In order to increase body mass during the 4–6 weeks before their departure from the Banc d'Arguin in spring, some waders, such as Dunlin, Knot, Bar-tailed Godwit, increased the total time they spent feeding. They fed more at night and fed throughout neap tides and at high temperatures, circumstances in which feeding activity was depressed in winter. In other species, however, such as the Little Stint, feeding time did not increase during the premigration period. In winter, feeding time and body mass in the 14 wader species studied were negatively associated, but this trend disappeared during the premigration period. Nocturnal feeding was particularly important in the smaller waders, but the larger waders also began to feed at night later in the season. It is suggested that only when pressed for time, waders will feed at times when the yield from feeding is low.

We examined to what extent insectivorous and granivorous passerines can obtain the calcium for shell formation and growth of the skeleton from their normal food, and to what extent they depend on additional calcium-rich material, such as snail shells and calcareous grit. We determined the calcium content of forest arthropods and seeds on Ca-rich and Ca-poor soils and the calcium content of females, eggs and nestlings of the Great Tit, Parus major, and calculated the calcium budget for females and nestlings. We found that female Great Tits do not store calcium in their skeleton before egg-laying. Thus, all calcium needed for egg-laying must be collected during the laying period. Even on Ca-rich soils the calcium intake from arthropods or seeds covered only 5–10% of the Ca requirements. It is demonstrated that these results also apply to other passerines, with the exception of a few species that include woodlice (Isopoda) and millipedes (Diplopoda) in their diet. These arthropods contain 10–13% (dry matter) calcium; a hundred times more than other arthropods. However, most passerines need Ca-rich material, in addition to their normal food, to meet their calcium demand.

A single population of Bar-tailed Godwits limosa lapponica was studied before departure from their wintering grounds on the Banc d'Arguin in Mauritania, and at a spring staging site in the Dutch Wadden Sea. On the Banc d'Arguin the godwits started gaining body mass in the course of March at rates of about 2.8 g d^-1 in ♂ ♂ and 3.2 g d^-1 in ♀ ♀, before leaving north between 25 and 27 April. In the Wadden Sea, body mass increased linearly over the entire staging period (29 April–31 May) at 5.6 g d^-1 in ♂ ♂ and 7.5 g d^-1 in ♀ ♀. Half of the mass increase was due to the deposition of fat, the other half to increases in fat-free tissue (mainly muscle protein). If the godwits flew at the heights with most tail wind assistance by varying their flight altitude up to 5.5 km, they gained an average wind assistance of 18 km h^-1, which would add considerably to their estimated air speed of 57 km h^-1. At an average ground speed of 57 18 = 75 km h^-1 the godwits would cover the required 4300 km in 57.3 h. During this flight, ♂ ♂ and ♀ ♀ were estimated to lose 136 g and 178 g respectively. Since half of these mass losses consisted of fat, they represent energetic equivalents of 3163 kJ and 3857 kJ respectively, leading to estimated flight costs of 55 kJ h^-1 for ♂ ♂ and 67 kJ h^-1 for ♀ ♀. The energy reserves stored on the Banc d'Arguin would not enable the Bar-tailed Godwits to cover the distance between W. Africa and the Wadden Sea in one flight without making good use of favourable high-altitude winds.

Wader species wintering on the Banc d'Arguin increased their body mass by about 40% during the 4–6 weeks before their departure in spring. This estimate is based on 1) the empirical fact that most waders which had finished or suspended their body moult into summer plumage were heavy and thus ready to take off, and 2) the assumption that only the heaviest birds in the population left, which allowed the mass of disappearing waders to be estimated from counts and the frequency distribution of body masses in samples of captured birds. The mass gain on the Banc d'Arguin is just over 1% per day, when expressed as a proportion of winter mass. A review of studies on waders preparing for migration shows that 1) the total migratory reserve adds 20–80% to winter mass, 2) the rate of mass gain is 0.1–4% per day and 3. the period of mass increase lasts four weeks on average, but longer if waders prepare for spring migration on the wintering areas. We suggest that all wader species leaving the B anc d'Arguin at the end of April and the beginning of May are able to reach SE. and NW. Europe without refuelling. This seems only possible if current equations to predict flight range systematically under-estimate this range, even when the energetic benefits of favourable winds at high altitude are taken into account.

Analyses of long-term data indicate that human-caused climatic changes are affecting bird phenology in directions consistent with theoretical predictions. Here, we report on recent trends in the timing of spring arrival and egg laying found within a western European Pied Flycatcher Ficedula hypoleuca population. Mean egg laying date has advanced over the past 20 years in this population. The advancement in egg laying date was stronger than the advancement of spring arrival, suggesting that Pied Flycatchers are changing these stages of their annual cycle at different rates. It could be shown that selection for earlier breeding had increased. Hence, the observed adjustment in laying date did not match the advancement of spring. Our findings raise general questions about the adaptability of migratory birds to rapid environmental changes. Adaptive advancement of reproduction in response to increasing spring temperatures and to the concomitant advancement of food supply could be held back, because annual breeding and migration cycles are controlled primarily by endogenous rhythms and photoperiodic cues which do not relate to temperature. Migrants may have several options for arriving earlier on the breeding grounds, including an increase in migration speed, earlier departure from the wintering area or a shortening of migration distance. Changes in migratory behaviour could be accomplished either by phenotypic plasticity or by selection on different genotypes. Although descriptive field data provide compelling evidence for changes in, and possible constraints on, the timing of breeding and migration, their explanatory power in predicting the limits of adaptation remains restricted. We review recent experimental approaches, which explicitly test the relative roles of genetic versus environmental factors in the adaptation of life-cycle timing to global environmental changes.