Influence of Nesting Habitat on Breeding Song Thrushes Turdus Philomelos

Bird Study (2007) 54, 221–229

Influence of nesting habitat on breeding Song Thrushes Turdus philomelos

KATHERINE M. KELLEHER1 and JOHN O’HALLORAN2* 1Fehily Timoney & Co., Core House, Pouladuff Road, Cork, Ireland and 2Department of Zoology, Ecology & Plant Science, University College Cork, Distillery Fields, North Mall, Cork, Ireland

Capsule Habitat significantly influences nest-site choice, breeding output and daily nest failure rates in Song Thrushes. Aims To describe nesting habitat of a stable Song Thrush breeding population, investigate habitat influence on breeding, and examine what variables influence nest-site selection by comparing habitat variables in nesting sites with random non-nesting sites. Methods Nesting habitat data were collected throughout Ireland in 2001–03 during a detailed study in County Cork and by volunteers across the country. Results Most nests were located 1.3–2.4 m from the ground. Nests built in April were significantly higher than those built in May. Nest-sites were mostly in trees, bushes or hedgerows and we highlight the role of garden habitat for nesting. Clutch size was significantly higher (mean ± sd) on farmland (4.5 ± 0.7) than in garden and parkland (4.0 ± 0.5) and woodland (4.0 ± 0.6). The number of hatched chicks was significantly greater in hedgerows (4.1 ± 0.9) than in trees (3.2 ± 0.7), but not in bushes (3.7 ± 0.9), while the number of fledged chicks was significantly lower in nests in trees (3.2 ± 0.7) than in hedgerows (4.2 ± 0.8) and bushes (3.8 ± 0.7). Daily nest failure rates were significantly lower for nests in trees and partly concealed nests across different nest periods. Nesting sites had significantly denser vegetation than non-nesting sites. Conclusion Dense vegetation is an important deciding factor for Song Thrushes when choosing a nest- site and may be an important consideration in conservation measures striving to maintain or restore Song Thrush populations.

Various resources can limit breeding bird populations, foraging areas through conservation policies in order to including the availability of nesting sites (Newton halt the decline of Song Thrush populations in rural 1994a, 1994b). Consequently management of nesting Britain. habitat has been identified as a conservation tool for Population trends for the Song Thrush in Ireland are endangered bird species such as the Eurasian Black unclear due to the lack of long-term data, but some Vulture Aegypius monachus in Greece (Poirazidis et al. decline is thought to have occurred during 1970–90 2004). In Britain, the Song Thrush Turdus philomelos (Gibbons et al. 1993, Tucker & Heath 1994). However, population has declined by 52% over the last 30 years the Song Thrush is not regarded as a species of conser- (Eaton et al. 2004), with the largest decline on farm- vation concern in Ireland (Newton et al. 1999), which land (65%) and slower decline in woodland (Thomson is supported by the recent Countryside Bird Survey et al. 1997, Baillie et al. 2002). The Song Thrush is now data (Coombes 2003). Given the current favourable classified as a species of conservation concern in conservation status of the Song Thrush in Ireland, we Britain (Gibbons et al. 1996), although there is some had a unique opportunity to describe nesting habitat of evidence of a recent recovery (Newton 2004). Peach et Song Thrushes, investigate the influence of habitat al. (2004a, 2004b) recently emphasized the importance on breeding output and daily nest failure rates, and of restoring suitable nesting cover adjacent to suitable examine habitat variables that influence nest-site selection. By providing an insight into the relationship *Correspondence author between Song Thrushes and their nesting environ- Email: [email protected] ment, this study will also help conservation measures

elsewhere, especially in areas of breeding population used in choosing these non-nesting survey sites. Areas decline. with seemingly appropriate nesting habitat, but where nesting had not occurred for the previous two breeding seasons, were identified on a map of the intensive study STUDY SITES sites. At these sites, observers threw a metal peg behind Nesting habitat data were collected in conjunction their backs and used its landing position as the survey with a Song Thrush breeding biology survey 2001–03 point for a non-nesting site. The intensive habitat from intensively monitored sites in County Cork, survey was conducted by the same observer at the end Ireland, and less intensively monitored sites country- of the breeding season to minimize disturbance to wide (Kelleher & O’Halloran 2006). Three intensive breeding birds that were already subjected to observa- study sites were located in County Cork at Fota tion as part of a breeding survey (Kelleher & Arboretum (51°53.5′N, 8°18.2′W), Millstreet Country O’Halloran 2006). In all cases, trees 5 m or less in Park (52°1.3′N, 10°0.8′W) and Willowbrook Farm height were classed as bush and shrub (after Crick (52°2.4′N, 10°3.3′W). These study sites consisted of 1992). garden/parkland, coniferous forests and mixed coppice, respectively. The less intensively monitored areas consisted of two sites in County Cork and a random Habitat measurement for intensive nesting habitat selection of sites countrywide where volunteers survey recorded nests (Kelleher & O’Halloran 2006). Standard methods for measuring habitat variables were adapted from Bibby et al. (1993). The height of the dominant habitat feature at the nest-site and the METHODS distance from the nest-site (or survey point for non- nesting sites) to the outer edge of the dominant habitat Data collection feature were measured using a measuring stick, tape or Clutch size, number of hatched and fledged young and clinometer (with the observer standing 20 m from the nest failure rates were collected during 2001–03 as feature). described in Kelleher & O’Halloran (2006). Nest fail- Percentage vegetation density above the nest-site ure was determined by calculating nest failure on a was estimated by eye, looking vertically upwards daily basis using the Mayfield method (Mayfield 1961, through a cylindrical sighting tube (length 11 cm, 1975). Nesting habitat data collection during 2001–03 internal diameter 4.5 cm) positioned 1 m from ground followed the BTO Nest Record Scheme methodology level at the nest-site (or at observer height, 1.6 m in (Crick et al. 1994) when nests were active. this case, from ground level for non-nesting sites). In 2002 and 2003 we undertook an intensive nesting Percentage vegetation density above the nest-site was habitat survey of 40 nests at the intensive study sites estimated in the same way at four points 1 m from the and recorded the following data: height of dominant nest-site, each point being 90° from a line linking the habitat feature at the nest-site (i.e. height of tree or nest-site and the previous point, and the results were bush), minimum and maximum distance of the nest- totalled and averaged. site to outer edge of dominant habitat feature, Vegetation density surrounding the nest-site was percentage vegetation cover above the nest-site (i.e. estimated using a chequered vegetation density board vegetation density over the nest), average percentage (18 × 18 cm), which was placed at the nest (or at cover of vegetation above the nest and 1 m from observer height, 1.6 m in this case, from ground level the nest-site, average vegetation cover (or density) for non-nesting sites) and facing towards the observer. surrounding the nest-site, percentage ground vegeta- The observer stood in front of the board and moved tion cover (i.e. bare soil, wood, dead or live vegetation) horizontally away until 50% of the board was covered within 1-m radius of the nest-site, percentage tree and by vegetation; the distance between the observer and bush and shrub cover within a 5-m radius of the nest- the board was recorded, where lower distance values site. indicate higher vegetation density and higher distance To assess habitat variables important to Song values indicate lower vegetation density. In order to Thrushes when choosing a nest-site, we also conducted represent vegetation density surrounding the nest-site, an intensive habitat survey during the same period at the same procedure was performed with the board 40 non-nesting sites. A stratified random approach was facing in four different directions, each 90° from the

previous direction, and the results were totalled and dominant habitat feature at nest-site, (ii) maximum averaged. distance of nest-site to outer edge of dominant habitat Percentage ground vegetation cover within a 1-m feature, (iii) average vegetation density surrounding radius of the nest-site and percentage tree and bush and the nest-site, (iv) percentage cover of vegetation shrub cover within a 5-m radius of the nest-site were density above the nest-site, (v) percentage ground both measured by eye. cover of bare soil and (vi) live vegetation within 1-m radius of nest-site.

Statistical analysis Analyses of nesting habitat variables and breeding RESULTS parameters were undertaken with logistic regression Nest height above ground models using generalized linear models (GLMs) (after Peach et al. 1999, Aebischer 1999, Siriwardena et al. The mean nest height (m) above ground was 1.71 ± 2000) as available in the PROC GENMOD feature of SAS 0.06 m (n = 93), with most nests located between 1.3 (release 8.2). This analysis uses maximum-likelihood and 2.4 m (Fig. 1). Nest height differed significantly ratio statistics to test for significance (SAS 2000). In all with the breeding season (χ2 = 10.96, df = 3, P < 0.05) cases, univariate models were conducted as the unbal- where April nests (1.89 ± 0.51 m, n = 22) were higher anced data set and the small sample sizes involved did than May nests (1.50 ± 0.39 m, n = 29) (χ2 = 10.71, df not support complex multivariate models. Nesting = 1, P < 0.01) but not March (1.58 ± 0.53 m, n = 12) habitat data for all years (2001–03) were combined for or June (1.62 ± 0.50 m, n = 10) nests. analysis. Clutch sizes and number of chicks at hatching and fledging were analysed with nest height above ground, Dominant nesting habitat dominant surrounding habitat, dominant habitat Approximately 50% of all nests recorded (n = 99) were feature at nest-site, nest concealment and rural and located within parkland and garden habitats and 41% suburban location using identity link function with in woodland and farmland combined (Fig. 2). For normal error distribution incorporated into a Type I records submitted by volunteers only (n = 49), most analysis for each model. A similar approach was used to nests were recorded in farmland (35%) followed by compare nest height above ground with dominant parkland and garden (33%) and woodland (16%) (Fig. surrounding habitat, dominant habitat feature at nest- 2). Nests (n = 89) were generally located in trees (40%) site, month (when first egg was laid), nest concealment (Fig. 2). and rural and suburban location. Daily nest failure rates, for egg (i.e. laying and incubation periods), nestling (i.e. period from when the first chick hatched Nesting habitat and breeding parameters to when the last chick fledged) and all (i.e. egg laying Mean clutch size in farmland (n = 11) was higher than to fledging) periods were analysed with nest height in garden and parkland (n = 32) and woodland (n = 12) above ground, dominant surrounding habitat, domi- (Table 1). Mean clutch size was higher in hedgerows nant habitat feature at nest-site, nest concealment, and rural and suburban location using logit link function with binomial error distribution incorporated into a Type I analysis for each model, where daily fail rates were modelled as the proportion of survival (0) or failure (1) and the total number of nest observation days. The additional habitat data collected from the intensive nesting habitat survey were used to compare habitat variables between nesting and non-nesting areas using Mann–Whitney tests throughout. To avoid the use of intercorrelated variables, we used a Spearman’s rank-order correlation to identify such variables and removed these from subsequent analysis. Figure 1. Percentage height frequency distribution (metres above The variables subsequently analysed were (i) height of ground) of Song Thrush nests in Ireland, 2001–03 (n = 93).

(n = 14) than in trees (n = 17) and bushes (n = 22) (Table 1). The mean number of hatched chicks was significantly greater in hedgerow (n = 13) than in trees (n = 14) but not bushes (n = 19) (Table 1). The mean number of fledged chicks was significantly lower at tree sites (n = 12) than hedgerow (n = 10) and bush sites (n = 13) (Table 1). Using Bonferroni’s correction (i.e. P < 0.01), differences in comparing mean clutch size between dominant habitat and differences in the number of fledged chicks between nesting sites remained significant.

Nesting habitat and nest failure rates Nest failure rates were significantly higher at tree nesting sites than at bush and hedgerow sites during egg (n = 16, 21, 12, respectively) and all nest periods (n = 26, 26, 16, respectively.) (Table 2). Nest failure rates were significantly higher at partly hidden nesting sites than at well hidden sites during egg (n = 27, 21, respectively), nestling (n = 19, 28, respectively) and all nesting periods (n = 34, 35, respectively) (Table 2). Figure 2. Dominant surrounding habitat, using all records (n = 99) Using Bonferroni’s correction (P < 0.01), differences and volunteer records only (n = 49), and dominant habitat feature in nest failure rates between nesting sites remain at nest-site (n = 89) for Song Thrush nests in Ireland, 2001–03. significant during all nest periods and differences in nest failure rates between nest concealment remain

Table 1. GLM test results for breeding parameters and nesting habitat variables for Song Thrush nests in Ireland 2001–03. Values are given as mean ± sd (n).

Variable Nest height (m) Clutch size Number of hatched chicks Number of fledged chicks

Dominant habitat Garden and parkland 1.8 ± 0.5 (45) 4.0 ± 0.5 (32) 3.6 ± 0.9 (23) 3.6 ± 0.7 (16) Farmland 1.5 ± 0.5 (17) 4.5 ± 0.7 (11) 4.1 ± 0.9 (11) 4.1 ± 0.6 (8) Woodland 1.8 ± 0.8 (23) 4.0 ± 0.6 (12) 3.4 ± 0.8 (16) 3.4 ± 0.8 (14) χ2 = 5.88, df = 2 χ2 = 8.21, df = 2 χ2 = 3.78, df = 2 χ2 = 4.59, df = 2 Dominant habitat feature Tree 1.9 ± 0.8 (36) 3.9 ± 0.4 (17) 3.2 ± 0.7 (14) 3.2 ± 0.7 (12) Bush and shrub 1.6 ± 0.4 (29) 3.9 ± 0.6 (22) 3.7 ± 0.9 (19) 3.8 ± 0.7 (13) Hedgerow 1.6 ± 0.4 (19) 4.4 ± 0.6 (14) 4.1 ± 0.9 (13) 4.2 ± 0.8 (10) χ2 = 2.42, df = 2 χ2 = 7.63, df = 2 χ2 = 6.86, df = 2 χ2 = 10.23, df = 2 Nest height (m) 0.7–1.2 na 4.1 ± 0.8 (12) 3.5 ± 0.9 (11) 3.8 ± 0.8 (9) 1.3–1.8 4.2 ± 0.6 (24) 3.8 ± 1.0 (23) 3.8 ± 0.9 (15) 1.9–2.4 3.9 ± 0.6 (16) 3.6 ± 0.8 (15) 3.5 ± 0.8 (14) χ2 = 1.82, df = 2 χ2 = 0.95, df = 2 χ2 = 1.20, df = 2 Nest location Rural 1.7 ± 0.6 (60) 4.2 ± 0.7 (32) 3.7 ± 0.9 (35) 3.7 ± 0.8 (30) Suburban 1.8 ± 0.6 (32) 4.0 ± 0.5 (23) 3.4 ± 1.0 (16) 3.7 ± 0.9 (9) χ2 = 3.80, df = 1 χ2 = 2.68, df = 1 χ2 = 1.21, df = 1 χ2 = 0.00, df = 1 Nest concealment Well hidden 1.7 ± 0.5 (43) 4.1 ± 0.6 (24) 3.7 ± 0.9 (30) 3.8 ± 0.7 (27) Partly hidden 1.6 ± 0.5 (40) 4.1 ± 0.5 (26) 3.7 ± 0.9 (16) 3.4 ± 0.9 (8) χ2 = 0.92, df = 1 χ2 = 0.03, df = 1 χ2 = 0.00, df = 1 χ2 = 1.67, df = 1

Significant values (P < 0.05) are shown in bold. P > 0.05 for non-significant values. na, Not applicable.

Table 2. GLM test results of daily nest failure rates of different nest periods and nesting habitat variables for Song Thrush nests in Ireland 2001–03. Upper and lower 95% CL are in parentheses.

Variable Egg stage Nestling stage All

Dominant habitat Garden and parkland 0.0763 (0.0465–0.1227) 0.0378 (0.0190–0.0738) 0.0574 (0.0384–0.0848) Farmland 0.0392 (0.0127–0.1146) 0.0209 (0.0052–0.0799) 0.0291 (0.0121–0.0679) Woodland 0.0567 (0.0215–0.1416) 0.0488 (0.0184–0.1229) 0.0525 (0.0264–0.1014) χ2 = 1.44, df = 2, n = 50 χ2 = 1.10, df = 2, n = 47 χ2 = 2.31, df = 2, n = 69 Dominant habitat feature Tree 0.1272 (0.0718–0.2152) 0.0769 (0.0350–0.1608) 0.1033 (0.0652–0.1599) Bush and shrub 0.0471 (0.0226–0.0956) 0.0397 (0.0190–0.0808) 0.0431 (0.0257–0.0714) Hedgerow 0.0367 (0.0138–0.0937) 0.0174 (0.0043–0.0668) 0.0276 (0.0125–0.0602) χ2 = 6.92, df = 2, n = 49 χ2 = 4.13, df = 2, n = 46 χ2 = 10.60, df = 2, n = 68 Nest height (m) 0.7–1.2 0.0723 (0.0328–0.1517) 0.0250 (0.0063–0.0945) 0.0491 (0.0247–0.0951) 1.3–1.8 0.0583 (0.0317–0.1050) 0.0558 (0.0312–0.0980) 0.0570 (0.0374–0.0858) 1.9–2.4 0.0430 (0.0162–0.1090) 0.0226 (0.0073–0.0678) 0.0310 (0.0149–0.0637) χ2 = 0.71, df = 2, n = 49 χ2 = 2.91, df = 2, n = 50 χ2 = 2.23, df = 2, n = 70 Nest location Rural 0.0530 (0.0303–0.0910) 0.0335 (0.0175–0.0631) 0.0424 (0.0278–0.0642) Suburban 0.0759 (0.0425–0.1318) 0.0496 (0.0238–0.1005) 0.0645 (0.0410–0.1001) χ2 = 0.78, df = 1, n = 53 χ2 = 0.62, df = 1, n = 50 χ2 = 1.76, df = 1, n = 73 Nest concealment Well hidden 0.0363 (0.0174–0.0741) 0.0168 (0.0063–0.0438) 0.0255 (0.0142–0.0454) Partly hidden 0.1103 (0.0676–0.1749) 0.0879 (0.0506–0.1484) 0.0991 (0.0688–0.1406) χ2 = 6.92, df = 1, n = 48 χ2 = 10.34, df = 1, n = 47 χ2 = 17.26, df = 1, n = 69

Significant values (P < 0.05) are shown in bold. P > 0.05 for non-significant values. significant during the egg period and all nest recently in Britain, Peach et al. (2004a, 2004b) high- periods. lighted the importance of nesting cover for Song Thrushes, especially dense woody vegetation, in conjunction with close proximity to suitable foraging Nesting habitat variables between nesting and habitats. Consequently, the importance of imple- non-nesting areas menting future agri-environment schemes that will The only habitat variable to differ significantly restore suitable nesting cover adjacent to suitable between nesting and non-nesting areas was vegetation foraging areas in a bid to tackle the decline of Song density surrounding the nest-site, where nesting areas Thrush populations in rural Britain have been high- had significantly higher vegetation density as indicated lighted (Peach et al. 2004a, 2004b). While the by the lower distance recorded between the observer unbalanced nature of our data set combined with small and the chequered vegetation density board (Table 3). sample sizes requires caution when interpreting our results, the influence of nesting habitat on breeding Song Thrushes is nonetheless shown. DISCUSSION Several studies in the UK have highlighted the importance of habitat for breeding Song Thrushes, including Nest height above ground hedgerows, trees, woody vegetation, ditches, gardens, Nest height above ground here is similar to some Song woods, grassland and scrub (Arnold 1983, Lack 1992, Thrush studies (Wesolowski & Czapulak 1986, Murray Green et al. 1994, MacDonald & Johnson 1995, Moles 2004) but lower than others (Dycrz 1969, Schnack & Breen 1995, Parish et al. 1995, Sparks et al. 1996, 1991). Differences in nest height are probably Mason 1998, Hinsley & Bellamy 2000, Peach et al. explained by differences in habitat type and structure 2004a). It has also been suggested that the positive between the studies as the extent of tree and shrub influence of hedge and tree habitat on Song Thrushes layer appears to influence Song Thrush nest height is probably related more to nest-site opportunities than (Dycrz 1969). Indeed, both Schnack (1991) and Dycrz to foraging opportunities (Sparks et al. 1996). More (1969) noted that Song Thrush nests were built higher

Table 3. Mann–Whitney test results of nesting habitat variables of Song Thrush nesting and non-nesting areas in Ireland 2002–03. Values are given as mean ± 95% CI.

Nesting areas Non-nesting Variable (n = 40) areas (n = 40) Mann–Whitney result

Height of dominant habitat feature (m) 6.93 ± 1.63 5.68 ± 1.33 W = 1748.0 Maximum distance to outer edge of dominant habitat feature (m) 18.16 ± 4.26 15.14 ± 3.66 W = 1721.0 Average vegetation density surrounding the nest-site (cm) 116.2 ± 22.64 178.5 ± 34.17 W = 1346.0, P = 0.008 Vegetation density above the nest-site (%) 99.87 98.00 W = 1682.5 Bare soil within 1-m radius (%) 2.92 4.25 W = 1665.5 Live vegetation within 1-m radius (%) 36.90 ± 15.14 40.52 ± 15.41 W = 1556.5

Significant value (P < 0.05) shown in bold. P > 0.05 for non-significant values. Calculation of 95% CI for both vegetation density above the nest-site and bare soil was not possible as proportions were >0.9 and <0.1 respectively. in habitats with taller vegetation (e.g. forests) than in within the Irish landscape rather than of Song Thrush habitats with lower vegetation (e.g. parks). Unlike nesting preference. The dominant habitat features Murray (2004), we found that Song Thrush nest height found at nest-sites in this study (i.e. trees, bushes and differed significantly through the breeding season (i.e. hedgerow) are not surprising and have been noted by monthly) with April nests higher than May nests. others (Cramp 1988, Wesolowski & Czapulak 1986). While Schnack (1991) also found such significant Other studies have shown that Song Thrushes select differences, the trend differed, with nests in June and field boundaries, gardens and woodland for nesting sites July higher than in April. Changes in nest height and home-ranges (Murray 2004, Peach et al. 2004a, through the season are most likely a reflection of 2004b) and the habitat features found here are typical changes in vegetation structure and growth. Some of such habitats. studies (Ludvig et al.1995, Burhans et al. 2002, Piper & Catterall 2004) have found significant relationships between nest height and predation rates, and although Nesting habitat and Song Thrush breeding this specific relationship was beyond the scope of our In contrast to this study, Pikula (1969) did not find study, daily nest failure rates in our study did not differ significant differences in Song Thrush clutch size significantly with nest height. between woodland and non-woodland habitats in for- mer Czechoslovakia. O’Connor & Shrubb (1986) found similar mean clutch sizes for Song Thrushes in Dominant nesting habitat British farmland (4.2), woodland (4.1) and suburban We found that most nests were located in garden and habitats (4.1) (comparable to our garden and parkland parkland habitat, but due to inherent biases when habitats, which mostly consisted of suburban areas) to selecting intensive study sites, the use of this habitat our study. We know that clutch size here varied with and woodland are likely to be over-estimated. This the season (Kelleher & O’Halloran 2006), and while appears to be confirmed by the fact that farmland is the this may have been a factor in differences between dominant nesting habitat from volunteer data, even clutch sizes in different habitats, the data are too few to though we must acknowledge that volunteer data also test for this interaction. had inherent biases including multiple records from the As found in this study, Pikula (1969) noted that same breeding pair within the same surrounding nesting sites influenced Song Thrush breeding, with habitat. However, despite these biases, the importance more chicks hatched and fledged in nests located in of gardens and parks for nesting Song Thrushes in trees and shrubs 2.5 m from the ground and in conifers Ireland is highlighted here, and has been found in other than in nests in shrubs and trees below 2.5 m and studies (Gregory & Baillie 1998, Mason 1998, Peach et deciduous trees, although nest height and predation al. 2004a, 2004b). The frequency of dominant habitat could be other confounding factors in this case. features at nest-sites should not suffer from the same Nest failure was mostly caused by avian predation biases since these features are generally available across during the egg stage (Kelleher & O’Halloran 2006). In all habitat types. However, the usage of these nesting agreement with our findings of Song Thrush daily nest sites is probably more reflective of their availability failure rates between nesting sites, Blackbird Turdus

merula nests are known to have significantly higher Bowen 1973, Chandler & Gregory 1976, Hulme & nesting success in hedgerows than in trees (Møller Barrow 1997) where Irish annual rainfall is up to 20% 1998). Interestingly, Hatchwell et al. (1995) found higher than the open lowlands of England (Keane successful Blackbird nests significantly less exposed 1986), again allowing for greater Song Thrush foraging than failed nests during the laying and incubation opportunities by creating easier probing conditions for stages but not the nestling stage, which is similar to our prey items such as earthworms. findings of daily nest failure rates and nest conceal- Differences in the Irish and British predator commu- ment. While Murray (2004) found that daily failure nity may also play a role in the differences in Song rates during Song Thrush nestling stage were signifi- Thrush nesting success, as fewer predator species are cantly higher for nests located in gardens than those in present in Ireland. However, despite such differences in woods, we did not find any such significant difference, the predator community, we do know that predation even though daily failure rates were higher in woodland rates for Song Thrush nests in Ireland are similar to nests than garden nests during the nestling period. Britain, although a more recent study indicates that Despite the fact that our study did not find signifi- predation on British farmland may now be higher cant differences in daily nest failure rates between (Kelleher & O’Halloran 2006). dominant surrounding habitats, it is interesting to note that farmland nests in our study had the lowest daily failure rates resulting in an overall nest success rate of Nesting habitat variables between nesting and 38% in farmland, 16% in garden and parkland, and non-nesting areas 19% in woodland. The fact that farmland nests in this In this study, vegetation density surrounding the nest- study had the lowest failure rates across all nesting site was significantly higher in nesting areas than in stages is surprising given that higher predation rates are non-nesting areas, which is in accordance with other known to occur in farmland than in woodland, and the studies where Song Thrush nests were located in areas lowland British Song Thrush population has declined of dense shrub when available (Dycrz 1963, 1969). The the most on farmland (Chamberlain et al. 1995, importance of dense nesting vegetation for Song Thomson et al. 1997, Baillie et al. 2002). Thrushes has recently been recognized by Peach et al. A recent study of Song Thrushes in Britain (2004a, 2004b) who recommend that future agri- (1995–97) found that overall nesting success of a stable environmental policies and schemes should incor- farmland population varied by 26–34% while a porate measures for improving nesting cover. Other declining farmland population varied by 17–30% (W. studies have found that Blackbird nesting areas have Peach pers. comm.). Therefore this study suggests that significantly denser vegetation cover than non-nesting Song Thrush nesting success is higher in Irish farmland areas (Hatchwell et al. 1995), re-affirming our findings than in British farmland. This difference in nesting for Song Thrushes in this respect. success between the two countries is probably due to a While this study did not investigate differences in combination of factors including extent of agricultural habitat variables between successful and non-successful intensification, habitat and climate. breeding attempts, Hill (1998) found that successful There has been less intensification of the Irish Song Thrush nests had more rank vegetation (i.e. over- agricultural landscape and on a smaller scale than in grown dense herbaceous growth, often with an Britain (Cabot 1985, Aalen et al. 1997, Lysaght 1989, extensive vertical element) surrounding their nests in Stanners & Bourdeau 1995). Consequently, the Irish comparison to its availability in the whole study site. landscape has a relatively dense network of mostly This further highlights the importance of vegetation unmanaged field boundaries (Lysaght 1989, Moles & density surrounding Song Thrush nests. Breen 1995, Aalen et al. 1997), which may provide good nesting opportunities. While permanent grassland Conclusions dominates the landscape of both countries, Ireland has a higher proportion than Britain (Tucker & Dixon While farmland habitat and nest concealment had a 1997) allowing for greater foraging opportunities, positive influence on breeding, nest-sites in trees had especially for earthworms (Gruar et al. 2003, Peach et both lower breeding output and higher daily nest al. 2004a). failure rates. In general, hedgerows and bushes were Ireland has a wetter and milder climate than equally important for breeding output and daily nest England, especially southeast England (Keane 1986, failure rates, emphasizing the importance of a bush and

shrub layer for breeding Song Thrushes. This therefore Their Conservation Status 2001. BTO Research Report No. 278. highlights the need to include bushes and shrubs, as BTO, Thetford. Bibby, C.J., Burgess, N.D. & Hill, D.A. 1993. Bird Census Tech- well as hedgerows, in conservation measures being niques. Academic Press, London. considered in areas of Song Thrush breeding decline. Bowen, D. 1973. Britain’s Weather. Its Workings, Lore and Forecast- Daily failure rates for Song Thrush nests on Irish farming. David & Charles, Newton Abbot. land seem to be lower than those of Britain. The most Burhans, D.E., Dearborn, D., Thompson, F.R. & Faaborg, J. 2002. Factors affecting predation at songbird nests in old fields. likely explanation for this is a combination of elements J. Wildl. Manage. 66: 240–249. within the Irish landscape including the relatively Cabot, D. 1985. The State of the Environment. An Foras Forbatha, lower agricultural intensification, dense network of Dublin. unmanaged field boundaries and milder, damper Chamberlain, D.E., Hatchwell, B.J. & Perrins, C.M. 1995. Spaced out nests and predators: an experiment to test the effects of weather. This study shows the importance of dense habitat structure. J. Avian Biol. 26: 346–349. vegetation cover in attracting Song Thrushes in Chandler, J. & Gregory, S. 1976. The Climate of the British Isles. choosing a nest-site. This finding should also be Longman, London. considered in any conservation measures designed to Coombes, D. 2003. Most species faring well. Five years on, the Coun- tryside Bird Survey shows few species in decline and positive trends maintain or support the recovery of Song Thrush for most. Wings no. 31 (winter). BirdWatch Ireland, Wicklow. breeding populations, especially in relation to any Cramp, S. (ed.) 1988. Birds of the Western Palearctic, Vol. 5. Oxford conservation policies involving the maintenance University Press, Oxford. and/or planting of suitable nesting habitat. Crick, H.Q.P. 1992. A bird-habitat coding system for use in Britain and Ireland incorporating aspects of land-management and human activity. Bird Study 39: 1–12. Crick, H., Dudley, C. & Glue, D. 1994. The Nest Record Scheme ACKNOWLEDGEMENTS Handbook. British Trust for Ornithology, UK. We acknowledge Enterprise Ireland and the Department of Dycrz, A. 1963. Comparative studies on the avifauna of wood and Zoology, Ecology and Plant Science, National University of park. Acta Ornithol. 7: 381–385. Dycrz, A. 1969. The ecology of the Song-thrush (Turdus philomelos Br.) Ireland, Cork, for funding this research. We are very grateful and Blackbird (Turdus merula L.) Ekol. Polska 17(2): 735–790. to all the volunteers countrywide who collected high quality Eaton, M.A., Noble, D.G., Cranswick, P.A., Carter, N., Wot- nesting data. We are also indebted to the following for ton, S., Ratcliffe, N., Wilson, A., Hilton, G.M. & Gregory, permission to use lands under their authority: Professor R.D. 2004. The State of the UK’s Birds 2003. BTO, RSPB and Raftery (Fota Arboretum), Dr Neil Stronach (Fota Wildlife WWT, Sandy. Park), Dr Jervis Good and Dr Fidelma Butler (Glinny Farm), Gibbons, D.W., Reid, J.B. & Chapman, R.A. 1993. The New Mr Jerry Sheehan (Millstreet Country Park) and Mr Patrick Atlas of Breeding Birds in Britain and Ireland 1988–1991. T. & A.D. Poyser, Staffs. Cronin (Willowbrook Farm). We would like to also thank Dr Gibbons, D.W., Avery, M.I., Baillie, S.R., Gregory, R.D., Gavin Fennessy (National University of Ireland, Cork), Dr Kirby, J., Porter, R.F., Tucker, G.M. & Williams, G. 1996. Gavin Siriwardena (BTO) and Dr Mark O’Brien (RSPB) for Bird Species of Conservation Concern in the United Kingdom, Chan- all their SAS advice, Derek Gruar and Dr Will Peach (both nel Islands & Isle of Man: Revising the Red Data List. RSPB RSPB) for all their invaluable Song Thrush advice, Dr Dave Conservation Review, Sandy. Leech (BTO) for help in tracking down nest observers, Green, R.E., Osborne, P.E. & Sears, E.J. 1994. The distribution of Andrew Gillespie and Tim O’Mahony (Millstreet Country passerine birds in hedgerows during the breeding season in relation to characteristics of the hedgerows and adjacent farmland. J. Appl. Park) for tip-offs on Song Thrush activity, and Gearoid and Ecol. 31: 677–692. Belen for assistance with habitat surveys. Gregory, R.D. & Baillie, S.R. 1998. Large-scale habitat use of some declining British birds. J. Appl. Ecol. 35: 785–799. Gruar, D., Peach, W. & Taylor, R. 2003. 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(MS received 20 September 2005; revised MS accepted 21 September 2006)