ANIMAL BEHAVIOUR, 2007, 74, 471e479 doi:10.1016/j.anbehav.2006.11.030 Extreme fattening by sedge warblers, Acrocephalus schoenobaenus, is not triggered by food availability alone NICHOLAS J. BAYLY*† *The Wetland Trust, U.K. ySchool of Life Sciences, University of Sussex, U.K. (Received 23 May 2006; initial acceptance 10 August 2006; final acceptance 6 November 2006; published online 22 August 2007; MS. number: 8976) The strategies adopted by birds during migration are expected to be strongly influenced by resource avail- ability and the efficiency with which these resources are used. The migratory strategy of the sedge warbler in northwest Europe has been linked to superabundances of plum-reed aphids Hyalopterus pruni that enable birds to accumulate extensive fat reserves and make nonstop flights to sub-Saharan Africa. Food availability was therefore expected to be the main determinant of whether sedge warblers accumulated ex- tensive reserves or not. In this study, birds were provided with an unlimited supplementary food source, but only 10 out of 24 birds accumulated large reserves. In view of this, temporal and geographical cues reflecting seasonal variations in aphid abundance are considered, in addition to food availability, to deter- mine whether extensive fuelling occurs. Optimality models of resource use by migrants predict two modes of fuelling behaviour, as seen here, if strategies have evolved to divide a journey into an optimal number of stages or if the perception of resources at future sites varies between birds or in time/space. If sedge warbler perception of future resources reflects the temporal and geographical variation in aphid abundance in northwest Europe, then the distribution of fuelling behaviours observed here is expected. Despite some birds accumulating large fuel reserves, they did not show a high degree of sensitivity to wind conditions when initiating migratory flights. Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Keywords: Acrocephalus schoenobaenus; migration; optimal fuelling behaviour; sedge warbler; stopover behaviour Migration is an energetically demanding process and While sedge warblers stopover for long periods and consequently migratory strategies are expected to be accumulate enormous fuel reserves (80e120% of their influenced by the ability to utilize various food supplies lean body mass) in some reedbeds in northwest Europe, and the distribution of those supplies both in space and the proportion doing so can be relatively small. Indeed, time (Alerstam 1990). The contrast between two migratory when aphid abundance is low, up to 85% of birds may birds, the reed warbler Acrocephalus scirpaceus and sedge remain at a site for 2 days or less, accumulating only small warbler illustrates this principle. Sedge warblers perform reserves (Bibby et al. 1976). This suggests that the spatial a remarkable long-haul migration from northwest Europe distribution of resources, i.e. reed aphids, influences fuel- to sub-Saharan Africa by feeding extensively on super- ling behaviour and that short stays and consequent rapid abundant food supplies, specifically reed aphids (e.g. Hya- movements between sites are a searching behaviour adop- lopterus pruni), while the reed warbler must stop and ted by birds looking for a resource rich reedbed (Bensch refuel two or three times to complete the same journey. & Nielsen 1999; Wernham et al. 2002). However, even This difference arises because reed warblers are less effi- where aphid infestations occur not all birds remain to cient at preying on reed aphids despite considerable eco- fuel extensively (Bibby & Green 1981), suggesting that logical and morphological similarity to the sedge warbler other factors influence which behaviour is adopted. (Bibby & Green 1981). Optimality models have been used to examine how migrants should respond to resource availability under various selection pressures during stopovers/fuelling Correspondence and present address: N. J. Bayly, 17 Walton Street, periods (cf. Alerstam & Hedenstro¨m 1998). The stopover Oxford, Oxfordshire OX1 2HQ, U.K. (email: [email protected]). behaviour of several long-distance migrants is consistent 471 0003e3472/07/$30.00/0 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. 472 ANIMAL BEHAVIOUR, 74,3 with time-minimization (Bayly 2006) and given the res- tricted temporal availability of reed aphids, whose abun- dance peaks in early August in the U.K. and is in rapid decline by late August (Bibby & Green 1981), the same is likely to be true of sedge warblers. While time-minimiz- ing models predict that fuel loads will increase with re- source availability/fuel deposition rate (FDR), stopover durations are expected to decrease, not increase as is true of sedge warblers (Alerstam & Lindstro¨m 1990). However, varying the assumptions of the model can change the outcome: for example, if migratory distance is considered finite then a stepped function arises because the journey is best divided into equal steps, rendering the same fuel load optimal over a range of FDRs (Weber & Houston 1997). In this case, fuelling duration increases in steps with FDR. Al- ternatively, fuel loads and fuelling durations are predicted to increase if decreasing resources are expected at future sites (Weber & Houston 1997). In addition to resources, Figure 1. Sedge warbler using remote weighing set-up. The perch is attached to the pan of the balance, which is reading 15.99 g. The weather is expected to influence stopovers (Weber et al. Perspex box around the bowl of mealworms is to encourage birds 1998) and given the extremely large reserves laid down to access the food from the perch. by sedge warblers, they are expected to be highly sensitive to wind conditions and to delay departure until condi- tions are favourable (Liechti & Bruderer 1998). To examine how resources influence fuelling behaviour, previous studies (e.g. Fransson 1998; Bayly 2006)andisex- free-living sedge warblers were attracted to an unlimited pected to give comparable results. In 2001 and 2002 birds supply of food at a reedbed in the U.K. and their body mass discovered the feeders by chance, while in 2003 and 2005, was recorded daily. The departure mass of birds utilizing the sedge warbler song was played close to unattended feeders food source was expected to reflect extensive fuelling if during the morning to increase the rate of discovery. abundant resources were the cue for this strategy. The departure loads and FDRs of experimental birds were compared to time-minimizing models to determine whether birds were responding to resources in an optimal Body Mass Recordings way. In conjunction with weather data, the sensitivity of departure decisions to wind conditions was also tested. Body mass, which reflects the mass of stored fuel (Redfern et al. 2004), was recorded using an electronic balance inserted into the feeder box (Ohaus Scout II, Ohaus Corpo- ration, Pine Brook, NJ 07058, U.S.A.; Fig. 1) and a video METHODS camera (Sony CCD-TR748E Hi8, Sony Corporation) accord- Study Site ing to the methods described in Bayly (2006). Remote weighing avoids the potentially confounding effects on The Pannel Valley Reserve (PVR) is located in southeast body mass and foraging behaviour associated with catching England (50540N, 0410E) and consisted of a 23 ha mosaic of and handling birds (e.g. Gosler 2001). Phragmites reedbed, sallow Salix scrub and open water, Recordings were made twice daily between 0700 and 1.7 km from the English Channel. Daily mist-netting at 1000 hours and 1700 and 2030 hours, with filming PVR between July and October has resulted in a mean sessions lasting ca. 45e90 min. Where possible, evening annual capture of 7965 sedge warblers (mean of 1995e2004). recordings were made in the hour preceding sunset. Videotapes were analysed on a colour television and the identity, time of visit (to the nearest minute), body mass Food Supplementation Experiments (to the nearest 0.01 g if possible, otherwise to 0.1 g) and the number of mealworms consumed on each visit were Sedge warblers were attracted to supplemental food during recorded. Some individuals could be identified either 1Auguste16 September 2001, 12 Julye30 September 2002, because they had a unique combination of colour rings 6Auguste2 September 2003 and 26 Julye12 September or because of a combination of the following features: 2005. Food was presented in a feeder consisting of a wooden the presence/absence of a metal ring, small plumage box (25 Â 40 Â 15 cm) attached to a stake (0.7e1.5 m high), differences, and by marked differences (>1.5 g) in body driven into the ground within the reedbed (Fig. 1). A plastic mass (e.g. of 24 birds: colour rings: two juveniles; metal bowl (10 Â 4cm),kepttoppedupwithatleast50meal- ring: 10 juveniles, one adult; no ring: 10 juveniles, one worms, Tenebrio molitor, from dawn till dusk, was inserted adult). Individuals that could not be confidently identified into a purpose-made hole on the box top. Feeders some- were excluded from the analyses. Licences for the trap- times had to be emptied overnight to avoid consumption ping, ringing and colour ringing of birds were obtained by small rodents. This set up is similar to that used in from the British Trust for Ornithology. BAYLY: SEDGE WARBLER MIGRATION STRATEGIES 473 Fuel Deposition Rates, Departure Loads which it was the number of days since the start of fuelling and Intake Rates (see above). On arrival at a feeder,