Migration is a period of exceptional energy demand (e.g. Berthold 1975, Blem 1980, Alerstam 1990). To meet this high demand, birds deposit substantial fat stores, which may reach 50% of total body mass among long-distance, intercontinental migrants (Blem 1980, 1990). The stores are depleted during migration, and birds are capable of replenishing fat stores at rates between 1-8% of their body mass per day (e.g. Dolnik and Blyumental 1967, Bairlein 1985, Biebach et al. 1986, Moore and Kerlinger 1987, Alerstam and Lindstrom 1990). If migrants are able to satisfy energetic demands and meet en route contingencies in a timely manner (cf. Alerstam and Lindstr6m 1990), they may experience a successful migration. Whereas adequate fat stores are critical for surviving during migration, birds that arrive on their breeding grounds with fat stores may be better able to cope with energetic demands that arise during the onset of the breeding season and to 'offset' time constraints associated with the brief breeding season at, especially, high latitudes (e.g. Slagsvold 1976, Jirvinen 1983, Ojanen 1984, Sandberg 1996). It is well established that the pre-breeding nutritional condition of parents affects reproductive success (Drent and Daan 1980, Price et al. 1988, Rowe et al. 1994). Such an effect is most dramatic among some waterfowl that breed at high latitudes and rely almost exclusively on pre-stored fat stores to produce a complete clutch of eggs (reviewed by Alisauskas and Ankney 1992). Although it is unlikely that a small passerine would arrive at the breeding grounds with energy stores sufficient to produce a complete clutch of eggs (cf. Perrins 1970), experiments in which food has been supplemented prior to egg laying provide compelling evidence that parental condition is a determinant of clutch size and/or laying date in passerines (see reviews by Davies and Lundberg 1985, Arcese and Smith 1988, Daan et al. 1988). Hence, fat stores acquired during migration should improve parental condition and influence reproductive success among passerine migrants. Despite the intuitive appeal of the above argument, empirical support for it is lacking. King et al. (1963) speculated that substantial fat accumulation for spring migration in the white-crowned sparrow Zonothricia leucophrys gambelli represents an adaptation for a faster migration and also for confronting inclement weather at the arrival on the breeding grounds. The arrival of female pied flycatchers Ficedula hypoleuca on the breeding grounds in northern Finland, with fat stores estimated at 14% of their body mass, prompted Ojanen (1984) to suggest that: Arriving with plenty of reserves thus assists with energy requirements for competition over nest-holes and for a rapid onset of breeding activities. More recently, Sandberg (1996) reported that pied flycatchers, bluethroats Luscinia svecica, and willow warblers Phylloscopus trochilus, three intercontinental migrants, arrive on their breeding grounds in northern Sweden with substantial fat loads, depending on species and sex, individuals on average carried energy stores sufficient to allow non-stop flights for an extra 240 to 500 km, if they would have been inclined to continue migrating. Moreover, when females and males arrived on the breeding grounds simultaneously, females on average carried significantly more fat than did males. Gudmundsson et al. (1991) hypothesized that overloading (i.e. to put on fat loads in excess of what is needed for the impending migration flight) at the last stopover site prior to arrival on the breeding grounds may confer an advantage during the breeding cycle. They predicted that it will be optimal to acquire extra energy stores at the penultimate stopover site as long as the fat deposition rate, devaluated by the concomitant flight costs, is greater than the expected rate of energy gain at the breeding destination.