Fat storage in male Willow Warblers in spring: Do residents arrive lean or fat?

Auk, The, Jul 1998 by Fransson, Thord, Jakobsson, Sven

THORD FRANSSON1,2,3 AND SVEN JAKOBSSON'

Early arrival on the breeding grounds confers important advantages for migratory birds during competition for resources and breeding opportunities (von Haartman 1968, Klomp 1970, Moller 1994, Wiggins et al. 1994). Arriving in good condition also can be important for arctic-breeding birds (Davidson and Evans 1988) because clutch size and timing of breeding may be constrained by parental condition (Rowe et al. 1994). Long-distance passerine migrants arrive at near-arctic breeding sites with larger fat stores than do medium- and short-distance migrants (Sandberg 1996). Recently, Sandberg and Moore (1996) advanced a number of hypothesized benefits to passerine migrants that arrive with extra energy on their breeding grounds, including improved breeding performance, relaxed handling-time constraints, and insurance against adverse conditions. Fat accumulation is not without cost, however. Migrants accumulate fat at stopover sites along migration routes, and time spent foraging at these sites strongly affects overall migration speed (Alerstam and Lindstrom 1990). Other potential costs that should select against excess fat accumulation include: (1) diminishing return of increased fat depots on potential flight distance (e.g. Lindstrom and Alerstam 1992) and (2) mass-dependent predation risk due to impaired flight performance and higher exposure to predators during intense foraging (e.g. Hedenstrom 1992, Houston and McNamara 1993, Witter and Cuthill 1993, Metcalfe and Ure 1995, Kullberg et al. 1996).

Despite the importance of fat, detailed information about fat content in migrant passerines when they arrive at breeding sites and during the transition from migration to breeding is rare (but see Ojanen 1984, Sandberg 1996). In this study, we investigated the amount of visible fat stores in male Willow Warblers (Phylloscopus trochilus) on the day they arrived at a breeding site in southern Sweden. The Willow Warbler is a long-distance migrant; birds that breed in northern Europe spend the winter in tropical Africa, some of them as far south as South Africa (Hedenstrom and Pettersson 1987). We compared visible fat stores of locally breeding birds with those of birds presumably heading farther north. Furthermore, we measured intra-individual changes in visible fat stores and body mass shortly after arrival to elucidate the dynamics of fat stores during the transition from migration to breeding.

Methods.-This study was conducted during 1992 and 1993 at a 30-ha site located about 1 km from the Baltic shore on the southernmost tip of Gotland Island (56o56'N, 18o11'E; Fig. 1). The site is almost surrounded by the Baltic Sea, and the open-sea distance to Poland in the south is about 240 km. A standardized banding program for migrants has been performed at the site since 1977 by the Sundre Bird Ringing Group (Fransson et al. 1995). Willow Warblers are common spring migrants at this site and make up about 25% of all birds trapped (Fransson et al. 1995).

Trapping was conducted daily from sunrise to noon from about 25 April to 8 June during both 1992 and 1993 using 35 mist nets. Mist nets were visited every 30 min; trapped birds were taken to a field laboratory for measurement and banding. Data collected included maximum wing length (1 mm; Svensson 1992), fat score, and body mass (0.1 g). Scoring of fat in the furcular cavity and on the belly followed Pettersson and Hasselquist (1985) and included seven stages from 0 (no visible fat) to 6 (belly covered with a thick layer of fat). Cross-checks of fat scores were made regularly among banders to minimize discrepancies (cf. Krementz and Pendleton 1990). Carcass analysis has shown that this visual scoring system, in conjunction with body mass, gives reliable estimates of fat deposition in Willow Warblers (Lundgren et al. 1995).

Most of the Willow Warblers that we trapped probably were on temporary stopover during migration to breeding areas farther north. This is suggested by subsequent recoveries of birds from as far north as 6930'N in Norway and by short-time recoveries from the Swedish mainland and southern Finland. One banded bird was recaptured a day later about 300 km to the north on an island in the Stockholm archipelago. We assumed that birds trapped during normal banding activities that showed no indication of belonging to the local breeding population were migrating birds. Males and females cannot be separated based on plumage characters, but they differ in size. We considered individuals with wing length >67 mm to be males (Svensson 1992). In spring, it is not possible to age Willow Warblers based on plumage characters (Svensson 1992).

Male Willow Warblers show a high degree of faithfulness to previous breeding sites (Jakobsson 1988), and about 50% of the males included in this study were banded at the study site during previous years. Males start to defend a territory very soon after they arrive on the breeding grounds, often within hours (Jakobsson 1988). Thus, it was crucial to catch birds as soon as possible after arrival. We therefore kept the study area under close surveillance, especially between 0400 and 1300 when most newly arrived males establish territories. Territorial males display by singing a species-specific song from the top of a small tree and are easy to spot when singing. When a new male was found, we attempted to trap him using song playback, a stuffed Willow Warbler, and a mist net. Captured males were immediately moved to the banding site for examination. Each territorial male was banded with a unique combination of colored leg bands, in addition to a metal band. We tried to retrap each color-banded individual within five days of arrival to reexamine its body mass and fat score. We successfully retrapped 15 birds for fat score examination and 14 for body mass (body mass data were missing for one of the retrapped birds).