Structural heterogeneity of linear habitats positively affects Barred Warbler Sylvia nisoria, Common Whitethroat Sylvia communis and Lesser Whitethroat Sylvia curruca in farmland of Western Poland

Figures & data

Table 1. Mean values, standard errors and ranges for habitat measurements of sections of linear habitats associated with the presence of a breeding territory of Barred Warbler, Common Whitethroat and Lesser Whitethroat.

	Barred Warbler	Common Whitethroat	Lesser Whitethroat
Heterogeneity	10.57 ± 1.65 (5–16)	6.80 ± 0.57 (2–16)	8.83 ± 2.02 (3–16)
Shrubs volume (m^3)	5151.70 ± 1201.56 (2362.5–11475.0)	2661.60 ± 466.87 (0.0–9261.0)	4851.69 ± 1309.62 (1452.0–9261.0)
Width (m)	14.29 ± 1.52 (9.0–21.0)	10.38 ± 0.60 (5.0–21.0)	10.17 ± 1.53 (5.5–14.0)
Brambles and nettles (m^2)	616.61 ± 78.26 (405.0–945.)	609.00 ± 66.97 (42.0–1370.3)	265.75 ± 69.97 (42.0–468.8)
Low vegetation (m^2)	162.86 ± 104.67 (0.0–653.0)	174.13 ± 36.31 (0.0–810.0)	4.13 ± 2.82 (0.0–16.5)
Ditch (n)	1.0 ± 0.38 (0–2)	0.90 ± 0.13 (0–2)	0.83 ± 0.17 (0–1)
No. of trees	17.29 ± 6.58 (5–56)	11.23 ± 2.47 (0–56)	1.67 ± 1.67 (0–10)

Table 2. The number (t) and density of breeding territories of studied Sylvia warblers within the 10.0 km² study area, and their number, number per km and percentage located in linear habitats within the study area.

	Barred Warbler	Common Whitethroat	Lesser Whitethroat
Total no. of territories in study area	8	40	9
No. of territories within linear habitats	7	30	6
Density in study area (t/km^2)	0.8	4.0	0.9
No. of territories/km of linear habitats	0.5	2.1	0.4
Percentage of total no. of territories located within linear habitats	87.5	75.0	66.7

Figure 1. The RDA ordination diagram (showing first – horizontal and second – vertical ordination axes) displaying the habitat preferences (black arrows) of Barred Warbler (BW), Common Whitethroat (CW) and Lesser Whitethroat (LW) among the seven linear habitat variables (grey arrows). The angle between axes of the ordination diagram and species and habitat variable arrows show the power of the correlation. Abbreviations of habitat variables are as follows: TRN – the numbers of trees, BRA – brambles and nettles, LVE – herbaceous vegetation, SHR – shrub volume, DIT – a ditch occurrence, HET – heterogeneity and WID – the width of the section.

Figure 2. Mean (±se, ±1.96*se) heterogeneity of occupied (grey boxes) and unoccupied (white boxes) sections of linear habitats of Barred Warbler (BW), Common Whitethroat (CW) and Lesser Whitethroat (LW). The heterogeneity of occupied and unoccupied sections was compared using Ranked-Sign Wilcoxon tests (Barred Warbler – T = 2.00, n = 7, P = 0.043; Common Whitethroat – z = 3.45, n = 30, P = 0.00053; Lesser Whitethroat – T = 2.00, n = 6, P = 0.028).

Table 3. Correlation coefficients between habitat variables and the first two axes in the RDA ordination.

Habitat variable	Axis I	Axis II
Heterogeneity	0.40	−0.09
Shrub volume	0.29	−0.13
Width	0.16	−0.07
Brambles and nettles	−0.05	0.15
Low vegetation	−0.27	−0.06
Ditch	−0.09	−0.06
No. of trees	0.06	0.04

Table 4. Results of Monte Carlo permutation testing of the significance of habitat variables affecting the distribution of studied Sylvia warbler species, with use of both individual and automatic selection procedures.

	Individual			Automatic
Habitat variable	%	F	P	%	F	P
Heterogeneity	50.3	9.20	0.002	50.3	9.10	0.002
Shrubs volume	26.8	4.87	0.007	11.1	2.18	0.11
Width	10.6	1.88	0.15	3.7	0.76	0.47
Brambles and nettles	6.9	1.27	0.27	7.9	1.55	0.19
Low vegetation	25.5	4.64	0.008	10.1	1.93	0.15
Ditch	3.7	0.61	0.58	13.2	2.63	0.07
No. of trees	4.8	0.81	0.43	5.3	1.33	0.31

Note: Percentage of variance in species data accounted for by a particular habitat variable is given.