Habitats of the song thrush Turdus philomelos in a largelyarable landscape

C. F. Mason*

Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, U.K.

(Accepted 21 April 1997)

Abstract

During three breeding seasons, 1994±96, the habitats of song thrushes were studied in north-east Essex,

U.K., in 10 tetrads (total 40 km2) of mainly farmland habitat and 35 woods of differing sizes (range 0.1±

57.0 ha). Within tetrads, only 6 (3.5%) territories were found in farmland. Gardens held 123 (71.5%)

territories, though this habitat made up only 2% of the total area, while 39 (22.7%) were in woodlands (1%

of total area). Population densities were much lower in the tetrads than in earlier studies; this may be an

indication of the severe population decline that the species has recently undergone. Densities in gardens

differed less from those of previous studies.

In the woodland study, the number of woods holding song thrush territories declined over the three

years, while the proportion of total territories in the six largest woods increased. Densities in the woods

were lower than those reported in earlier studies but the disparity was not as great as in farmland.

Extinctions in the smaller woods was 20±30% per annum, a rate similar to that for gardens: population

extinctions did not occur in the larger woods.

Some 40% of non-garden territories in the surveys of both tetrads and woodland plots were within 100 m

of a garden. Gardens may be currently acting as a refuge for song thrushes and an understanding of the

ecology of the species in different types of gardens may be important in current conservation efforts for the

species.

Key words: song thrush, Turdus philomelos, habitat selection, farmland, woodland, gardens

INTRODUCTION

The song thrush Turdus philomelos was considered 30years ago to be an abundant bird of woodlands, hedge-rows, parks and gardens in Britain (Simms, 1978;Cramp, 1988). However, the Common Birds Census(CBC) conducted by the British Trust for Ornithologysince 1962 has shown a decline in song thrush popula-tions both in woodland and in farmland habitats,especially since the 1970s (Marchant et al., 1990). Thedecline possibly began at least as early as the 1950s, asshown by an analysis of the annual number of nestlingsringed (Mason & Hussey, 1984). Thus the beginning ofthe decline may have pre-dated major agriculturalchanges, such as the widespread introduction of pesti-cides, the simpli®cation of landscape elements withinfarmland, and changes in cropping practices, all ofwhich might have been expected to have adverselyin¯uenced populations. The song thrush in Britain is

now considered a Red List species of high conservationconcern (RSPB, 1996) because its breeding populationhas declined by more than 50% over the last 25 years(Fuller, Hill & Tucker, 1993). Farmland bird species ingeneral are in decline in the U.K., some 86% of speciesshowing population losses (Fuller et al., 1995). Theproblem is not con®ned to Britain or Europe. ManyNorth American migrant passerines have shown majorpopulation declines for reasons thought to be operatingon the breeding grounds (Stotz et al., 1996). Similarfactors may be involved on both continents.

Despite the strong evidence of the decline in songthrush numbers, its causes are not well understood.They may include natural or anthropogenic factors, or acombination of both. There is a strong negative correla-tion between CBC indices and the number of freezingdays in the ®rst two months of the year (Baillie, 1990),but weather alone is not suf®cient to account for thecontinuing decline of the species (Baillie, 1993). Anthro-pogenic factors which may have affected populationsinclude simpli®cation of farmed landscapes, such as theremoval of hedges, changes in land use, in particular the

*All correspondence to: Dr C. F. Mason, Department of BiologicalSciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ

J. Zool., Lond. (1998) 244, 89±93 # 1998 The Zoological Society of London Printed in the United Kingdom

increased planting of winter cereals, and the use ofbiocides and especially molluscicides.

If changes in landscape and land use are involved inthe decline of the song thrush, it is clearly important forthe development of a conservation strategy to knowwhich components of the landscape are still utilized bypopulations and which are no longer capable of sup-porting them. The present paper describes the currentdistribution of song thrush territories in habitat typeswithin a largely arable region of eastern England.

STUDY AREA AND METHODS

The study was conducted in the Tendring district ofnorth-east Essex (51850'N, 1810'E), a peninsula of area325 km2, bounded by the Stour estuary to the north, theColne estuary to the south and the North Sea to theeast. The town of Colchester lies just to the west. Theclimate is more continental than in most of Britain, witha low annual rainfall (average 510 mm), much of itfalling in summer, higher summer temperatures andcolder winters than average for southern England. Theagriculture is predominantly arable, occupying some63% of the area, with an additional 13% of grassland,the majority of it improved. Woodland occupies 5.6% ofthe total, the built environment comprises 14%, withminor habitats (water, wasteland, etc.) making up theremainder.

Ten tetrads (26 2 km, total 40 km2) within thepeninsula were selected randomly. Before the start ofthe survey period, the areas of the major landscapefeatures (built, woodland, etc.) were determined frommaps (1:25,000). Field boundaries were mapped by ®eldsurvey and classi®ed as without shrubs, hedgerows <1.5m tall (whether continuous or with gaps), and hedges>1.5 m tall (continuous or with gaps). Land use wasdetermined on a ®eld-by-®eld basis each year duringbird surveys. A summary of the landscape and land usewithin the study area is given in Table 1.

All tetrads were visited on 4 occasions between mid-April and mid-July in 1994, 1995 and 1996. All roads,tracks and footpaths (which provide good access to allpotential habitat in the tetrads) were slowly traversed,surveys beginning in early morning and being com-pleted by 10:00 h GMT. Song thrushes were consideredto be holding territories if they were singing or carryingfood. The locations of all birds were plotted on tomaps (1: 12,500) and territories were assigned in rela-tion to the distribution of these locations. A singleregistration during the season was taken as evidence ofa territory.

Surveys were also conducted in a total of 35 woodsand woodland fragments (area range 0.2±60.0 ha), 16%of the total wooded area in the district, but none ofthem in the tetrads described above. The majority wereancient coppice-with-standards, receiving various levelsof management from actively coppiced to derelict. Afew were partly planted with conifers. Some had beenextensively cleared and replanted following storm

damage in 1987. A few of the smallest woods weredeciduous plantations. Each wood was visited on 4occasions from early morning to 08:00 h GMT betweenlate-March and mid-June in 1994, 1995 and 1996.Woods were slowly traversed, as with the tetrads butthe boundaries of the smallest woodland fragmentswere walked. Time spent surveying was dependent onthe size of the plot, with a minimum of 20 minutes persurvey being spent on the smallest plots. The positionsof song thrushes were plotted on maps as describedabove.

RESULTS

The total number and density of song thrushes recordedin the 10 tetrads in each year are shown in Table 2.Changes between years in population were examinedusing t-tests. The increase in population between 1994and 1995 was signi®cant (t = 4.49, d.f. = 18, P < 0.01)but the decline between 1995 and 1996 was not (t = 0.69,d.f. = 18 ns).

The habitat in which song thrushes were recorded inthe tetrads was placed into four categories: farmland(hedgerow and ®eld), woodland and scrub, orchard, andgarden and houses. (Table 1). These will be referred tobelow, for convenience, as farmland, woodland,orchard and garden. Data for the three years werecombined for analysis. The results are summarized inTable 2. Some 71.5% of territories comprised gardensand 22.7% were in woodland. Only 3.5% of territorieswere in farmland (all in hedges >1.5 m tall) and 2.3%(4 territories) in orchard. The number of territorieswithin each habitat was compared with the expectednumber in each habitat (the total number of territoriesmultiplied by the proportion of land occupied by eachhabitat). The results were signi®cantly different (w2 =2075, d.f. = 3, P<0.001). Song thrush territories weresituated signi®cantly more often in gardens thanexpected from the proportion of this habitat in thetetrads (w2 = 1517, d.f. = 1, P<0.001) and signi®cantlymore often in woods (w2 = 267, d.f. = 1, P<0.001).Territories occurred signi®cantly less often in farmland

Table 1. Habitats in the 10 study tetrads in Tendring district,north-east Essex. Roads and water were excluded from thetotals

Habitat Type Range

Hedgerows less than 1.5 m high (km) 23 0±4.1Hedgerows greater than 1.5 m high (km) 125 5.2±28.5Hedgerow density (m ha-1) 37.2 13.9±77.7Area of houses and gardens (ha) 80 5.4±9.8Area of woodland and scrub (ha) 41 0±14.2Area of farmland (ha) 3662 331±390% of winter tillage1 57 51±71% of spring tillage2 29 9±45% of other (grass, setaside3, etc.) 14 6±28

1Crops sown in autumn; 2crops sown in spring;3arable land taken out of cultivation

C. F. Mason90

than expected (w2 = 1204, d.f. = 1, P<0.001), thoughmuch of the area, being open cropland, was unsuitable(data are not available to provide a more realisticestimate of habitable hedgerow and ®eld area for statis-tical testing). The proportion of territories which werein orchards did not differ signi®cantly from the propor-tion of this habitat within the tetrads (w2 = 0.3, d.f. = 1,P>0.001). The number of territories in tetrads wassigni®cantly correlated with the area of woodlandwithin tetrads (r = 0.88, P<0.01) but not with the areaof garden (r = 0.42, ns). Of the territories in gardens,104 (86.3%) were in large, isolated gardens (such asfarmhouse or vicarage gardens), 15 (12.8%) were rowsof small, adjacent gardens, while a single territory wasassociated with a small isolated garden.

In the surveys of woodlands (Table 2), an increase inthe total number of territories in 1995 was followed by adecrease in 1996, mirroring the changes in populationsin the tetrads. The number of woods occupied in eachyear was 26 (74.3%), 24 (68.6%) and 18 (51.4%) in 1994,1995 and 1996, respectively. As the number of woodsoccupied declined, the percentage of territories in the sixlargest woods (>12 ha) increased (48.8% in 1994, 57.4%in 1995, 74.3% in 1996). None of the seven woods of lessthan 1 ha held song thrushes in all three years, whilethree of the seven never had territories. Nine of 22woods in the size range 1±10 ha had territories in allthree years and the six largest woods were occupied inall three years.

The annual colonization and extinction rates forwoods of different sizes and for gardens was determined(Table 3). Both colonization and extinction rates forgardens were similar to those for small woods. Some

40% of territories not within gardens were neverthelesssituated within 100 m of a garden.

DISCUSSION

The song thrush is known to be a particularly dif®cultspecies to census accurately (Tomia�ojc & Lontkowski,1989). Song is concentrated in the early morning andlate evening, with the most sustained song being givenby unpaired males (Slagsvold, 1973). Density may there-fore have been underestimated in the present study.However, four visits were made to each tetrad andthrush song, even short bursts, can be heard at somedistance in this rather open habitat. Similarly, the wood-lands were censused on four occasions while the largestwood, a public access nature reserve, was visited onmost days throughout the year, frequently at dawn anddusk, but no additional song thrushes were located. Thedensity estimates should be comparable with the ma-jority of studies in which, as in the present study, songthrushes were not the sole target species in censuses. TheCBC, for example, on which evidence of the song thrushdecline is based, does not require that counts be startedone hour before local sunrise, as recommended byTomia�ojc & Lontkowski (1989).

The distribution of territories between habitats isunlikely to be affected by any underestimate of density.If birds were present in farmland habitats but notsinging, then some of them would have been ¯ushed intransects along hedgerows. In addition, adults with¯edglings are very vocal when disturbed, their persis-tent, penetrating alarm note carrying over long dis-tances, but none was recorded in farmland.

The estimated density of song thrushes in the 10tetrads (Table 2) is lower than previous estimates ofdensity on farmland. For example, the average densityon CBC farm plots in Essex, before the main decline,was 18 territories km-2 (Cox, 1984). Densities in aCambridgeshire parish were 16.9 territories km-2

(Wyllie, 1976) and on a Suffolk farm 18.3 territorieskm-2 (Benson & Williamson, 1972). If pre-decline den-sities on farmland in Tendring were similar to these,then the decline is approximately 90%. Over the period1970±88, CBC data point to a 55% decline of songthrushes on farmland (Fuller et al., 1993), while therehas been a substantial further decline since that date(Balmer & Peach, 1996). With only six territories actu-ally within agricultural habitat in the 40 km2, the songthrush can be described as effectively no longer a farm-land bird in north-east Essex.

Houses with gardens held the majority of songthrushes within the tetrads (Table 2). In Wyllie's (1976)Cambridgeshire study, the overall density reportedabove equates to six territories km-2 in agricultural landand 91 territories km-2 in the village. Shrubb (1970)also emphasized the importance of farmsteads andgardens, which in his Sussex study area occupied 2% ofthe total area but held 46% of the population of all birdspecies associated with hedgerow habitat. Davies &

Table 2. Number and density of song thrush territories intetrads and woodlands in study area in north-east Essex, 1994±96

1994 1995 1996

Surveys in tetrads (40 km2)Overall number of territories 44 66 56Overall density (km-2) 1.1 1.7 1.4Number of territories in farmland 0 4 2Density (km-2) 0 0.1 0.05Number of territories in gardens 35 45 40Density (km-2) 43.8 60.0 50.0Number of territories in woodland 13 13 13Density (km-2) 31.7 31.7 31.7

Surveys in woodland (294 ha)Overall number of territories 41 47 35Density (km-2) 13.9 16.0 11.9

Table 3. Colonization and extinction of song thrushes (gainsand losses per year per site) in woodland and gardens

Colonization Extinction

Woods < 1 ha 0.20 0.21Woods 1±10 ha 0.05 0.22Woods > 12 ha ± 0Gardens 0.28 0.27

91Habitats of song thrushes

Snow (1965) reported densities in the Oxford BotanicGardens to be eight times higher than those of a nearbydeciduous wood.

The small woodland areas within the tetrads heldsong thrush densities of more than twice those found inthe independent surveys of woodlands. This differencein density re¯ects the small size of all the individualwoods within the tetrads, the presence of a territorywithin them having a disproportionate effect on density.Considering woodlands smaller than 5 ha in the wood-land survey, the density of song thrushes was 42 terri-tories km-2 (in a total of 43 ha), despite the relativelylow level of occupancy within individual woods. Themedian density of song thrushes in CBC woodland plotsbefore the decline was 28 km-2 (Hickling, 1983), while ina Suffolk coppice wood 19 territories km-2 were locatedin 1987 (Fuller & Henderson, 1992). The averagedensity of song thrushes in woodland in the presentstudy (12±16 km-2) suggests that, although declines mayhave occurred in woodlands, they have been less severethan those in farmland.

Within the tetrads, most of the territories locatedwere in gardens. However, of those that were not, 40%were within 100 m of a garden, as was also the case withthe woodland survey, so gardens could have providedfeeding opportunities. Garden habitat has previouslybeen shown to in¯uence the abundance of song thrushesin hedgerows near Oxford (Macdonald & Johnson,1995). Small woods and gardens have similar annualturnover rates (Table 3), 20±30% of sites being vacatedannually. By contrast, there were no losses from largewoods. Sparks, Parish & Hinsley (1996) also found highturnover rates in Cambridgeshire hedgerows, whichthey attributed either to poor quality habitat or to highpredation rates associated with edge habitats.

Farmland landscapes have been greatly simpli®edover the past 40 years, especially with the removal ofhedgerows. For example, in the parish of Wivenhoe,immediately adjacent to the study district, 49% ofhedgerows were removed between 1879 and 1960, with afurther reduction of 16.3% between 1960 and 1980.Between 1879 and 1980, 59% of boundary trees werelost (Mason, Elliot & Clelland, 1987). These are typicalof landscape changes within the Tendring district(unpubl. data). Current hedgerow density (average 37.2m ha-1, range 13.8±77.7 m ha-1 in the tetrads fromTable 1) is below the density of 60±80 m ha-1 recom-mended by Lack (1992) for maintaining numbers anddiversity of a range of bird species; only one tetrad laywithin this range. However, song thrush numbers havecontinued to decline over the last decade at a time whenthe rate of hedgerow loss has been reduced. There arestill many hedges in the study area with a suitablestructure for song thrushes (Green, Osborne & Sears,1994), but which did not support territories. In Cam-bridgeshire, song thrushes in farmland were most oftenassociated with hedges of height 4±5 m, especially wherethese bounded pasture (Sparks et al., 1996).

Land-use changes have involved a reduction in thearea of spring-sown crops, which may have reduced the

feeding opportunities for song thrushes and hence thecarrying capacity of the farmland (O'Connor & Shrubb,1986). So few song thrushes were found on farmland inthe present study that it was not possible to examine anyin¯uence of cropping practice. As Baillie (1990) pointsout, however, this is unlikely to be the only problembecause the decline is not restricted to arable land, asexempli®ed in the present study. The other majorchange in the environment is the greatly increased use ofpesticides and, of potential signi®cance to thrushes,molluscicides.

The changes in the agricultural environment havebeen so immense that they are readily implicated indeclines of species such as the song thrush. There may,however, be other natural factors, such as disease, ofwhich we know nothing: there is some evidence tosuggest that the decline of the song thrush may havebegun before agricultural change became a major forcein the environment (Mason & Hussey, 1984; Marchantet al., 1990). Whatever the causes, the song thrush inthis part of eastern England now appears to be largelyrestricted to woodlands and gardens. The populationmay be conforming to the ideal free distribution ofFretwell & Lucas (1970), exploiting the most pro®tableparts of the landscape at low population density. Wood-lands and gardens can be considered as ecologicalrefuges. Large gardens have many features providingideal conditions for the species throughout the year:mature trees, dense shrubberies, many with berry-bearing species, ¯owerbeds, vegetable plots and lawns,while the juxtaposition of buildings, walls and vegeta-tion provides ideal conditions for snails, utilized exten-sively by song thrushes when other foods are in shortsupply (Simms, 1978). Conversely, gardens may alsocontain more predators (especially cats) and be moredisturbed than other habitats; they may therefore act asecological traps and population sinks rather than assources for recolonizing other habitats (Best, 1986).

Within the Tendring district as a whole, the builtenvironment (including towns) occupies some 2.5 timesmore land area than woodland, so it may be of dispro-portionate importance for the survival of song thrushesin the area. Nothing is known of the densities of songthrushes in suburban and urban habitats in the district,though data presented in Simms (1978) suggest thatthey might be rather low. An understanding of theecology of song thrushes in the different types of gardenwould be valuable in helping to maintain populations inthis refuge, one where landowners are likely to begenerally sympathetic.

Note added in proof. Song thrushes were surveyed againduring spring 1997 in the 34 woods. Only 21 territorieswere located, 48% less than in 1996. Song thrushes werepresent in only 12 woods, compared with 18 in 1996.The most signi®cant feature of the last two springs hasbeen drought, with soil moisture less than 50% of thelong-term average in both of them. Thomson et al.(1997, J. Anim. Ecol. 66: 414±424) have recently shownthat there has been a signi®cant decrease in the survival

C. F. Mason92

of ®rst-year song thrushes as the population hasdeclined. Snails are extensively eaten by thrushes in dryweather when other large invertebrates, such as earth-worms, become unavailable, but snails themselves mightsuccumb to prolonged drought and their absence mightbe critical to inexperienced thrushes achieving indepen-dence of their parents in late spring and early summer.The impact is likely to be greatest in farmland, wherefeeding opportunities are limited in the modern agricul-tural landscape and the under-drainage of ®elds quicklydries out the land. Severe droughts also affect wood-lands, as evinced by the many dry woodland ponds overthe last two years. The impact of drought is likely to beleast in gardens where householders protect theirvegetables, ¯ower-beds and lawns with regular watering.It is worth noting that three extended periods of songthrush decline (as shown by Common Birds Censusdata) began during two consecutive years of markedlybelow average spring rainfall (1975/76, 1983/84 and1989/90).

I thank all those who gave me permission to carry outsurveys on their land. Dr S. M. Macdonald assisted withthe ®eldwork in tetrads.

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93Habitats of song thrushes