ANIMAL BEHAVIOUR, 2006, 72, 1035e1043doi:10.1016/j.anbehav.2006.02.020

Competing for position in the communal roosts of long-tailed tits

ANDREW MCGOWAN* , STUART P. SHARP†, MICHELLE SIMEONI† & BEN J. HATCHWELL†

*Centre for Ecology and Conservation, University of Exeter in Cornwall

yEvolution and Behaviour Group, Department of Animal and Plant Sciences, University of Sheffield

(Received 27 June 2005; initial acceptance 26 September 2005;

final acceptance 16 February 2006; published online 22 September 2006; MS. number: 8597R)

Communal roosting is widespread among social animals and has several hypothesized functions, includ-ing reduction of predation risk and thermoregulatory costs, and information sharing. The benefit derivedfrom roosting in close proximity to conspecifics is likely to depend on an individual’s position within theroost, but there have been few quantitative studies of the process of communal roost formation and thefactors influencing relative positions. We studied the communal roosting behaviour of temporarily captiveflocks of long-tailed tits, Aegithalos caudatus, a cooperative breeder that forms flocks during the nonbreed-ing season. Our objectives were to determine whether (1) individuals compete for particular positionswithin roosts, (2) individuals achieve consistent positions within roosts, and (3) an individual’s roost po-sition is a function of its dominance status. During roost formation birds were more likely to move to innerpositions regardless of whether they started in an outer or inner position. However, as the number of birdsin the roost increased during roost formation, birds in outer positions were less likely to relocate to an in-ner position. The same individuals occupied outer positions in a roost on successive nights, but there wasno consistency in the occupation of specific inner positions. Birds occupying outer roost positions weresignificantly less dominant than those occupying inner positions. Our results show that long-tailed titscompete for inner positions within communal roosts, and that an individual’s dominance status withina flock influences the outcome of this competition.

� 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Communal roosting behaviour is widespread amonganimals (Eiserer 1984; Lewis 1995; Anderson 1998) andin birds it is known to have evolved independently severaltimes (Beauchamp 1999). Communal roosts are thoughtto increase foraging efficiency by functioning as informa-tion centres (Ward & Zahavi 1973) or recruitment centres(Richner & Heeb 1996; Dall 2002), and they may alsoconfer benefits by decreasing predation risk or thermo-regulatory costs (Beauchamp 1999; DuPlessis 2004). Themagnitude of such benefits may depend on an individual’sposition within a communal roost, which in turn may beinfluenced by factors such as age, sex and dominancestatus (Swingland 1977; Summers et al. 1987; Feare et al.1995; Adams et al. 2000; Calf et al. 2002), although evi-dence of any benefits associated with specific roostingpositions is scarce.

Correspondence: A. McGowan, Marine Turtle Research Group, Centrefor Ecology and Conservation, University of Exeter in Cornwall, Trem-ough Campus, Penryn TR0 9EZ, U.K. (email: amcgowan@seaturtle.org). S. P. Sharp, M. Simeoni & B. J. Hatchwell are at the Evolutionand Behaviour Group, Department of Animal and Plant Sciences,University of Sheffield, Sheffield S10 2TN, U.K.

100003e3472/06/$30.00/0 � 2006 The Association for the

Communal roosting appears to be relatively common incooperatively breeding birds, although there have beenfew detailed studies (e.g. Chaplin 1982; Ligon et al. 1988;Williams et al. 1991; DuPlessis & Williams 1994; DuPlessiset al. 1994). Communal roosts are unlikely to function asinformation or recruitment centres in cooperatively breed-ing birds, as groups tend to be cohesive family-based unitsthat spend all of their time together. Thermoregulatorybenefits from group roosting are the most likely explana-tion for communal roosts in cooperative species becausestudies have shown that the energetic savings of commu-nal roosting can be as high as 37% relative to solitaryroosting (Riehm 1970; DuPlessis 2004). However, suchbenefits are unlikely to be acquired equally by all roostmembers, so an individual’s position within a roost mayhave important consequences for its energy budget. Stud-ies of the jungle babbler, Turdoides striatus (Gaston 1977),varied sitella, Daphoenositta chrysoptera (Noske 1985) andArabian babbler, Turdoides squamiceps (Zahavi 1990),have reported on the spatial organization of their commu-nal roosts, and in all three species the end positions intheir linear roosting huddles were occupied by the twomost dominant birds in the group, usually the top males.These structured roosts, with dominants adopting outer,

35Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

ANIMAL BEHAVIOUR, 72, 51036

potentially more costly positions while offspring adoptcentral positions, might be expected given that groupsin these species are composed of close kin. However, toour knowledge there are no published quantitative dataon the process of roost formation in these or any othercommunally roosting cooperative breeder.

We investigated the communal roosting behaviour ofthe cooperatively breeding long-tailed tit, Aegithalos cau-datus. Long-tailed tits spend the nonbreeding season inmixed-sex flocks that typically comprise adults and juve-niles of two or more nuclear families; flocks also containa proportion of unrelated immigrants that disperse be-tween flocks during their first winter (Glen & Perrins1988; Hatchwell et al. 2001a; Russell 2001). These flocksbreak down during the breeding season, when all birds at-tempt to breed in monogamous pairs; cooperation occurswhen failed breeders become helpers at the nest of an-other pair, assisting them in feeding nestlings and fledg-lings (Hatchwell et al. 2004). During the nonbreedingseason flock members roost together in tight linear hud-dles (Lack & Lack 1958) and it has been suggested thatthe thermoregulatory benefits of communal roostingmay be one of the principal reasons that this speciesshows cooperative behaviour, helpers effectively payingto join a family flock when they have failed to breedsuccessfully themselves (Glen & Perrins 1988; but seeMcGowan et al., in press).

Our principal objectives in this study were to investigatethe process of roost formation to determine whether (1)individuals compete for positions within communalroosts, (2) individuals achieve consistent positions withincommunal roosts, and (3) an individual’s roost position isa function of its dominance status within its flock.

METHODS

We studied the roosting behaviour of 13 temporarilycaptive, nonbreeding season flocks of long-tailed titsbetween October and January 2000e2002 (N ¼ 8 flocks)and 2004e2005 (N ¼ 5 flocks). Flocks of variable size(five of nine birds, three of eight birds, four of seven birdsand one of six birds; total ¼ 102 birds) were captured inmist nets (under British Trust for Ornithology permit)from two colour-ringed study populations in MeltonWood, Doncaster, U.K. (53�310N, 1�130W; N ¼ 7 flocks)and Rivelin Valley, Sheffield, U.K. (53�230N, 1�340W;N ¼ 2 flocks), or from unringed populations in woodlandsin the vicinity of Sheffield, U.K. (N ¼ 4 flocks). Flocks werehoused in an outdoor aviary (under English Nature licen-ce), at the Department of Animal and Plant Sciences, Uni-versity of Sheffield, Sheffield, U.K. The aviary wasapproximately 3 � 3 m and 2 m high (2000e2002) or3 � 1 m and 2 m high (2004e2005) and provided protec-tion from rainfall and some wind, although birds were ex-posed to ambient temperatures. The birds were fed withwax moth, Galleria mellonella, larvae, mealworms, Tenebriomolitor, and crickets, Gryllus bimaculatus, ad libitum. Waterwas provided for drinking and bathing at all times. Allbirds were weighed shortly after capture and again justprior to release at approximately the same time of day.

They all had access to food even in the presence of moreaggressive conspecifics and the majority of birds (91/102) gained weight over the duration of captivity (meanweight gain � SD ¼ 0.4 � 0.36 g, range �0.7e1.37 g.Each flock was kept for 7e10 days and then released atthe site of capture. Facilities for housing birds compliedwith Home Office regulations.

Communal Roosts

Flocks were allowed to acclimatize to captivity for 48e72 hbefore any observations were carried out. Roosting behav-iour was filmed with a video camera with an infrared nightvision function (Sony CCD-TR427E) positioned directlyabove a single roosting perch. In 2000e2002, this perch waslocated in an open-fronted wooden shelter (1.2 � 0.5 mand 0.8 m high) placed in the aviary. However, althoughwe removed all other perches from the aviary overnight,flocks did not always choose to roost on the roosting perchin this sheltered position within the aviary. Therefore, in2004e2005, flocks were captured just before dusk andplaced in a smaller cage (0.4 � 0.5 m and 0.3 m high) con-taining a single perch, then released back into the aviaryat dawn. Video recordings of roosting behaviour showedthat there was no difference in the duration or process ofroost formation in the two situations, so we pooled thedata, where appropriate, in analyses. In both situationsthe roosting perch was the width of the box (0.5 m) or thecage (0.3 m). The distance between the outer birds and ei-ther end of the perch varied each night, but on virtuallyall occasions the birds roosted fairly centrally on the perchand there was no contact between the end birds and a sup-porting wall. These walls were fine wire bars in 2004e2005.In the wild, long-tailed tits rarely roost in trees and seem toprefer dense clumps of brambles or bushes that are oftenlow to the ground (A. McGowan, personal observation).Other typical roosting perches in the field are fine branchesin holly bushes or in climbing plants such as clematis andivy (B. J. Hatchwell, personal observation). In this respect,the perches used in this study are a close representation ofthe natural roosting locations of long-tailed tits.

All birds were individually colour-ringed, but these ringscould not be seen during roost formation so a small(5 � 5 mm) piece of black card, uniquely marked withwhite enamel paint, was attached to the crown featherswith nontoxic glue. Symbols on the cards reflected theinfrared light emitted by the video camera, allowing usto determine the position of each bird in the roost. Weremoved the pieces of card from all birds prior to release.The tips of a few crown feathers were cut off duringremoval, but there was no visible effect of this on thebirds’ appearance.

In 2000e2002, we filmed seven flocks for a 4-h periodaround dusk (1 h before and 3 h after), ensuring that allmovements occurring during roost formation and finalpositions in the roost were recorded. A further 4 h of re-cording were carried out around dawn (2 h before and2 h after) to determine whether roost positions were con-sistent through the night and to record the break-up ofroosts. Therefore, a total of 8 h of recordings were made

MCGOWAN ET AL.: COMMUNAL ROOSTS OF TITS 1037

each night and roosts were filmed for 4e7 nights per flock.These recordings showed that on each night the spatial ar-rangement of birds within roosts was stable once a roosthad formed, and that roosts dissolved simply by birds firstremoving their heads from beneath their wings and thenpreening briefly, before leaving the roosting perch shortlyafter dawn. Therefore, in 2004e2005, we filmed for just1.5 h around dusk to record interactions during roost for-mation and the final roost position of all birds.

Roost formation was deemed to have commenced whena minimum of three birds began displaying signs ofroosting, huddling next to one another and jostling forposition before being joined by the other individuals inthe flock. A brief wing-shivering display by the first two tothree birds normally accompanied the early stages of roostformation. Therefore, it was clear when roost formationhad begun. The roost was deemed to be complete when allbirds were present and all movements between positionsin the roost had ceased.

For each night of filming of each flock, the finalposition of every bird in the roost was determined fromthe video recordings. In 2000e2002, the size of each flockwas held constant at its initial size throughout theobservations, allowing us to investigate the consistencyof positioning within roosts across a series of nights. Fromrecordings of these flocks, once roosts started to formwe noted all movements and attempted movements ofindividuals within the roost (hereafter all termed ‘re-locations’). An attempted movement was deemed tohave occurred when a bird left the perch and tried torelocate to another position but was prevented fromdoing so by one or more conspecifics and ended upback in the original position from which it had started.For each relocation, we recorded the number of birdspresent in the roost at the time of relocation, as well asthe starting and finishing positions within the linearhuddle of the individual that relocated. Therefore, therewere four potential types of movement within the roost:(1) from an inside position to an inside position; (2) froman inside position to an outside position; (3) from anoutside position to an inside position; and (4) from anoutside position to an outside position. In 2004e2005,flocks were progressively reduced in size after the first2 nights of filming for a separate investigation (B.J.Hatchwell & S.P. Sharp, unpublished data). We did notuse data from these flocks to analyse consistency ofpositioning within roosts or relocations during roostformation, but the 2 nights’ data on final roost positionsof the complete flocks were used.

Dominance Hierarchies

We determined the dominance hierarchy within eachflock from aggressive interactions that were recorded onvideo during roost formation and by direct observation ofinteractions over food. Pecks during roost formation werepredominantly aimed at the nape of the victim, and inthese interactions the aggressor and victim could bedetermined unambiguously. Interactions over food were‘tugs of war’ over a food item and birds were easily

identified as the aggressor (initially without food) andthe victim (initially with food); the winner of the in-teraction was the bird that obtained the food item. Thenumber of pecks and interactions over food varied widelybetween flocks. There were a mean � SD of 94.1 � 105.5pecks and 78.9 � 61.9 interactions over food per flock.For four of the 13 flocks there were insufficient interac-tions to generate a hierarchy for either pecks (N ¼ 3) orinteractions over food (N ¼ 1) alone. However, for theremaining 70 birds in nine flocks there was a highly signif-icant positive correlation between the dominance rankgenerated from pecks and that generated from inter-actions over food (Spearman correlation: rS ¼ 0.598,N ¼ 70, P < 0.001). Therefore, all pairwise interactionswere combined and a linear dominance hierarchy basedon dyadic interactions was constructed for each flock,with the individual that won the greater proportion of ag-gressive encounters in a dyad being regarded as dominant.Finally, we assigned the individuals in each flock a domi-nance score according to their position within the domi-nance hierarchy. The most subordinate individualreceived a score of 0, and the most dominant a score of1, and birds between received scores rising in incrementsof 1/(N � 1), where N ¼ flock size. Thus, a flock of sevenindividuals had scores of 0, 0.17, 0.33, 0.50, 0.67, 0.83and 1.0. In some cases individuals adjacent to each otherin the hierarchy could not be separated (for example, ifthey were not observed interacting with each other); inthese cases, tied individuals received the mean score ofthe adjacent positions.

Statistical Analyses

To investigate formation of roosts and the consistencyof roost position across different nights, we used data fromcomplete flocks that were filmed successfully for at least 4nights. This criterion excluded the five flocks observed in2004e2005 that we progressively reduced in size over thecourse of their captivity (see above), and one flock in2000e2001 that roosted in a position where it could befilmed on just a single night. The remaining seven flocks,containing a mean � SD of 8.1 � 0.90 birds, were filmedsuccessfully for 5.3 � 1.1 (range 4e7) nights.

Positions within roosts were defined as ‘outer’ (posi-tions on either end of the roost) or ‘inner’ (the rest) andthe observed number of nights spent in an outer positionby each individual was determined and compared withthe expected number of nights given an expectation ofrandom distributions in a chi-square test (Roscoe & Byars1971; Zar 1996). To determine whether specific innerpositions within the roost were occupied consistentlyby the same individuals on successive nights, we catego-rized each position relative to the outer and innermostposition. Thus a flock of eight birds had three innerpositions on either side of the midpoint. For flockswith an odd number of birds, the three central positionswere placed in the same category; thus a flock of sevenbirds had just two inner categories (the birds adjacentto the outer position and the three central positions).The observed number of nights spent in a specific inner

ANIMAL BEHAVIOUR, 72, 51038

position by each individual was determined and com-pared with the expected number of nights given an ex-pectation of random distributions in a chi-square test,as above.

For analyses of the relocation data we used generalizedlinear mixed models (GLMMs) with Poisson error distri-butions and a log link function, with Genstat Release 7.1(Genstat 2003). GLMMs allow both fixed and randomfactors as well as covariates to be fitted and the randomfactors control for the use of repeated measurements(Schall 1991). The significance of fixed terms in linearmixed models is calculated with maximum likelihoodsand is assessed by their Wald statistics, which are dis-tributed as c2 for each term fitted last in the model.The probability of relocating to a position changes asthe number of birds in the roost changes, so the ex-pected values for each type of movement were calculatedfor each group size, within each flock, with the mini-mum group size being three and the maximum beingtotal group size. The total number of each type of move-ment, for a given group size, was multiplied by the prob-ability of relocating to a given position. For example, ifthere were five birds in the roost, then the probabilityof relocating to an inner position from an outer positionwas 0.6 (3/5 inner positions/total positions) and 0.4 (2/5)for relocation to an outer position. If a total of 10 move-ments occurred from an outer position when there werefive birds in the roost and birds were relocating ran-domly, then six (10 � 0.6) relocations to inner positionswould be expected and four (10 � 0.4) to outer positions.Taking into account all the potential group sizes withineach flock then the maximum available sample size isthe number of different types of movement (four inthis case) multiplied by the number of different groupsizes within each flock multiplied by the number offlocks of a given group size. For example if all sevenflocks had nine birds and a flock of nine birds canhave groups of 3, 4, 5, 6, 7, 8 and 9 during roost forma-tion (seven different group sizes) and each of the fourtypes of movement can occur at these group sizes, thenthe maximum sample size would be 196 (i.e. 4 � 7 � 7).In this analysis, the observed number of occasions thateach type of movement occurred at each group sizewas used as the response term while the natural loga-rithm of the expected values was used as the offsetterm. The offset term is a variable that appears in thelinear predictor without a parameter and provides, foreach observation, a fixed offset from the estimated con-stant (Genstat 2003). ‘Starting position’ (inner or outer)and ‘direction of movement’ (to an inner position or toan outer position) were included as factors while the‘number of birds in the roost’ and ‘number ofnights the roost was recorded’ were included as co-variates in the starting model. ‘Flock’ was fitted as arandom factor and all the biologically meaningfulsecond-order interactions were included. The best-fittingmodel was constructed by stepwise deletion of the leastsignificant term until only significant terms (P < 0.05)remained in the model, following Crawley (1993).Means are presented � 1 SD throughout, unless otherwisestated.

RESULTS

Roost Formation and Relocations

A total of 1212 relocations were observed in the sevenflocks used for this analysis, an average of 33 per flockper night. There was a significant effect of direction ofrelocation on the observed number of relocations (Table1), with more relocations being directed towards theinner positions than expected when we controlled forthe number of birds in the roost (Fig. 1). Furthermore,there was a significant interaction between direction ofrelocation and starting position (Table 1); birds were2.4 times more likely to move to an inner position ifthey started in an outer position compared to birds start-ing in an inner position (Fig. 1). There was also a signif-icant interaction between the direction of relocation andthe number of birds in the roost (Table 1). Birds thatwere in an inner position were more likely to relocateto another inner position as the number of birds inthe roost increased (Fig. 2a), even when we controlledfor the number of birds in the roost. In contrast, birdsthat were in an outer position were less likely to relocateto an inner position as the number of birds in the roostincreased (Fig. 2b).

In these seven flocks, once relocations ceased and finalroost positions were established, we never observed anysubsequent changes in position during the remainingfilming after dusk or between filming bouts conductedafter dusk and before dawn the following morning. Thus,despite the extensive movements during roost formation,once established, roosts were stable overnight until theroost broke up at dawn.

In all seven flocks in which roost formation wasrecorded over at least 4 nights, all individuals, with theexception of one bird in flock 1, started the roost on atleast one occasion. There was no significant difference inthe average dominance status of those birds that startedthe roost across all nights per flock compared to thosebirds that did not start the roost (Wilcoxon signed-rankstest: Z ¼ �0.338, N ¼ 7, P > 0.7). Similarly, within each

Table 1. Results of generalized linear mixed model (GLMM) analysesinvestigating the relocations of long-tailed tits within a roost duringroost formation while controlling for group size

VariableWald

statistic df P

Flock <0.01 1 0.99Starting position 0.19 1 0.66Number of birds in roost 17.10 1 <0.001Direction of movement 186.25 1 <0.001Starting position*directionof movement

67.65 1 <0.001

Number of birds inroost*direction of movement

25.91 1 <0.001

Expected values were natural logarithm transformed and used as theoffset term. Flock was included as a random term. The number ofnights that a flock was recorded was nonsignificant and droppedfrom the model.

MCGOWAN ET AL.: COMMUNAL ROOSTS OF TITS 1039

flock there was no significant relation between dominancerank and the mean number of relocations per min thata bird made during roost formation (Spearman rank corre-lation: all flocks P > 0.2). Furthermore, there was no sig-nificant relation between the number of relocationsacross successive nights in any of the seven flocks (Spear-man rank correlation: all flocks P > 0.25), nor was therea significant relation between the start time of roostingacross successive nights in all 12 flocks for which wehave start times (Spearman rank correlation: all flocksP > 0.2).

Positions Within Roosts

Figure 3 shows that in four of the seven flocks observedfor at least 4 nights, the same two individuals always occu-pied outer roost positions. In the remaining three flocks,the same individuals usually occupied outer positionsbut either one or two other birds were also observed occu-pying an outer position on at least 1 night. In all sevenflocks the identity of birds occupying outer positionswas significantly nonrandom (chi-square tests: P < 0.05to P < 0.001). In contrast, when observed occupation ofspecific inner positions within the roost was comparedwith an expectation of random positioning, we foundno significant difference for 16/19 positions in sevenflocks (chi-square tests: all NS). In just two flocks (flocks3 and 4; Fig. 3c, d), some of the inner positions were oc-cupied by the same individuals more frequently thanexpected by chance (chi-square tests: P < 0.05). In sum-mary, the analysis of roost positions showed a significantnonrandomness in the identity of birds occupying outerpositions, but not of inner positions.

Direction of movement

Nu

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4

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18ObservedExpected

Insideposition

Insideposition

Outsideposition

Outsideposition

Start inside position Start outside position

445 632

4788

Figure 1. The effect of starting position and direction of movement

on the number of relocations within an average roost (X þ SE), when

we controlled for group size within a flock. Numbers in the blackbars represent the total number of relocations for each of the four

types of movement across all flocks. These relocations were observed

in a total of 38 group sizes from seven flocks for movements from an

inside position and 41 group sizes from seven flocks for movementsfrom an outside position.

Dominance and Roost Position

We determined a dominance hierarchy for each of the13 flocks, using a mean of 174 � 101 dyadic interactionsper flock, or 44 interactions per individual (combiningpecks and interactions over food, see Methods). In sixflocks a clear linear hierarchy was unambiguously deter-mined, but in the remaining seven flocks, the relativeranks of some individuals (mean ¼ 3.43 � 1.13 birds/flock) in adjacent positions in the hierarchy could notbe separated and were given the mean dominance scoreof the adjacent positions.

We recorded the final roost positions of 13 flocks fora mean of 3.92 � 1.85 nights per flock (range 1e7 nights).The two outer positions in the roost were occupied by thetwo most subordinate birds in a flock for 60.2 � 23.8%nights per flock (N ¼ 13) and by birds in the lower halfof the hierarchy for 78.5 � 24.4% nights per flock(N ¼ 13). The mean rank of birds occupying outer posi-tions was significantly lower than that of birds occupyinginner positions (Fig. 4).

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Figure 2. The relation between the number of observed relocations

minus the expected relocations (X þ SE) to an inner position with in-creasing group size when starting from (a) an inner position and (b)

an outer position. See Table 1 for statistics. A positive value on the

Y axis indicates that relocations occurred more often than expected.

Sample sizes for each group size are given above the error bars.

ANIMAL BEHAVIOUR, 72, 51040

(a)

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YRM

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L

Figure 3. (aeg) Roost position consistency for flocks 1, 2, 3, 4, 6, 7 and 8, respectively. Roost position 1 ¼ an outer position in the roost, roostposition 5 ¼ the central position within the roost for flocks 1, 4 and 6 and roost position 4 ¼ the central position within the roost for flocks 2, 3,

7 and 8. The number of nights of roosting varied between flocks with flock 2 recorded for 7 nights, flocks 1 and 3 for 6 nights, flocks 4 and 8 for

5 nights and flocks 6 and 7 for 4 nights. The size of the data points reflects the proportion of nights that a bird occupied a specific position.Note that the largest points for each flock are predominately in the outer position.

DISCUSSION

During roost formation individuals in outer positionsrelocated to inside positions more often than expected,although there was a significant decline in these move-ments as the number of birds in the roost increased.Within flocks, outer positions were occupied consistentlyby the same individuals on successive nights, but specificinner positions appeared to be occupied more or less

randomly. Birds that occupied outer roost positions wereof lower dominance status than those occupying innerpositions. Collectively, our results indicate that long-tailed tits prefer to occupy and actively compete forinner positions within communal roosts, and that theoutcome of this competition is determined to someextent by an individual’s status within a flock’s domi-nance hierarchy.

MCGOWAN ET AL.: COMMUNAL ROOSTS OF TITS 1041

Why should inner positions be preferred? The mostobvious explanation is that birds in inner positions havea lower effective surface area to volume ratio and thereforelower thermoregulatory costs. Riehm (1970) showed thata long-tailed tit can make an energy saving of 37% if itis the central bird of a trio and a 25% energy saving ifpart of a pair when compared to solitary individuals. Theenergetic benefit of communal roosting has also beenshown for a close relative of the long-tailed tit, the bushtit,Psaltriparus minimus, in which a huddling pair hadjust 79% of the overnight metabolic costs of a single birdat 20�C (Chaplin 1982). Similarly, DuPlessis et al. (1994)found that green woodhoopoes, Phoeniculus purpureus,huddling in groups of three or more expended less energyper unit body mass than single birds, so that at a minimumtemperature of 5�C a bird in a roost of four reduced its noc-turnal energy expenditure by 30% or more. Boix-Hinzen &Lovegrove (1998) showed in the same species that, com-pared to solitary birds, the energy consumption of hud-dling birds was 30.5% lower at 10�C and 7.9% lower at20�C. The energetic savings from huddling have alsobeen shown to increase with group size in grey partridges,Perdix perdix (Putaala et al. 1995) and in white-backedmousebirds, Colius colius (McKechnie & Lovegrove 2001).

Aside from gaining an inner position, birds couldmitigate thermoregulatory costs by reducing body tem-perature (Welton et al. 2002). Facultative hypothermic re-sponses to low ambient temperatures (also termednocturnal hypothermia and torpor) have been reportedin 95 species of birds (Lovegrove & Smith 2003; McKechnie& Lovegrove 2003). The majority of passerines investigatedrespond to energetic stress with moderate reductions inbody temperature and several passerine families are knownto show varying degrees of nocturnal hypothermia (Bucher& Worthington 1982), although it is not yet knownwhether it occurs in the Aegithalidae. A full understandingof the benefits of communal roosting in long-tailed tits andthe costs of adopting an outer position will require a more

Roost position

Outer Inner

Don

inan

ce s

core

0

0.1

0.2

0.3

0.4

0.5

0.6

Figure 4. The mean dominance score þSE of birds occupying outer

and inner positions in the communal roosts of 13 long-tailed tit

flocks (paired t test: t12 ¼ 4.21, P ¼ 0.001). The dashed line repre-sents the dominance score if birds occupied positions randomly.

detailed investigation of metabolic rates and temperatureregulation in different social and thermal environments.

Another potential benefit of communal roosting may bea reduction in predation risk. Inside positions may offerbenefits in terms of reduced predation risk because birds inouter positions would be more vulnerable to attack frommammalian predators. Conversely, birds in inside posi-tions may be more at risk from nocturnal avian predatorsas they have fewer directions in which they can escape.The level of predator attacks on roosts and the mortalityrisk associated with certain roost positions is unknownand studies of the escape responses of huddling groupswould be interesting. Nevertheless, the fact that long-tailed tits compete for inner positions suggests that theyoffer significant benefits, whatever they may be.

During roost formation there was a significant interac-tion between the direction of a relocation and roost size:birds on the outside were less likely to relocate from anoutside position as roost size increased. This probablyreflects the degree of settlement in the roost and theenforcement of final positions by other flock membersacting aggressively with repeated pecks towards birds thatattempted to move from outside positions. Despite all therelocations during roost formation, final positions werevery consistent across nights, suggesting that within therelatively stable social groups of long-tailed tits, there maybe little opportunity to avoid being in an outer position ifyou are a subordinate. Therefore, it is perhaps surprisingthat the competition for roost positions occurs on a dailybasis. On the other hand, although the core membership ofnonbreeding flocks of long-tailed tits is fairly stable incomposition, there is some lability with members of otherflocks and immigrants joining a flock, either temporarilyor permanently (Hatchwell et al. 2001a). Therefore, it maypay to compete for inner positions each night in casea more subordinate bird has joined the flock. It would beinteresting to add unfamiliar birds to the captive flocksto investigate further the outcome of this competitiveprocess.

Birds occupying outer positions had lower dominancestatus than birds occupying inner positions. Dominancestatus, or likely correlates of dominance such as age, hasbeen shown to influence roost position in a similar mannerin bronze manikins, Lonchura cucullata (Calf et al. 2002),starlings, Sturnus vulgaris (Summers et al. 1987; Feareet al. 1995), rooks, Corvus frugilegus (Swingland 1977),red-winged blackbirds, Agelaius phoeniceus (Weatherhead& Hoysak 1984) and bald eagles, Haliaeetus leucocephalus(Adams et al. 2000). However, among cooperatively bre-eding species, including Arabian babblers (Zahavi 1990;Bishop & Groves 1991), jungle babblers (Gaston 1977)and varied sitellas (Noske 1985) dominant group membersusually occupy outer positions in communal roosts. Long-tailed tits have an atypical social organization because inmost cooperatively breeding species, social groups are ex-tended families that occupy permanent exclusive territo-ries (Stacey & Koenig 1990), whereas long-tailed titnonbreeding groups are relatively large and usually com-prise parents and offspring from two or more breedinggroups, helpers and failed breeders of both sexes, as wellas a substantial fraction (ca. 30%) of immigrants that are

ANIMAL BEHAVIOUR, 72, 51042

probably unrelated to philopatric group members (Gaston1973; Hatchwell et al. 2001a; McGowan 2002). Thisnon-nuclear family structure of nonbreeding flocks oflong-tailed tits might result in communal roosts that,while conferring benefits to flock members, are more com-petitive than those of typical cooperative species, resultingin dominant flock members gaining access to preferredroost positions. This is not to say that familial identity orkinship will not be important in determining the domi-nance hierarchy within flocks and hence roost position.At present, it is not known what determines an individuallong-tailed tit’s dominance status because it is difficult tomake the appropriate observations in the field. Our obser-vations of captive birds indicate that females tend to besubordinate (McGowan 2002), but we have not yet deter-mined how sex interacts with age, size, condition and priorbreeding status in determining dominance. In the kin-selected cooperative breeding system of long-tailed tits(MacColl & Hatchwell 2004), it is also possible that related-ness within flocks might influence the dominance hierar-chy. It has already been shown that helpers prefer tohelp relatives (Russell & Hatchwell 2001), using vocalkin discrimination cues acquired during development(Hatchwell et al. 2001b; Sharp et al. 2005), and that kin-ship influences the spatial dynamics of winter flocks(Hatchwell et al. 2001a). Therefore, it is possible that kin-ship influences dominance status in long-tailed tits, justas it does in Siberian jays, Perisoreus infaustus (Ekmanet al. 1994). The next step in this investigation will be todetermine the role of relatedness, in addition to the factorslisted above, in determining dominance and the benefitderived from communal roosting.

Acknowledgments

We thank Andrew Russell, Andrew MacColl, Ian Hartley,Tim Birkhead and Dave Hazard for their invaluable inputand numerous landowners, including Doncaster andSheffield City Councils and Yorkshire Water, for allowingus to catch and watch birds on their land. We also thankDeborah Dawson and the NERC Sheffield MolecularGenetics Facility for the sexing of birds, two anonymousreferees whose comments helped to improve the manu-script, and Ceri Ashton, Lynn Fardon, Gemma Harvey,Donna Hemstock, Stacey Hewitt, Melissa Kent, NishaOwen, Claire Sterling, Keely Wilkins, James Wood andLiz Worthington for their assistance with behaviouralobservations. Financial support for this study was pro-vided by NERC, to whom we are most grateful.

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