Use of field margins managed under an agri-environment scheme by foraging Barn Swallows Hirundo rustica

Figures & data

Table 1. Grid references for the Barn Swallow colonies under investigation, the number of breeding pairs known to be present at each site and the number of grass and enhanced margins surveyed on each site.

Site	Grid reference	Pairs	Grass	Enhanced
1	SU 40834 48733	4	9	4
2	SU 32781 47386	8	11	2
3	SU 10227 71284	8	4	3
4	SU 09841 73504	7	10	4
5	SU 06111 64046	2	3	2

Figure 1. Estimated probability of observing a foraging Barn Swallow during surveys. Probabilities and standard errors were back transformed from the generalized linear mixed effects model estimates and account for the other variables modelled.

Table 2. Results of generalized linear mixed effects models of associations between field edge habitats and Barn Swallow presence (binomial model), total number of individuals (negative binomial model) and foraging activity (negative binomial model). All models included the random effect colony and control for the effects of cloud cover and visit.

Response	Explanatory	Estimate ± se	z-value	P
Swallow presence	Intercept (cereal, grass margin, June, verge)	3.19 ± 2.16	1.48	0.14
	Transect crop (oilseed rape)	−2.60 ± 1.78	−1.46	0.44
	Margin (enhanced)	1.33 ± 0.62	2.14	<0 . 05
	Neighbouring field (grassland)	−0.36 ± 0.59	−0.60	0.55
	Log (Percentage cloud cover + 1)	−0.94 ± 0.43	−2.17	<0 . 05
	Visit (July)	−0.15 ± 0.50	−0.30	0.76
	Hedgerow	0.89 ± 0.84	1.06	0.29
	Treeline	−0.31 ± 0.65	−0.47	0.64
Total number of individuals	Intercept (cereal, grass margin, June, verge)	2.85 ± 1.37	2.09	<0 . 05
	Transect crop (oilseed rape)	−1.38 ± 1.35	1.35	0.30
	Margin (enhanced)	0.64 ± 0.40	1.61	0.11
	Neighbouring field (grassland)	−0.44 ± 0.42	−1.06	0.29
	Log (Percentage cloud cover + 1)	−0.73 ± 0.27	−2.71	<0 . 05
	Visit (July)	−0.09 ± 0.33	−0.27	0.79
	Hedgerow	0.14 ± 0.49	0.29	0.77
	Treeline	−0.88 ± 0.43	−2.03	<0 . 05
Index of foraging activity	Intercept (cereal, grass margin, June, verge)	3.04 ± 2.11	1.44	0.15
	Transect crop (oilseed rape)	−0.61 ± 1.84	−0.33	0.72
	Margin (enhanced)	0.75 ± 0.61	1.23	0.22
	Neighbouring field (grassland)	−0.50 ± 0.61	−0.82	0.41
	Log (Percentage cloud cover + 1)	−0.55 ± 0.46	−1.20	0.23
	Visit (July)	0.03 ± 0.51	0.06	0.95
	Hedgerow	0.12 ± 0.64	0.19	0.85
	Treeline	−1.40 ± 0.65	−2.15	<0 . 05

Notes: The highest number of simultaneously foraging swallows observed within a one-minute period was used as a proxy for the total number of individuals using the transect. The total of each maximum count per minute was used to calculate an index of foraging activity.

Table 3. Comparison of alternative Dipteran models with details of the degrees of freedom (DF), AIC values, differences in AIC values and Akaike weights for all models.

Response	Model	df	AIC	AIC differences	Akaike weights
Swallow presence	Null	2	128.07	2.12	0.16
	Diptera abundance	3	125.94	0.00	0 . 46
	Length weighted Diptera	3	127.20	1.26	0.25
	Muscidae	3	128.69	2.74	0.12
Total number of individuals	Null	3	260.02	0.00	0 . 41
	Diptera abundance	4	261.17	1.15	0.23
	Length weighted Diptera	4	261.33	1.31	0.21
	Muscidae	4	261.95	1.93	0.16
Index of foraging activity	Null	3	356.18	3.74	0.08
	Diptera abundance	4	353.44	0.00	0 . 52
	Length weighted Diptera	4	353.09	0.65	0.37
	Muscidae	4	357.99	5.56	0.32

Table 4. Results of generalized linear mixed effects models of associations between field edge habitats, Diptera abundance and Barn Swallow presence (binomial model) and foraging activity (negative binomial model). All models included the random effect colony and control for the effects of cloud cover and visit.

Response	Explanatory	Estimate ± se	z-value	P
Swallow presence	Intercept (cereal, grass margin, June, verge)	1.68 ± 2.21	0.76	0.45
	Transect crop (oilseed rape)	−2.26 ± 1.63	−1.78	0.17
	Margin (enhanced)	1.24 ± 0.63	1.96	<0 . 05
	Neighbouring field (grassland)	−0.31 ± 0.61	−0.51	0.61
	Log (Percentage cloud cover + 1)	−0.77 ± 0.43	−1.78	0.07
	Visit (July)	−0.08 ± 0.50	−0.15	0.88
	Hedgerow	0.76 ± 0.83	0.92	0.36
	Treeline	−0.37 ± 0.66	−0.56	0.58
	Diptera abundance	0.34 ± 0.21	0.76	0.45
Index of foraging activity	Intercept (cereal, grass margin, June, verge)	0.09 ± 1.79	0.05	0.96
	Transect crop (oilseed rape)	−0.35 ± 0.88	−0.39	0.69
	Margin (enhanced)	0.49 ± 0.56	0.87	0.38
	Neighbouring field (grassland)	−0.26 ± 0.58	−0.46	0.65
	Log (Percentage cloud cover + 1)	−0.21 ± 0.38	−0.56	0.58
	Visit (July)	0.14 ± 0.47	0.31	0.76
	Hedgerow	−0.17 ± 0.63	−0.26	0.79
	Treeline	−1.82 ± 0.61	−2.98	<0 . 01
	Diptera abundance	0.67 ± 0.22	3.01	<0 . 01

Table 5. Results of generalized linear mixed effects models of associations with Diptera abundance (negative binomial model) and Muscidae abundance (negative binomial model) Colony is included in the model as a random effect, with transect ID nested within colony.

Response	Explanatory	Estimate ± se	z-value	P
Diptera abundance	Intercept (cereal, arable, June, verge)	2.18 ± 0.81	2.69	<0 . 01
	Enhanced margin	0.80 ± 0.27	2.96	<0 . 01
	Grass Margin	0.56 ± 0.19	2.98	<0 . 01
	Oilseed rape	1.32 ± 0.29	4.57	<0 . 001
	Neighbouring field (grassland)	−0.13 ± 0.37	−0.34	0.73
	Log (Percentage cloud cover + 1)	−0.42 ± 0.17	−2.51	<0 . 05
	Visit (July)	−0.06 ± 0.16	−0.40	0.69
	Hedgerow	0.28 ± 0.39	0.71	0.47
	Treeline	0.76 ± 0.33	2.29	<0 . 05
Muscidae abundance	Intercept (cereal, arable, June, verge)	−0.51 ± 0.66	−0.77	0.44
	Enhanced margin	−0.31 ± 0.31	−1.00	0.32
	Grass Margin	−0.06 ± 0.21	−0.27	0.79
	Oilseed rape	0.54 ± 0.29	1.88	0.06
	Neighbouring field (grassland)	−0.25 ± 0.26	−0.96	0.34
	Log (Percentage cloud cover + 1)	0.01 ± 0.13	0.07	0.94
	Visit (July)	−0.73 ± 0.16	−4.54	<0 . 001
	Hedgerow	0.30 ± 0.29	1.05	0.30
	Treeline	0.36 ± 0.23	1.51	0.13

Table 6. Result of linear mixed effects model of associations with log transformed length weighted Diptera abundance. Colony is included in the model as a random effect, with transect ID nested within colony.

Response	Explanatory	Estimate ± se	t-value	P
Length weighted Diptera abundance	Intercept (cereal, arable, June, verge)	2.66 ± 0.59	4.48	<0 . 001
	Enhanced margin	0.59 ± 0.32	2.53	<0 . 05
	Grass Margin	0.49 ± 0.16	3.14	<0 . 01
	Oilseed rape	0.99 ± 0.25	4.01	<0 . 001
	Neighbouring field (grassland)	−0.27 ± 0.27	−0.98	0.33
	Log (Percentage cloud cover + 1)	−0.28 ± 0.12	−2.33	<0 . 05
	Visit (July)	−0.18 ± 0.12	−1.45	0.15
	Hedgerow	0.28 ± 0.29	0.97	0.34
	Treeline	0.44 ± 0.26	1.74	0.09