The decline of theWillowTit in Britain

Alex l. G. Lewis, Arjun Amor, Elisabeth C. Chormon

ond Finn R. P StewortABSIMCT Numbers of the BritishWillowTit Poecile montonus kleinschmidtihave decreased dramatically in recent decades.This paper outlines the three

main hypotheses for the decline - increased competition, increased predationand changes in habitat availability - and describes the fieldwork and analyses

undertaken to try and understand its cause.The results so far are notconclusive but indicate that the loss of young, damp woodland may havebeen important. Possible positive management strategies are suggested.

IntroductionRonge ond habitotThe British race of the Willow Tit Poecilemontanus kleinschmidti is found in England,Wales and parts of southern Scotland, includingDumfries & Galloway and North Lanarkshire(Clyde). The species as a whole is foundthroughout much of the Western Palearctic andeast through northern Asia as far as fapan, whereit is found largely in boreal forest with conifers(Snow 1954; Alerstam et aL 7974; Snow &Perrins 1998; Sellers 2002). A recent study innorthern Finland (Sitrczyk et a\.2003) showedthat in a 'mosaic forest landscape', Willow Titspreferred mature woodland (and pine bogs) andavoided young stands. In Britain, however, thespecies is associated almost exclusively withearly successional habitats such as those foundalong river valleys, overgrown flooded gravel-pits and wet woodland (Snow & Perrins l99B).

Breeding ecologyThree separate but related studies of Willow Titswere commissioned by the RSPB and took placein 2005 and 2006, and form the basis of thisreview. One of these was carried out in theMidlands (South Yorkshire, Derbyshire andNottinghamshire) and involved the location andobservation of 56 Willow Tit nests during thetrvo breeding seasons (Lewis unpubl.). This was

the first study to observe such a high number ofnatural nests in Britain and thus provides aninsight into the species'breeding ecology.

Willow Tits remain in their territories fromtheir first breeding year and start to sing inlanuary when the weather is good (Maxwell2007). Between April and May, they excavate a

nest hole in dead wood. The most frequentlychosen tree species in the Midlands study areawere willow Salix spp. (n=20), E\der Sambucusniger (n=70) (plate 21 1) and Silver Birch Betulapendula (n=16) (plate 212),bttt nests were alsofound in Alder Alnus glutinosa (n=3), HawthornCrataegus monogyna (n=2), Hazel Corylusayellana (n=2), Wild Cherry Prunus ayium(n=1) and a conifer (n=i). The nest hole is

excavated in either a dead tree or a dead part ofa live tree, or occasionally in a felled trunk (p1ate

213). Wiilow Tits can nest successfully in a trunkwith a diameter as small as 5 cm (measured atthe part of the trunk corresponding to the baseof the nest chamber), although the meandiameter recorded was 12 cm.

Willow Tits line their nests with material suchas feathers, animal hair, thin strips of bark andthe fluf$' seeds from the heads ofbthrsh Typha(plate 214). The female incubates for approx-imately 14 days (Maxwe1l 2007). Of 56 nesrsstudied, it was possible to inspect only 20internally (with an endoscope; plate 215) so that

386 o Erirish Birds 102 . July 2009 . 386-393

The decline of the Willow Tit in

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2l l-2a 3. Willow Tit Poecile montonus nests in Elder Sombucus niger (21 I ), Silver Birch Betuto pendula (212)and a felled Hawthorn Crotoegus monogyno (213), Nottinghamshire, April 2006.

clutch size could be determined. Clutch size wasfound to be 8.8 + 0.4 (mean + I s.e.) and this wasnot related to tree diameter (as measured above).In all cases, other than that described in Lewis &Daniells (.2007), i rvas impossible to calculatebrood size and number of fledglings owing to:(i) the way that the chicks were invariably sittingon top of one another inside the chamber; and(ii) the need to avoid excessive disturbance thatprolonged viewing may have caused.

After the eggs hatch, the chicks remain in thenest for approximately 18 days (plate 216). Oncefledged (plate 217), they remain in theimmediate area for up to 20 days, while theycontinue to be fed by both parents (Maxwell2007). At the end of the breeding season, BritishWillow Tits differ from their Europeancounterparts ir-r that they do not form gregarioussocial groups, neither rvith conspecifics nor withother tit species (Perrins 1979; Sellers 2002), and

remain in the breeding territory all year roulrd.The few ringing reco\reries for this speciessupport the finding t1.rat Willow Tits aresedentary (but that juveniles in late summer orautumn will occasionally move more than 5 km;Sellers 2002).

Populotion statusAn analysis of Con"rmon Birds Census (CBC) andBreeding Bird Survey (BBS) data has shorvn thatthe British Willow Tit underwent a drarnaticdecline of880/o betrveen 1970 and 2006 (Eaton etal. 2008). Ringing data provide further evidence ofa substantial population decline (Perrins 2003). Infact, of all the species mor.ritored adequately by theBBS, the Willow Tit showed the greatest decline(77o/o,1994 2007; Risely et a\.2008). The RepeatWoodland Bird Survey, which looked atpopulation changes in woodland habitat only, alsorecorded a severe decline, of 7070 between the mid

British Eirds I 02 . Juty 2009 . 386-393 387

The decline of theWillowTit in Britain

1980s and 2003/04 (Hewson efaL 2007). As well as

a reduction in numbers, the Willow Tit has

suffered a marked contraction in its range in

Britain and has been iost entirely from manycounties (Gibbons et al. 1993 Smith er al. 1993).

Owing to its population decline, the Willow Titappears on the UK's 'Red list' of birds ofconservation concern (Eaton ef al.2009) and is a

priority species in the UK Biodiversity ActionPlan. However, despite the decline in numbers and

range contraction, Willow Tits still appear to be

doing well in parts of the Midlands. Consequently,

much of the recent work has focused on these

apparently stable populations.

Potential threots t o Willow Tit populotions

Research into the decline of the Willow Tit has

focused on three hypotheses: increased

competition, increased predation and habitatchange.

CompetitionThe excavation of a nest hole can be a noisyactivity, as both birds often call repeatedly toeach other. Typically, the process is also visuallyobvious as it involves much physical activity and

continual production of small wood chippings.An excavating Willow Tit pair is thus vulnerabieto detection by competitively superior BlueCyanistes caeruleus and Great Tits Parus major.Ifeither of these species finds a Wil1ow Tit nest

ho1e, they can oust the occupants with littledifficulty (Maxwe1l 2002). Excavating a nest hole

is a time- and energy-consuming process and

losing a nest hole following its completion canresult in breeding failure. Between 1995 and2000, Maxwell (2002) monitored 30 Willow Titpairs using both natural nests and nestboxes. Helound lhat only ten ofthese pairs were success-

fu1. Of the 20 unsuccessful pairs, 18 had theirnest cavity taken over by Blue Tits and two byGreat Tits. Blue and Great Tit populations have

increased in the UK between 1970 and 2006(B1ue Tits by 33o/o and Great Tits by giolo)

(Eaton ef al. 2008). Consequently, interspecificcompetition for nest-sites may have increased

and contributed to the Willow Tit's decline.

PredotionOnce a nest hole has been excavated and lined,Willow Tits can still be noisy around theirbreeding site, leaving them vulnerable todetection by Great Spotted WoodpeckersDendrocopos major, whrch are accomplished at

extracting prey from rotten wood (Wesolowski

2002; Fuller et aI.2005). This species will destroya Willow Tit nest with ease and take the eggs orchicks (Lewis unpubl.) (plate 218). Willow Titsare single-brooded and if predation occurs at

the chick stage, the pair is extremely unlikely tobreed again that year. Even if predation occurs at

the egg stage, the pair will have limited resources(of time and energy) for another attempt.Numbers of Great Spotted Woodpeckers have

increased dramatically in the UK (by 3l4o/o

between 1970 and 2006; Eaton et al.2008), andWillow Tits may have suffered a correspondingincrease in predation rate.

HobitotWillow Tits are traditionallyassociated with damp,scrubby arers in Britain(Perrins 1979; Snow & Perrins1998). Despite the steep pop-ulation decline, they can stilloccur dt relatively high densi-

ties in some brownfield sites

and disused gravel-pits, where

such habitat is often charac-teri:tic (plate 2 l9) (Lewis

unpubl.). However, thesedisused industrial sites have

become less common inrecent decades, having been

lost to (for example) urbandevelopment and agriculturalclearance (Ban et aI.1993).

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e2 I 4. EightWillowTit Poecile montonus chicks in a nestbox,Wiltshire, May

2005.The bedding is made up of feathers and thin strips of bark.

388 Eritish Birds 102 . July 2009 . 386-393

The decline of the Willow Tit in Britain

This apparent wasteland does not have the same

perceived biodiversity value as more establishedhabitats such as woodland (Mortimer er al.

2000) and the loss of large areas of such scrub

could be driving the Willow Tit's deciine.

Iesting the hypotheses ond exploringthecouses ofthe declineData analysis

Siriwardena (2004) showed that the majordecline in Willow Tit numbers had occurred inwoodland and farmland, and that populationsin wet habitats had remained stable. In addition,no significant negative relationships were foundbetween the abundance of Willow Tits andnumbers of Great Tits, Blue Tits or GreatSpotted Woodpeckers over the same time period(apart from between Willow Tits and Great

Spotted Woodpeckers on farmland; see below).Siriwardena (2004) concluded that furtherresearch, particularly into the possible role ofhabitat in the species' demise, was needed,

prompting the RSPB to undertake the three

research projects described here. In addition tothe Midlands breeding study, a small-scale studywas carried out to investigate Willow Tithabitat selection by comparing occupied and

unoccupied woods in an area of northNottinghamshire with a relatively high densityof Willow Tits. This project also looked at

differences in habitat within indivrdual woods,

between occupied and unused areas. A separate,

larger-scale study tested the competition,predation and habitat-changehypotheses outlined above, bycomparing woods that hadbeen abandoned by WillowTits with those that were stilloccupied. These two studies

used the same basic fie1d

methodology, but at differentgeographical scales.

The smoll- ond lorge-scole

studiesThe small-scale study tLewiset al. in press) was carried outin north Nottinghamshire. inan area of 18 x 26 km whereWillow Tits still occur in good

numbers. The apparent stabil-ity and relatively high densiry

oi Willow Tils in this regionincreases the probability that

unoccupied patches represent unfavourablehabitat; in areas which are sparsely popuiatedowing to factors other than habitat (such as

predation), many favourable sites may be unoc-cupied and thus wrongly classified as

unfavourable. A large number of habitat vari-ables were quantified (inciuding tree diameter,

tree-stem density and tree species composition,canopy cover, soil-water content, dead-woodabundance and vegetation cover at varyingheight bands) in both occupied and unoccupiedsites, and in areas within each occupied sitewhere Willow Tits were present and absent.

The large-sca1e study (Lewis et aI.2007) tookplace throughout central, southern and easternEngland. It aimed to determine whether ninewoodland sites that were known to have been

abandoned by Willow Tits (five years or morepreviously) differed from nine that were stilloccupied, in terms of competitor numbers,predator numbers and/or habitat. The numbersof Blue Tits, Great Tits and Great SpottedWoodpeckers were recorded and habitatmeasurements taken. These measurements were

taken around past or present Willow Titlocations rather than throughout the wood, inorder to ensure that the habitat sampled was ina relevant area.

In terms of habitat, both of these studies

showed that occupied sites had a significantlyhigher soil-water content than unoccupied orabandoned sites. In addition, the small-scale

study showed that occupied sites were of a

2 1,5. An endoscope is used to inspect the contents of aWillowTit Poecile

montonus nest at Bennerley Marsh, Nottinghamshire, May 2006. One personis directing the fibre-optic tube while the other views the chamber contents.

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rBritjsh Birds I 02 . July 2009 . 386-393 389

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significantly earlier successior-ral stage (definedby a combination of factors indicative of tree age,

including tree diameter and canopy height) thanunoccupied sites. Furthermore, Wil1ow Titstended to be found in most young patches ofwoodland but rvere also present in older patches

if these patches had moist soils. However, theywere almost completely absent from older, drierwoods. In addition, unoccupied sites tended tohave more coniferous trees. Within occupiedwoods, Wil1ow Tits were located in areas withiess Bracken Pteridium aquilinum and morecover between 2 and 4 m in height - the mid-level understorey. The large-scale study found nodifferences in numbers of Blr-re Tits, Great Titsand Great Spotted Woodpeckers betweenoccupied and abandoned sites.

The Midlonds studyResults from the Midlands study showed that, ofthe 56 Willow Tit nests monitored over twobreeding seasons, breeding success was highwith only 17 nests (30%) failing. Breedir.rg

success was similar between the two breedir-rg

seasons (2005: 660/o X lo/o (n=24),2006:72o/o +B% (n=32)). Great Spotted Woodpeckerpredation caused ten of the 17 failures (seven atthe chick stage and three at the egg stage). There

was no evidence of any of these pairs re-nestingfollowing predation. Only one nest failure was

due to mammal predation (by a Weasel Mustelanivalis;Lewis & Daniells 2007), and no failuresdue to ousting by competitors were recorded.The cause of failure for six nests was unknown.

I nterp retotio n of resultsThe sma11- and large-sca1e

srudie: pror ide quantitative\upport for the observalionthat Wiliow Tits in Britainselect early successionaldeciduous woodland and thatthey are often associated withdamp habitats (Snow &Perrins 1998). Their prefer-ence for damp woodland (thatwith a relatively high soil-sater content) was shown inboth studies and for earlysuccessional woodland in thesnall-scale study. The fact thatabandoned sites tended to bedrier than occupied sitessuggests that drier sites are

either less likely to retain theirWillow Tits or that they have

become drier and lost them as

a result. In the small-scalestudy, measurements of treediarneter lrom occupied site>

showed that, according toForestry Commission yieldmodels, stands between 15

and 25 years old had thehighest probability ofoccupancy. Both studies also

supported Siriwardena's(200a) finding that WillowTits have declined dramat-ically in woodland andfarmland but not in damphabitats. The reasons for this

216 & 217. A colour-ringedWillowTit Poeci/e montonus bringingcaterpillars back to chicl<s in the nest, and a chick ready to fledge;

Bennerley Marsh, Nottinghamshire, May 2005.

390 British Birds I 02 . luly 2009 . 386-393

The decline of theWillowTit in Britain

species' association with young, damp woodlandare currently being investigated (see below).Such sites may offer greater nestingopportunities: willow and birch are more likelyto occur in greater cluantity in damp conditionswhile higher rates of timber decay in such areas

may facilitate nest-hole excavation. Sr-rch

conditions may also present Willow Tits withlai ourable leed ing opport uni Lies.

Neither study was able to establish the reason

for the Willow Tit's decline with certainty, butthe finding that abandoned sites were

significantly drier than occupied sites providesperhaps the strongest clue that the drying out ofwoodland could be driving the species'decline.It is impossible to quantift the decline in earlysuccessional, damp woodland over the last fewdecades; no figures exist for either past orpresent coverage (UK Wet WoodlandBiodiversity Action Plan wwwukbap.org.ul<).Hower.er, various factors known to contribute tothe loss of such woodiand (such as the loweringof water tables, pollutior-r from agricuitural run-off and climate warming) are all known to have

increased in recent years (UK Wet WoodlandBiodiversity Action PIan).

\;\&y do Wiilow Tits prefer young, dampwoodland? Early successional woodlandsupports proportionately fewer Blue and GreatTits; for example, Smart er al. (2007) showed

that Blue Tits were more abundant in olderwoods, which contain more natural cavities fornest-sites. Competition for nest-sites is not new,

but mav well have increased in recent vears as

populations of Blue and GreatTits have grown. Although no

direct link was found between

abundance oI oLher liL species

and the Willow Tit decline(Siriwardena 2004), competi-lors may.till be influencingthe Willow Tit's selection ofhabitat. Great SpottedWoodpeckers were the main

nest predator olWillow Tits inthe Midlands study. Thiswoodpecker requires trees

with a minimum diameter oflB cm for nest ercavation(Smith 1997) and youngwoodland will thus supportlower densities (Kosihski2006). Willow Tits maytherelore select (or survive

better in) habitats which support iowerpopulation densities of this predator.

The results do not show that the GreatSpotted Woodpecker has been instrumental inthe Willow Tit's decline. However, an increase inwoodpecker numbers within the Willow Tit'spreferred habitat would be 1ike1y to increasepredation rates. Moreover, although no overallrelationship was found between the \,\,'illow Tit'sdecline and the Great Spotted Woodpecker's

increase between 1970 and 2005, Siriwardena(2004) did find a significant negative

relationship between the two species orr

farmland sites. Willow Tit habitat ir.r farmlandareas usually consists of small, scrubbywoodlands, orchards or hedgerows and Amaret al. (2006) showed that Great SpottedWoodpecker numbers had increased by 440o/o insmall farm woods over the past 20 years (eight

times more than in other broadleaved woods).The same study showed that the probability ofan increase in numbers of the Willow Tit's mostsignificant predator was higher in youngerwoodlands - the Willow Tit's preferred habitat.Nonetheless, in the large-scale analysis nodifference was found in Great SpottedWoodpecker abundance between woodsoccupied and abandoned by Willow Tits.

Ongoing ond future reseorchInvestigations into habitat selection by WillowTits are ongoing and have now been extended tocover more of the UK. Establishing consistenthabitat preferences across a wide area should

2 I 8. Willow Tit Poecile montonus nest in willow So/ix destroyed by G reatS potted Wood pe cker D e n d rocop os mojor, Ogston Res e rvo i r, D erbys h i re,

May 2004. All the chicks were eaten and the nest abandoned.

.e

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Eritish Birds I 02 . July 2009 . 386-393 39t

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of a structured survey) andsuch an assessment should be

undertaken before and afterany intervention.

lnterim manogementrecommendotionsWet featuresWillow Tits are traditionaliyassociated with wet woodlandand the comparative studiesshowed that more Willow Titswere found at sites with a

high soil-moisture content.Features such as ponds and

219. Daneshill Gravel-pits, Nottinghamshire, April 2006, a site where streams should therefore con_several pairs ofWillowTits Poecile montonus stili breed successfully. tribute to making a site more

improve our understanding of whether habitatloss has been responsible for driving the decline.Once the characteristics of suitable Willow Tithabitat are more clearly understood, it will becritical to establish whether a shortage of suchhabitats exists where declines have been mostevident. If habitat is abundant, then otherfactors must be responsible for the decline andwoodland management will not help. However,if a lack of suitable habitat appears to be instru-mental in the species'decline, such managementlvill be crucial. Further analyses of key BTO data(such as that from Constant Effort Sites and theNest Record Scheme) are also being undertakento improve understanding of any demographicchanges linked to the species'decline.

There are also several key areas relating toWillow Tit ecology that are worthy of furtherstudy. Although the species appears to favourdamp scrub and woodland, the reasons for thisremain unclear. As outlined above, this mayreflect the physical structure of such habitatresulting in reduced competition and/or nestpredation. However, it also may be related tofood availability. Until more is known, inparticular about the species'diet throughout theyear, establishing the importance of such habitatwill be difficult.

Given the current gaps in knowledge, theefficacy of any habitat management measures(such as provision of nestboxes, creation of wetfeatures and/or cultivation of areas of scrub)must be monitored carefully. A good estimate ofthe number of Willow Tits at a site can beobtained by playing a recording of their call andsong at specific points throughout the site(although this should be carried out only as part

favourable as Willow Tit habitat.

Young trees

Willow Tits are traditionally associated withearly successional woodland and have beenshown to favour sites with trees between 15 and25 years o1d. Managing an area to includesubstantial areas of such trees should enhancethe prospects for Willow Tits.

The protection of brownfield sitesBrownfield sites in the Midlands often supporthigh densities of Willow Tits, possibly becausesuch areas often support young trees andwaterbodies. These areas have minimalprotection from development and are thereforeconstantly at risk of being lost. As well as

supporting Willow Tits, such areas providehabitat for other Red-listed species such as theBr"rllfinch Pyrrhula pyrrhula, Turtle DoveStreptopelia turtur and Reed Bunting Emberizaschoeniclus. Protection of such areas will help tomaintain key populations of Wi11ow Tits.

Nestbox plocement * o red herring?Neslbores are often put up in an attempt to

increase Willow Tit abundance and/or breedingsuccess in an area. However, there is no evidenceto suggest that dead wood is a limiting factor: nodifference in dead-wood abundance was foundbetween sites that had been abandoned byWillow Tits and those occupied by them (Lewis

et a\.2007).A survey ofthe River Clyde showedthat the number of Willow Tit pairs haddeclined by 50% between 2000 and 2003, yet nosl.rortage of dead wood was found (Maxwell2007). Recent habitat-related studies of

392 Brirish Birds I 02 . july 2009 . 386-393

The decline of the Willow Tit in Britain

woodland have also shown that, overall, the

dead-wood resource has ir-rcreased (e.g. Amar er

aI.2007). A reduction in dead wood is therefore

unlikely to be the cause of the species' decline. Ifthere is local concern that dead wood is lacking,

the resource should be measured (see Lewis ef al.

2007) and compared with that of the occupied

sites in the north Nottinghamshire populationwhere the availability of dead wood is knownnot to be a problem.

Acknowledgments

Thrs study was funded by the Royal Soc ety lor theProtection of Brds, Natural England (through the Action

for Brrds n England Partnersh p) and ForestryCommission Engand.We are ertremely grateful to Ken

Smth ofthe RSPB for guidance and a the landowners

and managers who a lowed us access to thelr landthroughout the three year study.We aso thank DaneCord -Piec, Laura Daniells, Derek Gruan Chris Hil Erica

Morey, .lacqui Weir and Davld Wood for help wth thefe dwork and all the volunteers who generousLy gave up

t me to support the proledr n a variety ol ways.

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