The importance of food availability at stopover sites is further shown by the following types of findings, drawn from a range of different studies:
1. Birds were more likely to stay at sites where food was plentiful, and move on rapidly from sites where food was scarce (Bibby & Green 1981, Spina & Bezzi 1990, Ottich & Dierschke 2003).
2. When birds stayed at a site, their refuelling rates (as judged by weight gains) were often correlated with spatial and temporal variation in food supplies
Figure 27.3 Relationship between food supply (aphid density) at a stopover site: (left) tendency to stay; and (right) rate of migratory weight gain in Sedge Warblers Acrocephalus schoenobaenus. From Bibby & Green (1981).
(Figure 27.3; Bibby et al. 1976, Bibby & Green 1981, Cherry 1982, Piersma 1987, Prop & Deerenberg 1991). In one study involving Red Knots Calidris canutus, fuelling rates increased with latitude, in parallel with the greater 'harvestable biomass' available at higher latitudes in spring (Piersma et al. 2005).
3. Mean stopover durations were inversely correlated with food supplies, as birds put on weight more slowly where food was scarce than where food was abundant (Figure 27.3; Piersma 1987, Russell et al. 1992).
4. Birds that arrived at particular sites with low body reserves stayed longer than those that arrived at the same sites with larger reserves (Dolnik & Blyumental 1967, Cherry 1982, Bairlein 1985a, Pettersson & Hasselquist 1985, Biebach 1985, Biebach et al. 1986, Moore & Kerlinger 1987, Dunn et al. 1988, Serie & Sharp 1989, Loria & Moore 1990, Ellegren 1991, van Eerden et al. 1991, Kuenzi et al. 1991, Morris et al. 1994, Yong & Moore 1997).
5. Some populations could not accumulate body fuel for spring migration until food availability increased in some way (for Whimbrel Numenius phaeopus see Zwarts 1990).
6. Fuel deposition rates or body weights declined in cold or wet weather expected to reduce feeding rates (Ash 1969, Ormerod 1989, Schaub & Jenni 2001b).
7. The provision of supplementary food to migrants in the field led to increased rates of fuel deposition compared to rates in unfed birds (for Bluethroat Luscinia svecica see Lindstrom & Alerstam 1992, for Greater Whitethroat Sylvia communis see Fransson 1998, for Robin Erithacus rubecula see Danhardt & Lindstrom 2001, for Garden Warbler Sylvia borin see Bauchinger 2002).
8. Rates of fuel deposition most commonly reported (1-3% of lean mass per day) were much lower than the maximum rates (>7% of lean mass per day) recorded from individuals in the same population. This finding implied that most individuals fed at rates too low to fuel at the maximum possible rate (Lindstrom 2003).
Whether all this matters depends on how fuelling rates affect migration timing and subsequent performance. Some birds are apparently unable to accumulate sufficient body reserves in time to migrate at an appropriate date. They may then remain in wintering areas (e.g. Dijk et al. 1990), or at stopover sites (e.g. Spendelow 1985), or in breeding areas where they may die as winter approaches (e.g. Kolunen & Peiponen 1991). Such findings emphasise the extreme consequences of inadequate food supply to successful migration.
In general, migrants seem to stay at a site for some days if they have a good chance of replenishing their depleted body reserves there, but move on if conditions are unfavourable for replenishment, presumably in search of more suitable habitat elsewhere (Rappole & Warner 1976, Biebach 1985, Kuenzi et al. 1991). This inference is supported by feeding experiments in wild birds (for Northern Wheatear Oenanthe oenanthe see Dierschke et al. 2003), and by laboratory experiments with Spotted Flycatchers Muscicapa striata and Garden Warblers Sylvia borin, in which migratory restlessness was greatly reduced during periods of weight gain (Biebach 1985, Gwinner et al. 1985b). More specifically, birds with high fuel reserves, or with low fuel reserves and insufficient food, showed migratory restlessness, whereas birds with low food reserves and abundant food did not (Chapter 12). In natural conditions, stopover duration depends not only on food levels, but also on weather (as birds wait for conditions favourable for migration), predation and disturbance, the arrival-departure schedules of other individuals, and the location of the stopover site with respect to a sea or other barrier. These other factors often influence the relationships between arrival weight and stopover duration, or rate of weight gain and stopover duration, and from many stopover sites departing birds can show a wide range of fuel loads. In landscapes with many potential stopping places, birds might pause and feed for a fairly constant period each day, and then fly whatever distance their reserves allow before they stop again, the durations of flights rather than the stopovers being the main variable in the journey (Schaub & Jenni 2001a).
Rates of feeding and weight gain are often greatest in lean birds, and then slow as body mass increases towards the departure value, a trend found in captive birds as well as wild ones (Rappole & Warner 1976, Kuenzi et al. 1991, Yong & Moore 1993, Moore 1994). This is partly because lean birds take greater risks than fatter ones: they may spend less time scanning for predators, feed for longer each day, more actively, and in more dangerous places (Metcalfe & Furness 1984, Loria & Moore 1990, Moore & Aborn 2000, Ydenberg et al. 2002). But the maximum rate at which food can be obtained and processed is still limited by the available feeding time (set by daylength, tidal rhythms or prey activity), and the type of food, and by bird features, such as crop capacity and digestive throughput (Chapter 5). The bird must also do things other than forage, all of which limit the rate of weight gain.
The assumption that long stopovers and associated low rates of weight gain slow the overall progress of migration is supported by observational data. For example, autumn migratory movement by six passerines between traps 12-50 km apart was 1.5-3.0 times faster in fat birds than in lean ones (Blyumental, in King 1972). When weight gain is slow at early staging sites, delaying departure, it could reduce fattening rates at later sites if food supplies have already been depleted there (the domino effect of Piersma 1987). On this basis, and in the absence of weather-induced delays, differences between individuals in start dates could be magnified during the course of a journey. Some studies at particular stopover sites have indicated a progressive lowering of body mass and fat levels among successive samples of birds caught during the course of the spring migration. Individuals that passed through earlier in the season were heaviest, and those that passed through latest were lightest: as for example, in three out of four thrush species caught in spring at a stopover site in coastal Louisiana (Yong & Moore 1997), and among Steppe Buzzards Buteo b. vulpinus and Levant Sparrowhawks Accipiter brevipes caught in spring at a stopover site in Israel (Yosef et al. 2002, 2003).
The same has been found for birds arriving in their breeding areas, with the earliest arrivals being the heaviest, as in Barn Swallows Hirundo rustica (Moller 1994), Garden Warblers Sylvia communis (Widmer & Biebach 2001), American Redstarts (Smith & Moore 2005), and Common Terns Sterna hirundo (Dittman & Becker 2003). Such trends could result from the effects of competition relegating lighter birds to the later part of the spring migration period. However, not all species would be expected to show decline in the mean weights of samples taken through the spring migration season: at some stopover sites food supplies increase through spring (through growth or reproduction of prey items), and bird densities decline, as passage comes to an end. Both these changes could provide enhanced feeding conditions for the later migrants (for greater fattening rates in the latest of two godwit populations to pass through the Netherlands in spring, see Drent & Piersma 1990).
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