Woodpeckers differ from most birds in several ways. They have powerful bills and excavate nesting cavities, and compared with most other birds of equivalent size they have relatively small clutches, relatively small eggs, and short incubation periods whereas the chicks remain in the nest for a relatively long time. Based upon the literature, I have compared the reproductive patterns of seven «true» Nordic woodpeckers (Picinae) with that of eight secondary hole-nesting species (particularly five passerines). The mean incubation period of the woodpeckers (12.4 ± 2.2 days) does not differ significantly from that of the secondary hole-nesters (17.6 ± 6.3 days; t-test, p=0.057). However, the incubation period of Picus canus and P. viridis (15.5 days) differs from the mean of the remaining woodpeckers (11.1 days; p<0.001). If the two Picus species are omitted from the analysis, the mean incubation period of the woodpeckers is shorter than that of the secondary hole-nesters (2.3 days; p=0.045). The secondary hole-nesting woodpecker species Jynx torquilla also has a shorter incubation period (12 days) than any of the other secondary hole-nesters. The mean nestling period of the woodpeckers (24.3 ± 2.9 days) does not differ from that of the secondary hole-nesters (22.7 ± 6.9 days; p=0.57). However, the woodpeckers have a longer nestling period (p=0.013) than the five passerines (mean 18.5 days). The ratio nestling period:incubation period differs, being 2.01 in woodpeckers and 1.31 in secondary hole-nesters (p<0.001). Woodpecker eggs are small and thin-shelled with a mean weight, related to the body mass of females, of 6.2 ± 2.0, and they are significantly lighter than those of the eight secondary hole-nesters (9.5 ± 2.0; p=0.007). The mean eggshell thickness in relation to egg volume is less in woodpeckers (1.3 ± 0.5) than in the secondary hole-nesting passerines (2.4 ± 0.7; p=0.01). The mean clutch size of the woodpeckers (5.5 ± 1.5 eggs) does not differ from that of the eight secondary hole-nesters (5.9 ± 2.2 eggs; p=0.15). However, if the two Picus species are omitted because of their tendency to reuse nest holes and their clutch size (mean 7.3 ± 1.1) that is larger than that of the other woodpeckers (mean 4.9 ± 0.9; p=0.03), the woodpeckers have a smaller clutch size than the secondary hole-nesters (p=0.04). The mean clutch weight related to the female body mass is lower in woodpeckers (0.33 ± 0.11) than in the secondary hole-nesters (0.57 ± 0.24, p=0.03). Hypotheses proposed to explain why reproduction in woodpeckers differs from that of other hole-nesters are differences in nest predation, limitation in nest sites of the secondary hole-nesters, energy costs expended in excavation, differences in food supply, adult survival and life duration.
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