Importance of Sitka Spruce Picea Sitchensis Seed and Garden Bird-Feeders for Siskins Carduelis Spinus and Coal Tits Periparus Ater

Bird Study (2007) 54, 236–247

Importance of Sitka Spruce Picea sitchensis seed and garden bird-feeders for Siskins Carduelis spinus and Coal Tits Periparus ater

AILSA J. MCKENZIE1,2*, STEVE J. PETTY3, MIKE P. TOMS4 and ROBERT W. FURNESS1 1Institute of Biomedical and Life Sciences, Graham Kerr Building, University of Glasgow, Glasgow, G12 8QQ, UK, 2School of Biology and Psychology, Ridley Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK, 3Ecology Division, Forest Research, Northern Research Station, Roslin, Midlothian, EH25 9SY, UK and 4British Trust for Ornithology, The Nunnery, Thetford, Norfolk, IP24 2PU, UK

Capsule Siskins and Coal Tits were present in gardens more often in years of poor Sitka Spruce cone crops, with synchrony across the UK. Aims To examine how the relative abundances of Siskins and Coal Tits in a garden in western Scotland varied annually with the size of the spruce cone crop, and to determine whether this pattern was similar nationally. Methods A ten-year mist-netting data set from Tarbet, Argyll and Bute was used to test the hypothesis that some species utilize supplementary food in gardens more often in years with poor cone crops than in years when crops are large. Annual peaks in the relative abundances of four species (two that ate conifer seed and two that did not) in Tarbet were correlated with a Sitka Spruce masting index. BTO Garden BirdWatch data from six regions of the UK were correlated with the Tarbet data, the cone index and each other to determine the extent of synchrony. Results Both Siskins and Coal Tits were present more often in the Tarbet garden in years with poor cone crops, with relative abundances significantly negatively correlated with cone crop for both species (r = –0.739 and –0.832, respectively). Correlations between regions using BTO data showed abundances of species in gardens to be broadly similar across the UK. Independent scores of cone abundance in Kielder and Galloway were correlated. Correlations between BTO data, cone index and Tarbet garden data were mostly high and almost all in the same direction, also indicating a degree of synchrony in cone production across the UK. Conclusion Both Siskins and Coal Tits appear to be influenced by changes in cone abundance, switching to feed on supplementary food in gardens more often in years with few cones than in mast years. While such a relationship has been shown previously for Siskins, for Coal Tits this is a novel result.

Over the last 80 years, the area of forest in the UK has Petty & Avery 1990). Conversely, many woodland increased from 5.0% of the land surface in 1924 to species have benefited, including some with previously 11.5% in 2004 (Forestry Commission 2004). Most of restricted distributions, or even species that had ceased this increase has been accomplished by establishing to breed in Britain (Jardine 1993, Knox 1993, Petty forests of non-native species on marginal agricultural 1996). land in the uplands (Petty & Avery 1990). The estab- Sitka Spruce Picea sitchensis, a native of northwest lishment of such habitats has been criticized because of America, has been one of the main tree species used the negative impact on several moorland bird species, in Britain’s reforestation programme because of its some of high conservation value (Nature Conservancy exceptional growth rate and high quality timber Council 1986, Stroud et al. 1987, Thompson et al. 1988, (Cahalan 1987, Rook 1992). Today it constitutes 29% of the total forest area of the UK, and 47% of the for- *Correspondence author. est area in Scotland (Forestry Commission 2004). It Email: [email protected] produces prolific cone crops at intervals of three to

five years that appear to be synchronized over large broadleaved forests (Perrins 1979), but similar to geographical areas (Matthews 1955, Philipson, 1987, Marsh Tits Poecile palustris and Willow Tits Poecile Petty et al. 1995, Lurz et al. 2000). Sitka Spruce has montanus (Perrins 1979). relatively small seeds compared to widespread conifer The practice of feeding birds in gardens has become species native to Europe such as Scots Pine Pinus more popular over the last 30 years, yet it is only now sylvestris, Norway Spruce Picea abies and European that its effects are being considered scientifically Larch Larix deciduas (Staines et al. 1987). Sitka Spruce (Cannon 1999, Bland et al. 2004). It has been suggested is also one of the first conifers to shed its seed in that when natural foods are scarce, birds will utilize autumn. Therefore, it is of considerable importance to supplementary foods to a much greater extent than better understand how the potentially enormous food when natural foods are plentiful (Brittingham & resource on Sitka Spruce is utilized by Britain’s wildlife, Temple 1988, 1992a, Chamberlain et al. 2005). This as many characteristics differ from those of native was found to be the case for Siskins, with virtually no European conifers with which conifer seed-eating birds using feeders in good cone years (Shaw 1990). species have co-evolved. Years when the highest numbers of Siskins were In northern Europe, many birds that exploit conifer observed in the forest coincided with years when the seed exhibit rapid population growth rates in years with cone crop was particularly good (Shaw 1990). a good cone crop, due to extended breeding seasons and Although a similar relationship has been suggested in low mortality (Newton 1972). In poor cone years, a Coal Tits, no analysis has been undertaken to investi- range of adaptations have evolved to overcome periods gate whether they too switch between cones and when conifer seed is unavailable, such as nomadic garden feeders depending on cone abundance. movements (Newton 1970), scatter-hoarding of conifer In this paper, we use a large data set of birds mist- seed (Moreno et al. 1988, Perrins 1979) and switching netted in a garden in western Scotland (Tarbet), a to other foods (Newton 1972). location surrounded by extensive Sitka Spruce forests, In Britain, an increasing range of bird species has to investigate relationships between cone crops and the been shown to exploit Sitka Spruce seed (Shaw & occurrence of Siskins and Coal Tits at bird-feeders. Our Livingstone 1991). Two of the best known examples hypothesis was that both species would use feeders in are Common Crossbill Loxia curvirostra and Siskin greater numbers when the cone crop was poor. As the Carduelis spinus, both of which have greatly increased data set contained information on a large number of their breeding ranges in Britain in response to the other species, a further two species (Blue Tit and Great increasing area of cone-producing forest, with numbers Tit) were used as controls. It was predicted that dramatically increasing in good cone years (Jardine the abundance of both would be unrelated to Sitka 1993, Knox 1993). A number of studies have shown Spruce cone crop, as both species feed predominantly that population size and breeding performance of both in deciduous woodlands (Perrins 1979). species are closely linked to the coning frequencies of As cone crops appear to be synchronized over large spruce, both in Britain and mainland Europe geographical areas, it is likely that the cone data used (Reinikainen 1937, Newton 1972, Petty et al. 1995, in this study were representative of coning frequencies Virkkala 2004). A range of other bird species utilize over a larger area, comprising at least northern England Sitka Spruce seed when it is abundant, including Coal and southern Scotland. To investigate this possibility, Tits Periparus ater. Coal Tits have also increased their we used the British Trust for Ornithology’s (BTO) breeding range in Britain, as conifer forests are their Garden BirdWatch (GBW) data from six regions in the preferred breeding habitat, although the species occurs UK to see if peaks in reporting rate of the four species widely in other habitats outside the breeding season. (Coal Tit, Blue Tit, Great Tit and Siskin) were Similarly to other tits, Coal Tits feed largely on insects synchronized with the mist-netting data from Tarbet during the breeding season, but exploit seeds at other and the cone crop index. The GBW data were times of the year and have been observed feeding on independent of the mist-netting records and were Sitka Spruce seeds, usually from cones on trees (Perrins collected by a large number of people over an extensive 1979, Shaw & Livingstone 1991). They are known to area, thus permitting regional patterns to be explored. scatter-hoard seeds in conifer forests (Brotons & The likely effect of there being significantly less Sitka Haftorn 1999, Brotons 2000a, 2000b), unlike the Spruce present in the south of England compared to ubiquitous Blue Tit Cyanistes caeruleus and Great Tit northern Britain were incorporated into the analysis by Parus major, species characteristic of temperate using a general linear model (GLM).

MATERIALS AND METHODS Kielder Forest, Northumberland (Petty et al. 1995). A similar assessment was carried out in Galloway by G. Study area and catching protocol Shaw from 1982 to 2001 after which time R.W. Furness Birds were caught in a mist-net and ringed by R.W. continued recording in Argyll. Petty and Shaw estab- Furness in a garden in Tarbet, near Loch Lomond in lished criteria for a cone crop index running from zero western Scotland (56°12′N, 4°42′W), between January to 10 (see Table 1 for definitions of each increment). 1995 and October 2004. The net was set close to bird- The index used in this analysis comprised the Petty feeders and captured birds were removed, identified, data during 1994/5–1997/8, the Shaw data during sexed and aged (if possible), ringed or ring read, 1998/9–2001/2 and the Furness data during 2002/3 and weighed and released. Birds were caught on between 2003/4. Although data were taken from different areas one and 15 (mean = 4) days in most months, when the of the UK, evidence suggests that cone crops tend to be weather was not wet or windy. Catching effort varied synchronous over large geographical areas. This is and tended to be low in summer. During the study supported by the data from the 16-year overlap of cone period a total of 27 713 birds were caught from 47 scoring in Kielder and Galloway, which gives a cone different species. score correlation of 0.67 (P < 0.01) between these locations.

Standardizing data The data for Siskin, Coal Tit, Blue Tit and Great Tit Data analysis (hereafter referred to as ‘the four species’) were Each of the four species displayed a marked standardized to control for variations in catching effort. winter/spring peak in abundance in the Tarbet garden, This was done by calculating numbers of the four and the value of this peak was recorded for each year species caught per calendar week per year and dividing and correlated with the Sitka Spruce cone index. The this by the total number of birds caught per calendar peak value was at times difficult to discern, especially week per year. Thus, the statistic used is the proportion for Blue Tits and Great Tits as they fluctuated less of total catch represented by the focal species. The obviously than the other two species and were also more common ‘catch per unit effort’ (CPUE) method present for much of the year. was not deemed suitable in this instance due to variation in the duration of catching sessions and effects of weather on hourly catch rates. Also, a weekly measure BTO data corresponded with the BTO method of recording Since 1995, the BTO has encouraged its members to ‘reporting rate per week per year’, with which the data report bird sightings from their gardens throughout were compared. Days when the number of birds caught the year. As this data set is based on reporting rate in the garden was five or fewer were removed, as these (presence of a species) rather than abundance of were predominantly days when the weather became individual species, it is relatively crude. However, it has unsuitable for catching and may well have been unrep- highlighted some interesting trends (Chamberlain et al. resentative, as well as subject to sampling error. 2004, 2005). This data set can be used to see whether Siskins and Coal Tits display a similar pattern of peaks and troughs Sitka Spruce cone index in other areas of the UK to those seen in the study area, During 1974–98, Sitka Spruce cone crops were assessed and by so doing an inference can be drawn about cone by S.J. Petty over one week in mid-September, in crop synchrony across the UK. The BTO data were

Table 1. Definition of cone index (adapted from Petty et al. 1995).

Index value Cone crop Definition

0 Nil No cones visible on upper tree canopy when viewed with binoculars 1, 2 Very low No cones visible at a casual glance and very few found by search with binoculars 3, 4 Low Occasional clusters visible on a few trees in some blocks 5, 6 Medium Occasional clusters visible in all blocks 7, 8 High Clusters prominent in all blocks, some trees with multiple clusters 9, 10 Very high Multiple clusters easily visible in all blocks, colour of tree tops swamped by cones

independent of the mist-netting data set used here. and cone index to determine how much of an influence Thus, if patterns were in agreement with those found in this has on significance. the Tarbet data, it is likely that any biases within that data set will not have affected the overall pattern. The annual peak reporting rate values for the four RESULTS species were obtained from the BTO for six regions of the UK (north and south Scotland, and NE, NW, SE Tarbet mist-net data and SW England) in order to compare reporting rate Siskin between regions and reporting rates with both the While relative abundance varied widely both within Sitka Spruce index and the Tarbet data. Pearson corre- and among years, the overall pattern among years was lations were carried out on the data and, where generally consistent and characterized by a spring peak appropriate, probabilities were combined to reduce the (February–late May). This was followed by a marked number of r-values and eliminate autocorrelation decline, often to zero during the summer months, (Tables 6–8) where before peaking again in the autumn, though with far smaller magnitude. The largest spring peaks were –2Σln P = combined probability seen in 1995, 1998 and 2004 (0.39, 0.40 and 0.68, respectively) and the smallest in 1996 and 1997 (0.03 with df = 2k (k = number of separate tests and proba- and 0.08) (Fig. 1a). bilities). As there is significantly less Sitka Spruce available in the south of England than in the other Coal Tit areas considered (less than 0.2% of total woodland Relative abundance varied more than for Siskins, both compared with 36% and 48% in northern England and within and among years. There was a pattern in the Scotland, respectively), the values for percentage Sitka data, but this was less clear than for Siskins. The peak Spruce cover (Forestry Commission 2001a, 2001b) tended to occur in winter, typically during were introduced into a GLM along with reporting rate November–December, but was sometimes as late as

Figure 1. Relative abundance of the four species expressed as a proportion of the total number of birds caught per week, 1995–2004. (a) Siskin; (b) Coal Tit; (c) Blue Tit; (d) Great Tit.

February, with abundance then remaining relatively high into late winter–early spring, with at times a smaller autumn peak. The largest peaks were seen in 2003 and 2004 (0.49, 0.57) and the smallest in 1997 (0.14) (Fig. 1b).

Blue Tit and Great Tit Relative abundance of these two species varied widely although the pattern was less clear than for either Siskins or Coal Tits. The peak in abundance tended to occur during late winter–early spring (January–March), but abundance remained relatively high throughout Figure 2. Sitka Spruce cone index, 1994/5–2003/4. the year. There was often another peak later in the year but the timing of this varied between July and significant (87%), despite the fact that owing to September (Fig. 1c, d). the relatively small sample size, the threshold for significance is high (0.6).

Sitka Spruce cone index Coal Tit Cone index As with the BTO Siskin data, reporting rate peaks Between 1994/5 and 2003/4 the crop varied from 1 between 1995 and 2004 for the six regions indicated a (very low) in 2002/3 and 2003/4 to 8 (high) in 1996/7 degree of correlation across the country (Fig. 3b). and 1998/9. During the ten-year period there were The annual peak in reporting rate was also positively three peaks, in 1996/7, 1998/9 and 2000/1 (Fig. 2). correlated across a number of UK regions. Of 15 correlations, nine (60%) were significant. Although Cone index versus Tarbet data the range of correlations was greater for Siskins, with The relative abundance of both Siskins and Coal Tits values as low as 0.154 (S Scotland and SE England), in the garden was significantly negatively correlated four of six non-significant coefficients were greater with cone crop (r = –0.739 and r = –0.832, respectively, than 0.47 (Table 3). both P < 0.05). In contrast, neither Blue Tit nor Great Tit abundance was correlated with the cone index (r = Blue Tit and Great Tit 0.46 and r = –0.03, respectively, both non-significant). Reporting rate peaks between 1995 and 2004 for the six regions indicated a degree of correlation across the country for Blue Tits (Fig. 3c). Analysis showed that BTO data versus BTO data the annual peak in reporting rate was also positively Siskin correlated across a number of UK regions, with nine of Plotting peaks of reporting rate from the BTO’s GBW 15 correlations significant (60%) (Table 4). However, data for the six regions, from 1995 to 2004, indicated a as Blue Tits are a very common species in gardens, degree of correlation across the country (Fig. 3a). reporting rate was always close to 1. For Great Tits, Indeed, statistical analyses showed that these peaks a similar pattern was observed (Fig. 3d); however, were significantly positively correlated across a number only four of 15 correlations were significant (27%) of UK regions, with 13 from 15 correlations (Table 2) (Table 5).

Table 2. Correlations in BTO reporting rate between UK regions for Siskins.

N Scotland S Scotland NE England NW England SE England

S Scotland 0.837, P < 0.05 NE England 0.868, P < 0.05 0.914, P < 0.05 NW England 0.738, P < 0.05 0.715, P < 0.05 0.826, P < 0.05 SE England 0.703, P < 0.05 0.452, ns 0.633, P < 0.05 0.643, P < 0.05 SW England 0.626, P < 0.05 0.439, ns 0.625, P < 0.05 0.615, P < 0.05 0.984, P < 0.05 ns, Not significant, P > 0.05.

Figure 3. Trends in reporting rate annual peak values across six regions of the UK using BTO Garden BirdWatch data. Thick line, N Scotland; thin line, S Scotland; thick dashed line, NE England; thin dashed line, NW England; thick dotted line, SE England; thin dotted line, SW England. (a) Siskin; (b) Coal Tit; (c) Blue Tit; (d) Great Tit.

BTO data versus cone index with the Sitka Spruce cone index, peaks of reporting rate were significantly negatively correlated with cone Siskin index in two of the six regions (N and S Scotland). When BTO data for the six regions were correlated Correlations for the other five regions varied more with the Sitka Spruce cone index, peaks of reporting widely than for Siskins (–0.565 to 0.123), although rate were significantly negatively correlated with the values for NE and NW England were relatively close to cone index in only one of the six regions (N Scotland). significance (–0.565 and –0.506, respectively) (Table Nevertheless, the r-values with the five other regions 6). As with the Siskin data, when probabilities were were all negative and ranged between –0.49 and –0.58 combined, the relationship was significant to the 0.01 (Table 6). When probabilities were combined, the level (Table 6). relationship was significant to the 0.01 level (Table 6). Blue Tit and Great Tit Coal Tit When the BTO data for the six regions were correlated When the BTO data for the six regions were correlated with the Sitka Spruce cone index, peaks of reporting

Table 3. Correlations in BTO reporting rate between UK regions for Coal Tits.

N Scotland S Scotland NE England NW England SE England

S Scotland 0.719, P < 0.05 NE England 0.473, ns 0.773, P < 0.05 NW England 0.607, P < 0.05 0.810, P < 0.05 0.889, P < 0.05 SE England 0.211, ns 0.154, ns 0.521, ns 0.474, ns SW England 0.560, ns 0.695, P < 0.05 0.825, P < 0.05 0.851, P < 0.05 0.772, P < 0.05 ns, Not significant, P > 0.05.

Table 4. Correlations in BTO reporting rate between UK regions for Blue Tits.

N Scotland S Scotland NE England NW England SE England

S Scotland –0.045, ns NE England 0.360, ns 0.337, ns NW England 0.756, P < 0.05 –0.137, ns 0.60, P < 0.05 SE England 0.773, P < 0.05 –0.168, ns 0.681, P < 0.05 0.961, P < 0.05 SW England 0.783, P < 0.05 –0.086, ns 0.656, P < 0.05 0.968, P < 0.05 0.978, P < 0.05 ns, Not significant, P > 0.05.

Table 5. Correlations in BTO reporting rate between UK regions for Great Tits.

N Scotland S Scotland NE England NW England SE England

S Scotland 0.737, P < 0.05 NE England –0.098, ns 0.154, ns NW England 0.293, ns 0.191, ns 0.370, ns SE England 0.135, ns 0.082, ns 0.171, ns 0.699, P < 0.05 SW England 0.188, ns 0.286, ns 0.440, ns 0.628, P < 0.05 0.771, P < 0.05 ns, Not significant, P > 0.05.

Table 6. Correlation coefficients between BTO reporting rate peaks and Sitka Spruce cone index.

Siskin Coal Tit Blue Tit Great Tit

N Scotland –0.624, P < 0.05 –0.800, P < 0.05 –0.062, ns –0.391, ns S Scotland –0.487, ns –0.798, P < 0.05 –0.035, ns –0.267, ns NE England –0.523, ns –0.566, ns 0.291, ns –0.234, ns NW England –0.486, ns –0.505, ns 0.467, ns 0.349, ns SE England –0.579, ns 0.123, ns 0.395, ns 0.456, ns SW England –0.523, ns –0.346, ns 0.482, ns 0.336, ns Combined probabilities 26.857, P < 0.05 32.431, P < 0.05 12.089, ns 13.334, ns ns, Not significant, P > 0.05. rate were not significantly correlated with cone index probabilities shows an overall significant relationship in any of the six regions. Coefficients varied widely (P < 0.05) between the data (Table 8). between –0.060 and 0.489 for Blue Tits and between –0.389 and 0.456 for Great Tits (Table 6). When prob- Coal Tit abilities were combined, the relationship was not The BTO abundance peaks correlated significantly with significant in either species (Table 6). the Tarbet data for two regions (N and S Scotland, 0.708 and 0.620, respectively). Other correlation values varied Influence of Sitka Spruce cover widely between –0.225 and 0.480 (Table 8). Combining The GLM results showed that Sitka Spruce cover did probabilities shows an overall significant relationship (P not influence the relationship between cone index and < 0.05) between the data (Table 8). reporting rate for any species (P > 0.05) (Table 7). Table 7. General linear model output for the interaction between cone index, reporting rate and regional Sitka Spruce abundance. BTO data versus Tarbet data Coefficient se coefficient T-value P-value Siskin BTO abundance peaks correlated significantly with Siskin 0.069 0.081 0.850 0.400 Tarbet data for only one region (N Scotland, r = 0.65). Coal Tit 0.022 0.032 0.710 0.479 Blue Tit –0.004 0.004 –1.09 0.279 However, correlation values for all other regions were Great Tit –0.012 0.014 –0.87 0.390 positive and moderately strong (Table 8). Combining

Table 8. Correlations between Tarbet and BTO data for the four species over the six regions.

Siskin Coal Tit Blue Tit Great Tit

N Scotland 0.653, P < 0.05 0.707, P < 0.05 0.181, ns 0.473, ns S Scotland 0.543, ns 0.620, P < 0.05 0.081, ns 0.466, ns NE England 0.454, ns 0.447, ns 0.572, ns 0.026, ns NW England 0.414, ns 0.479, ns 0.676, P < 0.05 0.430, ns SE England 0.507, ns –0.225, ns 0.597, ns –0.171, ns SW England 0.427, ns 0.272, ns 0.606, P < 0.05 0.131, ns Combined probabilities 24.235, P < 0.05 23.190, P < 0.05 24.084, P < 0.05 11.815, ns ns, Not significant, P > 0.05.

Blue Tit and Great Tit certainly true for conifer seed as in such conditions the For Blue Tits, the BTO abundance peaks correlated cone scales open to allow birds to remove seeds. Shaw significantly with the Tarbet data for three regions (1990) found that trapping Siskins in a garden was (NW, SE and SW England, r = 0.676, 0.603 and 0.616, most successful under cool, cloudy conditions, i.e. respectively). The correlation values for other regions when cones were most likely to be closed. However, as fell between 0.09 and 0.57 (Table 8). BTO Great Tit wet, humid conditions are the norm for the Tarbet peaks did not correlate significantly with the Tarbet study area it can be argued that climatic fluctuations data in any region, correlation values falling between are so slight that such bias is minimal. Also, as –0.17 and 0.47 (Table 8). Combining probabilities discussed in previous studies (Lukas et al. 1996), mist- shows an overall significant relationship (P < 0.05) nets are most apparent to birds on sunny days, thus between the data (Table 8) for Blue Tits, but no such making dry, overcast days with little wind optimal for relationship exists for Great Tits (Table 8). catching, and not the dry, sunny days that are optimal for cone opening. This too may help ameliorate bias.

DISCUSSION Standardization and Sitka Spruce cone index Data collection The weekly proportion method was used to standardize While mist-netting is a fairly common method of the data because this does not require a uniform estimating bird abundance, it can be misleading as catching protocol per day. The effort of one day of biases obviously exist (Peach et al. 1998, Poulin et al. catching was not standard, thus the data were unsuit- 2000, Proffitt et al. 2004). Certain species may exhibit able for standardization by other methods such as behaviour that makes them less likely to be caught, for CPUE. Also, the BTO data used are expressed as example flying too high or learning to avoid the net weekly reporting rate and can thus be more easily (Silkey et al. 1999), as was observed during the present compared with the Tarbet data when expressed study. Such biases make estimates of absolute similarly. abundance and population structure made through The validity of the Sitka Spruce index may be subject mist-netting unreliable (Lukas et al. 1996). However, as to bias, as assessments were undertaken in three mist-netting was used only to determine the relative different locations and by three different people. This abundance of birds, while biases will undoubtedly still introduces an element of subjectivity because although exist these should be similar over years, and thus leave crops were assessed using the same protocol, the proto- relative numbers unaffected (DeSante et al. 1999). col in itself was somewhat subjective. However, this Indeed, Peach et al. (1998) compared long-term mist- was the only index available and as long as results net data with changes in territory counts on Common are interpreted with caution, it should not pose a Bird Census plots and found long-term changes to be significant problem. Furthermore, in the 16 years in similar in most species. which cone score was estimated independently in Another potential bias is linked to weather condi- Kielder and Galloway, there was a highly significant tions. Mist-netting could only be carried out on dry positive correlation in the annual score between these days with little wind. This introduces a bias, as natural locations. In the months following the completion of food tends to be more abundant on dry days; this is this study, Broome & Poulson (2006) have published

Forestry Commission data on Sitka Spruce coning Coal Tit from the Forest Condition Monitoring scheme. This The correlation between Coal Tit abundance and cone produces a much more intensive data set from a wider crop index was also significant. This is a novel result geographical area. The index used in the present study (although a similar relationship has been found correlates well with this data set, suggesting strong between the species and beechmast abundance, Glue synchrony across the UK. 1982). Surprisingly, this is an even stronger relationship than observed for Siskins. While a relationship was predicted, it was expected to be less robust than for Relationship with Sitka Spruce cone index Siskins, as Coal Tits feed on a wider range of foods and Siskin are thus likely to be less dependent on a single resource. The annual peak in relative abundance in the garden This result is especially interesting given that it is only was significantly negatively correlated with the cone relatively recently that Coal Tits in the UK have been index. This agrees with the expectation that in years of reported feeding on Sitka Spruce seed. low natural food abundance, birds enter gardens more So why should this species be so different from the often to take advantage of supplementary food than in other tits? Neither Blue Tits nor Great Tits displayed years of high natural food abundance. These findings any relationship with cone crop fluctuations. Therefore are in agreement with Shaw (1990), which is the only it appears that Coal Tits have developed a liking for truly comparable study for Siskins in Sitka Spruce; in this novel conifer habitat, which may enhance their that study birds were most abundant in the forest in winter survival because they can exploit the periodic the heaviest cone years, when they had virtually no abundance of spruce seed, whereas in other environ- interest in peanuts. A similar relationship was found in ments Coal Tits are often dominated by Blue Tits and Finland with Norway Spruce where Siskin numbers Great Tits. varied four-fold in successive years and seven-fold over five years in response to cone abundance (Haapanen 1966). Evidence for cone crop synchrony across the UK Shaw (1990) also considered breeding success in When the Tarbet and BTO data sets for Siskins and relation to cone abundance. Laying began two months Coal Tits were compared across six regions of the UK, earlier in years of heaviest cone crop relative to the both species showed a degree of correlation across the poorest years, which had been shown previously in country, although this was more marked for Siskins Finland (Haapanen 1966). Post-breeding flocks of than for Coal Tits (13 of 15 versus nine of 15 when P < juveniles were also more numerous in the forest in good 0.05). This was perhaps unsurprising as Siskins, which crop years, suggesting that good crops promoted not are irruptive, rely more heavily on tree seeds than do only earlier breeding but also more successful nestling/ Coal Tits (Newton 1972, Shaw 1990, Petty et al. 1995). fledgling survival (Shaw 1990). While constraints of So was this apparent synchrony due to fluctuations in the present study did not allow any analysis of breeding cone abundance or some other factor? Correlating status, it would be interesting to explore whether a regional peaks with the cone crop index gave mixed similar relationship could be determined using garden results. For the Siskin data, the reporting rate in only data, by establishing how important supplementary one region (N Scotland) showed a significant negative food is to both adult and nestling survival in both good relationship with cone crop. However, due to a small and bad cone years. Various studies in mainland Europe sample size, the threshold for significance is relatively have shown the provision of artificial food to enhance high (r = > –0.6). Correlation coefficients for the autumn recruitment, overwinter survival, breeding remaining five areas of the UK ranged between –0.49 numbers and breeding date of many small passerines and –0.58 and were all in the same direction. (van Balen 1980, Jansson et al. 1981, Kallander 1981). Combining the probabilities from the six regions gave The effect of severe winter weather has been shown to a highly significant relationship between the two data be reduced in species making use of feeders (van Balen sets. A longer time span of data may well resolve this et al. 1987), with Great Tit, Willow Tit and Crested Tit aspect. Lophophanes cristatus displaying an increased recruit- A similar relationship was seen with Coal Tits, this ment rate and adult survival after the provision of extra time with N and S Scotland both correlating food in autumn and winter (Suhonen et al. 1992, negatively with the index. However, coefficients for Perdeck et al. 2000). the other regions were spread more widely than with

Siskins, with only NE and NW England values close to On the other hand, an increasing number of species significance. This highlights the probability that Coal are being recorded feeding on Sitka Spruce seed (Shaw Tits were taking advantage of a wider range of foods. 1990), and it appears to be an important food for a They are one of the most sedentary species in the UK number of these because of certain characteristics. (Gosler 2002), so they rely on food available within Although it has relatively small seeds, its cones are their restricted breeding and winter ranges, rather than quite small and flimsy, and little effort is required travelling elsewhere. Consequently, if that area to pull seeds out from between the cone scales, contains little Sitka Spruce, there is unlikely to be a particularly during dry periods when the scales open significant correlation between Coal Tit numbers and rapidly (Staines et al. 1987). Similarly to other tit cone abundance. species of the boreal forest, Coal Tits scatter-hoard seed Regional data on Blue Tit and Great Tit abundance when it is abundant (Brotons 2000a, Brotons & in gardens showed no significant relationship with the Haftorn 1999) and they do the same with Sitka Spruce cone index for any region; this was expected as neither seed in the UK (S.J. Petty pers. obs.). Sitka Spruce species feeds on Sitka Spruce seed. covers more of Britain than any other coniferous These correlations fail, however, to consider one species, and in good cone years a vast amount of seed is important factor: the amount of Sitka Spruce available available. It also produces good cone crops at two to in the region. Clearly this is likely to influence how three times the frequency of Norway Spruce (Petty et tight a relationship exists between reporting rate and al. 1995). When cones are present on other conifer cone index. For example, in the south of England the species, Sitka Spruce seed is utilized early in the season percentage of Sitka Spruce available is very small before birds move onto more profitable tree species: (0.02% of total woodland, Forestry Commission Norway Spruce first and then Scots Pine. For example, 2001a), therefore it was not expected that birds would in Kielder Forest, Northumberland, when both Sitka respond to fluctuations in the same way as in Scotland, Spruce and Norway Spruce produced large crops, where 48% of the total woodland area is Sitka Spruce Common Crossbills and Siskins fed mainly on (Forestry Commission 2001b). However, while a GLM Sitka Spruce seed in autumn and early winter before analysis found the relationship between cone index and switching to Norway Spruce in late winter and early reporting rate to be significant, the interaction spring (Petty et al. 1995). A similar sequence has been between index, reporting rate and regional Sitka observed elsewhere (Shaw 1990, Marquiss & Rae Spruce abundance was not significant for any species. 1994). The clear relationship may therefore be more represen- If Sitka Spruce sheds most of its seed in autumn and tative of a general coning trend across many different early winter, this poses a problem for the analysis of conifer species. Siskin abundance, as the birds do not tend to reappear in numbers in forests until early spring. However, there is a weather effect to consider as cones need dry, bright Sitka Spruce seed as food for wildlife and windy conditions to open (Shaw 1990). While Two factors appear to make Sitka Spruce seed a less Siskins and Coal Tits can theoretically utilize Sitka attractive food resource than other conifer seed. First, Spruce seed from September, it is likely that at least most conifer-seed-dependent species in the UK have some cones will remain closed and retain their seed for co-evolved with native European conifers, all of which much longer. In dry autumns and winters, most seed is have much larger seeds than Sitka Spruce (see intro- probably shed by the end of January, but in damp duction). Thus, feeding on the small seeds of Sitka conditions much can remain in cones until April (S.J. Spruce may involve extra foraging costs. Second, Sitka Petty pers. obs.). Mair (1973) calculated the percent- Spruce seeds are shed before those of Norway Spruce age of seed dropped by Sitka Spruce over a season in and Scots Pine, the two most important species for a two geographically separated forests, one in Inverness- range of conifer-seed-eating birds and mammals in shire and the other in Roxburghshire. In Inverness- northern Europe (Reinikainen 1937, Haftorn 1959, shire, 40% of the seed was dropped in October– Newton 1970, 1972, Andrén & Lemnell 1992, Wauters November, 51% in December–January and 9% in & Lens 1995). Thus, the availability of Sitka Spruce February–April. However, the proportions in Roxburgh- seed may be mismatched with the peak energy require- shire were quite different: 2% in October–November, ment of seed-dependent species prior to and during 20% in November–December and 72% in February– breeding. April. Thus, even in one year there can be a huge

difference in the timing of seed fall. As the present Blue Tits and Great Tits, which feed primarily in study was carried out in western Scotland, it is likely broadleaved trees. By drawing inferences from the that seed fall occurred in a ratio even more extreme BTO’s GBW data, there was also evidence of than the Roxburghshire pattern, owing to the oceanic synchrony in bird abundance across the UK. However, climate. due to a relatively small sample size (ten data points), The timing of seed dispersal is also likely to differ it is hard to make definitive statements; a longer time within forests, with seed shed earlier along edges series of BTO data may resolve this aspect. exposed to drying winds and sunshine, although Here we report the first evidence of the importance such an effect has not been quantified. Birds will of Sitka Spruce seed to Coal Tits during the winter, undoubtedly exploit this spatial variation in seed including the tendency for Coal Tits to visit gardens retention. Thus, Siskins moving back into forests in more often in years when cone crops fail. These are February and March may well encounter many pockets significant findings about the ecology of Coal Tits in a where cones have retained their seeds, especially in novel habitat, about which little is known, but clearly more oceanic parts of Britain or following mild, wet show the importance of conifer seed. As more of the winters. UK’s forests start producing cones, it is important to better understand the influence of conifers (particularly Sitka Spruce) on UK wildlife in general. The food resource for birds in gardens It appears that gardens have become an important habitat for Siskins and Coal Tits. This is a significant ACKNOWLEDGEMENTS finding, as relatively few studies have considered the We thank Geoff Shaw for making available his data on importance of supplemental feeding for populations. spruce cone crops and Chris Quine for comments on an Private gardens in the UK cover approximately twice earlier draft. We are very grateful to the many thousands of the area of nature reserves and as such appear to be a volunteers who have contributed to the BTO/CJ Garden very important resource (Chamberlain et al. 2004). It is BirdWatch. The project is funded by volunteer's subscrip- tions and is supported by CJ WildBird Foods Ltd. interesting that although food may be provided year- round in gardens, birds seem only to use it when absolutely necessary, in this instance, in poor cone REFERENCES years. This has been found elsewhere, with Black- Andrén, H. & Lemnell, P. 1992. 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(MS received 14 January 2006; revised MS accepted 25 September 2006)