher than the probability of doing so along ones including grass margins (see Figure 1). This may be due to the higher aerial inverte- brate abundance recorded in this habitat, as the total fly count was 36% higher along floristically enhanced than along grass margin transects, indicating that floristically enhanced margins may be an important feeding resource for swallows occupying arable farmland. Our measurement of swallow foraging activity, however, found no differences between the two margin types, but foraging activity decreased significantly with increasing boundary height (see Figure 2). This implies that providing floristically enhanced margins for foraging swallows may be more effective when placed either mid-field or along field edges with low or no hedgerows. These findings suggest that there may be a role for AES in the conservation of
swallows although more research is needed to determine whether AES can influence colony size or improve breeding success. This information will play a role in formulating arable farmland conservation strategies for this species.
www.gwct.org.uk
Foraging swallows were more likely to be present along margins containing wildflowers and legumes when compared to grass-only margins. The availability of swallow invertebrate food items was 36% higher on transects containing floristically diverse margins. Foraging activity decreased significantly with increasing boundary height.
Niamh McHugh
Aerial invertebrates were sampled using a kite net. © Niamh McHugh/GWCT
ACKNOWLEDGEMENTS
We are grateful to Belinda Bown and Jasmine Clarke for their assistance in the field and to the landowners who allowed us access to their land to conduct this study.
GAME & WILDLIFE REVIEW 2016 | 57
Swallow foraging activity
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