The Woodpecker's Cavity Microenvironment: Advantageous or Restricting?

Abstract

We studied the nesting biology of the Syrian woodpecker (Dendrocopos syriacus) in Israel, emphasising its physiological aspects and the environment of the tree hole nest cavity (~ 2 litres in volume). Eggs (mean mass = 5.4 ± 0.4 g SD) are laid once a day during April and May to produce a mean clutch size of four (range 3-5). Active incubation starts after the last egg is laid and lasts 11 days. Both hatching and fledging are spread over 2-3 days. Parents share incubation during the day but only males incubate at night. Both parents share the feeding of the nestlings equally. Males occupy and guard the empty nest at night all year round. Incubation and fledging periods add up to a normal fledging time for the species. The young are fed by their parents outside the nest for another month.

Mean egg temperature was not significantly different between day and night, averaging 34.2°C (± 4.3 SD). Mean egg water loss was 1.4 % d⁻¹ of the initial egg mass prior to the onset of incubation and 0.5% d⁻¹ during incubation. The extrapolated total water loss of the eggs was 13.1% at the end of incubation. This indicates a good match of cavity humidity and the water vapour eggshell conductance of 1 mg·(d Torr)⁻¹ (=100 mg·(d kPa)⁻¹), where, in spite of the higher egg temperature during active incubation, the egg water loss rate is reduced due to the presence of the incubating parent.

Gas composition at the bottom of the nest initially decreased by ~ 1.75 % for O₂ whereas CO₂ increased by ~ 1.20 %, due to clutch activities until the nestlings were about 15 days old. There was little change in nest gas composition between 15 and 22 days of rearing. From that time on until fledging O₂ concentration increased and CO₂ decreased due to nestlings’ up and down activity in the nest shaft and the fact that they fledge asynchronously at the age of 26 days.