The timing of birds' breeding seasons: the Perrins hypothesis revisited especially for migrants

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