site and survey characteristics
The samples of pheasant woods and control woods contained similar numbers of each woodland type (combined regions: χ24 = 6·53, P = 0·18, southern region: χ24 = 4·80, P = 0·31, eastern region: χ24 = 4·90, P = 0·30). There was no difference in mean survey dates between treatments (F1,155 = 0·06, P = 0·82) but mean survey date was 5 days earlier in eastern woods (F1,155 = 2·75, P = 0·001). On average surveys took 67 min to complete and there was no difference in duration between treatments, although surveys typically lasted 10 min longer in the southern region (treatment F1,155 = 1·29, P = 0·26, region F1,155 = 17·88, P < 0·001).
In the height category 31 cm–1 m there were significant treatment (Wald = 14·88, d.f. = 1, P < 0·001) and regional (Wald = 21·78, d.f. = 1, P < 0·001) effects. In pheasant woods in the southern region, herbaceous vegetation was recorded in this height category in 64·4 ± 2·8% (mean ± 1 SE) of sampling stations compared with 52·6 ± 3·5% in control woods. In this height category in the eastern region, herbaceous vegetation was recorded in 45·9 ± 4·0% of sampling stations in pheasant woods compared with 37·4 ± 3·9% in control woods. There were no differences in structure of herbaceous vegetation cover between treatment or region in any other height categories.
In the height category 0–10 cm, R. fruticosus was more abundant in pheasant woods (southern region, pheasant woods: 12·8 ± 2·2%, control woods: 7·9 ± 1·7, eastern region, pheasant woods: 18·8 ± 3·1%, control woods: 14·5 ± 2·6%, Wald = 6·00, d.f. = 1, P = 0·01). There was also a significant regional effect with more R. fruticosus recorded in eastern woods (Wald = 4·49, d.f. = 1, P = 0·03). R. fruticosus was also more abundant in pheasant woods in the height category 11–30 cm (southern region, pheasant woods: 26·9 ± 3·7%, control woods: 20·8 ± 3·6%, eastern region, pheasant woods: 21·9 ± 3·3%, control woods: 16·8 ± 2·9%, Wald = 4·35, d.f. = 1, P = 0·04). There were no differences in the abundance of R. fruticosus over 30 cm in height between regions or treatments. There were no differences in abundance of woody shrubs between treatments or regions except in the height category 1–2 m, where there was a significant region × treatment interaction (Wald = 7·74, d.f. = 1, P = 0·005). This was due to more shrubs being recorded in this height category in pheasant woods in the east (pheasant woods: 16·1 ± 1·9%, control woods: 10·9 ± 1·7%) and in control woods in the south (pheasant woods: 19·8 ± 2·0, control woods: 25·0 ± 2·8).
Higher levels of tree canopy cover were recorded in control woods than pheasant woods (southern region pheasant woods: 76·3 ± 1·8%, control woods: 83·6 ± 2·1%, eastern region pheasant woods: 78·2 ± 2·5%, control woods: 80·3 ± 2·4%, Wald = 6·26, d.f. = 1, P = 0·01), indicating that pheasant woods had a more open structure. There was no regional difference in levels of canopy cover (Wald = 0·01, d.f. = 1, P = 0·91), but there was a significant date effect with higher levels of canopy cover later in the survey period (Wald = 17·39, d.f. = 2, P < 0·001).
Overall, between 22% and 32% more birds were observed in pheasant woods than control woods (Table 1). Higher numbers of birds in the ‘warbler’ group, and woodpigeons were recorded in pheasant-managed woods in both regions. There were also regional differences in numbers of many bird groups with a tendency for more birds in southern woods (Table 1). There was a significant region × treatment effect for the ‘others’ group, with higher numbers recorded in pheasant woods in the south and control woods in the east. There was a decline in the total number of birds recorded per wood through the survey period (Wald = 35·11, d.f. = 2, P < 0·001). Date was a significant factor for the finch group with more birds recorded in earlier surveys (Wald = 43·64, d.f. = 2, P < 0·001). There was a significant negative correlation between pigeon numbers and woodland area (Wald = 6·78, d.f. = 1, P = 0·009). Woodland area did not significantly influence any of the other bird groups.
The overall composition of bird groups differed significantly between pheasant-managed and control woodlands (Λ = 0·926, F5,153 = 2·42, P = 0·04) (Fig. 1). A ranking matrix (Table 3) showed that relative to control woods, woodpigeons constituted a larger proportion of the bird community in pheasant woods. There was also a significant regional difference in the composition of bird groups (Λ = 0·879, F5,153 = 4·17, P = 0·001) with a higher proportion of woodpigeons in southern woods relative to eastern woods. There was no difference in the overall composition of bird groups between randomly and non-randomly selected woods (Λ = 0·979, F5,153 = 0·648, P = 0·66).
Figure 1. Percentage compositions (mean ± 1 SE) of bird groups in pheasant managed woods (n = 81) and control woods (n = 78) during May–July 2004.
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Table 3. Ranking matrix for the relative proportions of bird groups in woodland interiors in pheasant-managed woods (n = 81) and control woods (n = 78) in southern and eastern England, summer 2004. Pheasant woods are represented by the rows and control woods by columns. + indicates the row group constituted a higher proportion than expected relative to the column; – indicates the opposite. A triple sign indicates that the difference was significant at P < 0·05
|Pigeons||+ + +||+ + +||+||+ + +||+||5|
|Others||+||+ + +||–||+||–||4|
Species richness did not differ between pheasant and control woods (Wald = 3·09, d.f. = 1, P = 0·08) but was higher in southern woods than eastern woods (mean species per wood ± 1 SE, 7·51 ± 0·25 and 6·95 ± 0·25, respectively, Wald = 7·27, d.f. = 1, P = 0·007). There was a significant effect of date category (Wald = 25·37, d.f. = 2, P < 0·001) and a negative relationship with woodland area within 1 km (Wald = 4·21, d.f. = 1, P = 0·04). The time of day that surveys were conducted also influenced species richness, with more species recorded in surveys started before 12·00 (Wald = 4·89, d.f. = 1, P = 0·03). Simpson's diversity index differed between regions (Wald = 7·05, d.f. = 1, P = 0·008), with a higher diversity index recorded in southern woods. Date was a significant factor, with a higher index recorded in earlier surveys (Wald = 17·00, d.f. = 2, P < 0·001). Wind was also a significant factor, with a lower index associated with higher wind categories (Wald = 6·48, d.f. = 2, P = 0·04).
For the bird groups where we found significant treatment effects, we conducted further REML analyses to determine the influence of vegetation characteristics on bird numbers. There was a negative correlation between total bird numbers and amount of tree canopy cover (Wald = 4·99, d.f. = 1, P = 0·03) (Fig. 2a). Region (Wald = 30·57, d.f. = 1, P < 0·001) was again a significant factor influencing total bird numbers. There was a significant negative correlation between amount of canopy cover and pigeon numbers (Wald = 5·24, d.f. = 1, P = 0·02) (Fig. 2b). Region was also a significant factor (Wald = 9·53, d.f. = 1, P = 0·002). There were positive correlations between numbers of warblers and the amount of vegetation (Wald = 4·93, d.f. = 1, P = 0·03) and the amount of R. fruticosus (Wald = 13·77, d.f. = 1, P < 0·001). There was also a negative correlation between warbler numbers and amount of tree canopy cover (Wald = 4·23, d.f. = 1, P = 0·04) (Fig. 2c).
REML analysis of deer browsing yielded a treatment × region effect (Wald = 4·42, d.f. = 1, P = 0·04). There was more evidence of deer browsing (% sampling stations with browsing recorded) in pheasant woods in the south and more in control woods in the east (southern region, pheasant woods: 22·5 ± 2·4%, control woods: 16·2 ± 1·8%, eastern region, pheasant woods: 18·7 ± 2·9%, control woods: 22·1 ± 2·4%). Date category (Wald = 52·63, d.f. = 2, P < 0·001) and woodland area (Wald = 6·27, d.f. = 1, P = 0·01) were also significant. Examination of the data for deer species seen indicated that muntjac deer Muntiacus reevesi Raf. was the most abundant deer species recorded in eastern woods and roe deer Capreolus capreolus L. the most abundant species in southern woods.