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Perrins (1970) galvanized thinking on the timing of birds' breeding seasons by pointing out that most individuals laid too late for the offspring to profit fully from the seasonal peak of food abundance, and suggested that the proximate cause was a shortage of food for the female when forming the eggs. This idea (the food constraint hypothesis) stimulated field experiments with supplementary food, and also catalyzed analysis of the seasonal trends of fitness for both parents and offspring. Most experiments resulted in minor advances in laying date hinting that other factors are also important, and the fitness comparisons underlined the view that laying date must be viewed as a trade-off between opposing seasonal trends affecting parents (later is better) and offspring (the earlier the better). Experimental advances of laying have revealed fitness costs (lowered survival) for the parent(s) counteracting the higher output of young. Laying date is thus best considered as an individually based compromise (individual optimisation hypothesis), with exact timing subject to local environmental control. Recent analyses of climate change (warming trend in spring) confirm responses in many but not all bird species, and where detailed data are available the advance documented in laying date seems often to fall short of the shift in the food peak. Migrants in particular may be unable to speed up their spring travels, in some species on account of time conflicts with moult. The larger bodied migrants breeding in the Arctic face severe time constraints as they must lay relatively early on the snowy tundra. Postulated to rely on endogenous stores to lay eggs and incubate them, finer resolution of their travel schedules by means of satellite telemetry combined with investigation of the isotopic signature of body tissues, eggs, and food sources show a mixed strategy to prevail: transport what you can ('capital') and supplement this on the breeding grounds ('income'). Stores acquired at stopover sites are of crucial importance for both survival and breeding and any'mismatch' caused by uncoupling of rates of climate change along the migratory pathway may have profound effects on the population level.

We present data on the natural variation in daily energy expenditure (DEE, measured by the doubly labelled water technique) of individual female Great Tits Parus major tending their brood near the northern border of the species' distribution. We expected that longer foraging days would result in a higher energy expenditure. Female DEE, visitation rate and the duration of the active period were measured at the nestling age of 12 days. There was a positive relationship between the duration of activity period and brood size. Female DEF was positively associated with the duration of the activity period and the number of young in the nest. Energy expenditure was not related to temperature or feeding rates. Despite the longer foraging day and lower ambient temperatures, female DEE did not differ from the values obtained in central Europe. This result suggests that there are differences in foraging cost and/or mode with latitude.

We studied temporal variation in reproductive performance of Marsh Harriers Circus aeruginosus in two land reclamations in The Netherlands, i.e. South Flevoland and the Lauwersmeerpolder, embanked in 1968 and 1969 respectively. The number of breeding pairs in Flevoland rapidly increased to a maximum of 350 pairs (± 1 pair km-2) in 1977, followed by a sharp decline in the 1980s due to large-scale cultivation. The same trend was observed in the Lauwersmeer, although colonization was retarded and peak densities were reached later. In both study areas mean clutch size as well as the number of fledglings per nest decreased in the course of the twenty years of study. Two factors were responsible for this decline in reproductive output: (1) decrease of food abundance in the course of the years, and (2) an increase of nest predation, mainly by the Red Fox Vulpes vulpes. Superimposed on these long-term changes, annual fluctuations in density of the Common Vole Microtus arvalis had a considerable effect on the number of breeding pairs, as well as the fledgling production. Mean annual clutch size was associated positively, and laying date negatively, with the average annual temperature during the pre-laying phase. Our results indicate that the early stages of succession, during the first decade after reclamation, are characterized by high prey abundance (i.e. vole 'plagues') and low densities of ground predators, and offer favorable breeding conditions for Marsh Harriers. The simultaneous negative effects in recent years of less breeding habitat, decreased prey abundance and increased predation on reproductive output, exert pressure on these populations. The intense nest predation (50% of all nests), as recorded in the Lauwersmeer since 1990, may eventually cause local extinction in the absence of, relatively safe, inundated breeding habitat.