capercaillie and black grouse productivity
The annual productivity of capercaillie at Abernethy Forest ranged from 0 to 2·9 chicks female−1 (mean 0·71, n= 11; Fig. 2). Poisson regression showed that there was a significant interaction between site and year when Abernethy was compared with the reference sites (χ2 = 166·8, d.f. = 10, P < 0·001; Fig. 3). During 1989–93 and 1997–99, productivity at the reference sites was higher than at Abernethy Forest but during 1994–96 the opposite was true. This shows that the higher productivity at Abernethy Forest during 1994–96 was independent of the general pattern of productivity across Scotland.
Figure 2. Annual breeding productivities (chicks per female) of capercaillie (circles) and black grouse (squares) at Abernethy Forest. Predator control took place between 1992 and 1996. Numbers of female black grouse (above) and capercaillie (below) are shown.
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Figure 3. Breeding productivity (chicks per female) of capercaillie at Abernethy Forest (circles) and other forests (squares, with 95% confidence limits). Predator control at Abernethy Forest took place between 1992 and 1996.
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The productivity of black grouse at Abernethy Forest ranged from 0·4 to 4·7 chicks female−1 (mean 2·17, n = 11; Fig. 2), greater than that for capercaillie. Annual variations in black grouse productivity were significantly correlated with capercaillie productivity (rs = 0·84, P < 0·005, n= 11).
egg predation by captive predators
The fragments of shells from hen eggs eaten by captive red foxes, pine martens and crows showed that red foxes crunched the eggs so that no large fragments remained. The median size of the largest pieces was 0·44 g (interquartile range 0·29–0·91 g). Even when a large part of the egg shell appeared intact it was found to be cracked and held together only by the membrane. Crows tended to make one hole through which the majority of the contents were removed. Some yolk remained in the shell. The distribution of the largest fragments was bimodal because crows sometimes broke the egg into two large pieces. The median size of all the largest pieces was 5·0 g (interquartile range 2·2–5·7 g). Pine martens made larger holes in eggs than crows. However, they licked the egg clean so no pool of yolk remained. The median mass of the largest pieces was 1·2 g (interquartile range 0·7–2·3 g).
Crows made distinctive marks in the wax eggs, comprising paired dents separated by varying distances between the two dents of a pair. Sometimes the dents were together when the bill was closed, and up to 1 cm apart if the bill was gaping. The dents tended to be distributed around the equator of the egg since this part lies uppermost. The wax eggs given to a red fox were chewed and showed slicing marks in the wax. Pine martens left paired canine marks in the wax.
Thus, it was possible to classify depredated eggs as ‘crow-depredated’ or ‘mammal-depredated’ from the size of the largest fragment, the presence of yolk (if found fresh) and from marks left in the wax egg. Only fresh eggs were used in these experiments. Clearly, a larger hole would be needed to extract the chick from a well-incubated egg. Therefore, we may have underestimated the number of wild capercaillie eggs taken by crows.
predation rates on artificial nests
The predation rates on the artificial nests along the two transects were correlated (rs = 0·77, P < 0·05, n= 9). Overall, predation rates were high during 1991–93, low in 1994 and 1995, and high again during 1997–99 (Table 3).
Table 3. Predation rates on artificial nests at Abernethy Forest. Years with predator control are shown in bold. Each transect had 24 nests
|Year||Transect||Daily predation rate on nests||Standard error||Percentage chance of predation in 28 days||Number depredated||Number taken by crows||Number taken by mammals|
|1991||1||0.1942||0·0390||99·8||20|| 8|| 0|
|1992||1||0·1565||0·0300||99·1||23|| 8|| 0|
|1992||2||0·0323||0·0088||60·1||13|| 8|| 0|
|1994||1||0·0198||0·0057||42·8||12|| 8|| 2|
|1994||2||0·0129||0·0045||30·5|| 8|| 5|| 0|
|1995||1||0·0133||0·0047||31·2|| 8|| 5|| 0|
|1995||2||0·0220||0·0060||46·3||13|| 0|| 9|
|1996||2||0·0166||0·0055||37·4|| 9|| 8|| 0|
|1997||1||0·0594||0·0126||82·0||21|| 2|| 3|
|1997||2||0·0260||0·0069||52·1||14|| 2|| 1|
|1998||1||0·0896||0·0187||92·8||21|| 1|| 2|
|1998||2||0·0497||0·0121||76·0||16|| 2|| 1|
|1999||2||0·0248||0·0071||50·6||12|| 5|| 2|
|Interaction|| 230·1|| 4·1||8||< 0·001|
In many cases when an artificial nest was depredated, the clutch of hen eggs was removed and not found elsewhere and the wax egg had no marks on it, so the predator could not be identified. Cameras at artificial nests in 1999 and 2000 showed that on all 21 occasions when all the hen eggs were removed and the wax egg was untouched, the predator was a pine marten. Therefore, some of the ‘unknown’ losses of artificial nests can probably be attributed to pine martens, at least after 1994.
At other depredated artificial nests, the size distribution of the largest fragments of hen eggs indicated that crows were mainly responsible for predation where the predator was known (Table 3). Some nests depredated by crows were identified from eggs found away (greater than 20 m) from the nest, where no signs had been left on wax-filled eggs. Therefore, some of the ‘unknown’ category also referred to crows. Occasionally, the wax-filled egg was removed completely from the nest site, the attached string having been broken or pulled out of the wax. These were probably taken by mammals because considerable strength would have been required.
Using the data for which artificial nest losses could be assigned to crows or mammals, minimum daily predation rates for each year showed that crow predation was greatest during 1991–93 (Fig. 4). The small peak in 1996 occurred when the number of breeding crows was low but non-breeding birds were present (Table 2). Predation by mammals showed a contrasting pattern, and increased over the study period (Fig. 4). These indices were correlated with counts of predators, confirming their reliability as measures of predator activity. Thus, the daily predation rates by crows on artificial nests were correlated with the numbers of crow territories (rs = 0·88, P < 0·005, n= 9), and the daily predation rates by mammals on artificial nests were correlated with the number of pine marten sightings (rs = 0·79, P < 0·02, n= 9) but not with counts of fox scats (rs =−0·64, NS, n= 8). The correlation with pine martens indicated that they, rather than red foxes, were the main mammalian predators of artificial nests.
Figure 4. Minimum daily predation rates by crows (circles) and mammals (squares) on artificial nests. The vertical lines show 95% confidence limits. Predator control took place between 1992 and 1996.
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Most depredated hen eggs were found where they were originally placed or within a few metres, but some were also found at shell dumps made by crows, up to 2·95 km away. Groups of depredated eggs sometimes included eggs from more than one artificial clutch. Depredated capercaillie eggs were also found casually and at shell dumps in crow territories. Numbers attributed to crows found each year varied from 0 to nine capercaillie eggs (mean 2·4 annum−1). Smaller numbers of crow-depredated black grouse eggs were found (mean 1·0 annum−1, range 0–4).
Estimates of the number of capercaillie eggs that were depredated by crows were 18–35 in 1991, 71–158 in 1992 and 55–118 in 1993 (Table 4). Thus, minimum estimates of capercaillie clutches lost to crows were three to five in 1991, 10–23 in 1992 and eight to 17 in 1993. These estimates could only be made for 1991–93 because no hen eggs from artificial nests were found away from nests in other years.
Table 4. Numbers of artificial clutches and eggs lost to predators, numbers of depredated hen eggs found at greater than 20 m from their original site, numbers of depredated capercaillie eggs found, and estimates of total number of capercaillie eggs taken by crows. Maximum and minimum estimates of capercaillie eggs taken by crows (a and b) are given. The (a) estimate is based on the number of hen eggs from artificial nests known to have been taken by crows plus the number that potentially could have been taken by crows (i.e. those taken by unknown predators). Estimate (b) is based only on artificial clutches known to have been taken by crows. –, no count or estimate. Years with predator control are shown in bold
|Year||Total number of artificial eggs (clutches) depredated||Number of artificial eggs (clutches) lost to crows||Number of hen eggs (dumps) found depredated by crows||Number of capercaillie eggs (dumps) found depredated||Number of capercaillie eggs (dumps) eaten by crows||Estimate of the total number of capercaillie eggs (÷7 = nests) depredated by crows|
|1990||–||–||–||5 (5)||3 (3)||–||–|
|1991||207 (35)||107 (18)||12 (12)||4 (4)||2 (2)|| 35 (5)||18 (3)|
|1992||210 (36)|| 95 (16)||12 (6)||9 (4)||9 (4)||158 (23)||71 (10)|
|1993||266 (45)||123 (21)|| 9 (5)||5 (5)||4 (4)||118 (17)||55 (8)|
|1994||104 (20)|| 76 (13)|| 0||1 (1)||1 (1)||–||–|
|1995|| 93 (21)|| 18 (5)|| 0||2 (2)||2 (2)||–||–|
|1996||163 (28)||133 (23)|| 0||3 (3)||1 (1)||–||–|
|1997||198 (35)|| 24 (4)|| 0||1 (1)||1 (1)||–||–|
|1998||210 (37)|| 7 (2)|| 0||0||0||–||–|
|1999||193 (34)|| 29 (6)|| 0||1 (1)||1 (1)||–||–|
A two-way (year × season) Poisson anova of red deer counts indicated significant variation among years (likelihood-ratio test statistic, χ2 = 456, d.f. = 10, P < 0·001) and between the March and October counts (χ2 = 241, d.f. = 1, P < 0·001). Counts in March and October were similar in 1989 and 1990, but the October counts declined by 1992 and remained low (Table 2). March densities also declined across years but not as steeply as the October counts. Combined October and March counts declined significantly from 1989 (rs =−0·53, P < 0·02, n= 22). The average March density was about five red deer km−2 by 1999, approximately half the 1989 density.
relationships of capercaillie and black grouse productivity with environmental variables
Univariate Poisson regression showed that capercaillie productivity at Abernethy Forest was negatively related to the predation rate of the artificial nests, and with the amount of rainfall in the first 20 days of June and in the whole of June (Table 5).
Table 5. Univariate Poisson regression relationships between capercaillie and black grouse productivity, and independent variables
| ||Capercaillie||Black grouse|
|t||Residual d.f.||P||t||Residual d.f.||P|
|Artificial nest predation||−3·25||7||< 0·02||−2·13||7||NS|
|Crow predation on artificial nests||−0·47||7||NS||−0·24||7||NS|
|Mammal predation on artificial nests||−0·73||7||NS||−0·46||7||NS|
|Number of breeding crows||−0·68||7||NS||−0·66||7||NS|
|Red fox abundance (scats)||−0·37||6||NS|| 0·46||6||NS|
|June rainfall (first 20 days)||−2·59||9||< 0·05||−2·04||9||NS|
|June rainfall (last 20 days)||−2·20||9||NS||−3·07||9||< 0·02|
|June rainfall (total)||−2·96||9||< 0·02||−2·85||9||< 0·02|
|Late May temperature||−0·99||9||NS|| 1·38||9||NS|
|Mid-April temperature|| 1·30||9||NS|| 0·81||9||NS|
|Relative mid-April temperature|| 1·75||9||NS|| 1·48||9||NS|
|Red deer density (March)|| 0·64||9||NS|| 0·32||9||NS|
|Vaccinium height||−0·06||5||NS|| 1·07||5||NS|
Multiple Poisson regression showed that capercaillie productivity was significantly and negatively related to the overall rate of predation on artificial nests and also negatively related to June rainfall (Table 6a). The effect of the two-way interaction between these two variables was not significant (χ2 = 0·93, d.f. = 1, NS).
Table 6. (a) The relationship between capercaillie productivity at Abernethy Forest and June rainfall (mm) and the percentage of the artificial nests depredated in 28 days. Percentage of deviance accounted for = 90%. (b) The relationship between capercaillie productivity at Abernethy Forest and June rainfall (mm) plus the minimum daily predation rates on artificial nests by crows. Percentage of deviance accounted for = 92%. (c) The relationship between black grouse productivity at Abernethy Forest and total June rainfall (mm) plus the minimum daily predation rates on artificial nests by crows. Percentage of deviance accounted for = 84%
|Intercept|| 3·722|| 0·6617|| || |
|June rain|| −0·0264|| 0·00763||−3·46||< 0·02|
|Predation on artificial nests|| −0·0454|| 0·01034||−4·39||< 0·01|
|Intercept|| 6·633|| 1·511|| || |
|June rain|| −0·1037|| 0·02641||−3·93||< 0·02|
|Crow predation||−209·1||53·15||−3·93||< 0·02|
|Interaction|| 2·219|| 0·660|| 3·36||< 0·05|
|Intercept|| 3·107|| 0·556|| || |
|June rain|| −0·0314|| 0·00755||−4·16||< 0·01|
|Crow predation|| −65·32||20·87||−3·13||< 0·05|
|Interaction|| 0·6611|| 0·2158|| 3·06||< 0·05|
When the variable selection procedure was repeated with the predation rates on artificial nests by crows and mammals as candidate variables instead of the overall rate, the selected model included a negative effect on capercaillie productivity of June rainfall; the negative effect of predation rate on artificial nests by crows was significant only when the two-way interaction between these variables was considered (Table 6b and Fig. 7). The interaction between predation rate by crows and June rainfall was positive, so that the magnitude of the negative effect of crow predation was reduced when June rainfall was high. Thus, capercaillie productivity was highest in dry Junes when crow predation on artificial nests was low. No relationship between capercaillie productivity and predation rate on artificial nests by mammals was detected, indicating that crow predation may have been more important than mammal predation.
Figure 7. The relationship between capercaillie and black grouse productivity (chicks per female) and total June rainfall for different years (91 = 1991, etc.) at Abernethy Forest. Squares indicate years in which crows and red foxes were culled and circles show years with no predator control. The filled and open symbols indicate years when crow predation on artificial nests was above and below or equal to the median, respectively. The solid and dashed lines are the regression lines for the models (Table 6) when the average predation rate by crows was low (less than or equal to the median) and high (greater than the median), respectively.
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Poisson regressions were also carried out to examine patterns in black grouse productivity. Univariate analyses indicated significant negative relationships between black grouse productivity and rainfall in the last 20 days of June and in the whole of June (Table 5). In multiple regression, initially excluding predation rates on artificial nests by crows and mammals, no two combinations of variables were significant. However, there was a relationship between black grouse productivity and June rainfall, plus crow predation on artificial nests, plus their interaction (Table 6c and Fig. 7). Again, the negative effect of predation rate on artificial nests by crows was significant only when the two-way interaction between these variables was considered.