Avian breeding populations have been shown to be regulated by territorial behaviour, often creating a surplus of non-breeding individuals. However, most evidence is of a male non-breeder surplus, whereas for a surplus to actually buffer a population both non-breeding males and females should be present. Here, we provide descriptive and experimental evidence for the existence of a population buffer consisting of mostly male and potentially also female Pied Flycatchers using nest box areas. First we show that local recruits often do not breed in their first year, with 23% of all recruiting males observed breeding in their first year, and 51% of females. When accounting for mortality in the years prior to observed first breeding, we estimate that only 9% of all first-year males breed locally, and 29% of first-year females. Similar percentages of first-year flycatchers skipping breeding have been observed in other study populations. We show that in the year of new establishment of our nest box plots, most known-aged flycatchers were first-year birds (77%), whereas after establishment, recruiting immigrants from the same source population were mostly older (28% first-year birds). An experimental removal of paired flycatchers from one study plot in two years (19 and 58 individuals removed) resulted in complete replacement by males and females. Male but not female replacements were younger than removed individuals. These results imply that a non-breeding surplus is present in Pied Flycatcher populations. The average later age at first-breeding in males compared to females, suggests that this non-breeding surplus is strongly male-biased. Skipping breeding in the first year(s) is not just caused by shortage of suitable nesting sites, as we observed on average 12% of males defending a nest box without pairing up with a female. Using stable isotopes ratios, we show that non-breeding first-year individuals do not stay at their African wintering grounds. Competition for nest sites is one cause for refraining from breeding, as shown by our experiments, but cannot be the sole cause, as many nest boxes remain unused in a season, and up to 20% of territorial males defend a nest box without pairing up with a female. We hypothesize that many young flycatchers arrive too late for breeding and are therefore not seen in their first year. Indeed first-year Pied Flycatchers that do breed/defend a nest box arrive on average later at the breeding grounds, and we argue that the non-observed group arrives even later. The causes of their later arrival could be the need for learning, lower quality wintering sites resulting in later departure, and/or a trade-off between low breeding success and the costs of early arrival. These could be general factors in long-distance migrants, and this pleads for a better understanding of how migration develops during ontogeny.
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Vol. 105 • No. 1