Water economy in bird nests

Summary

Nest humidity ($P_{{\text{H}}_2 {\text{O}}}$) under an incubating bantam hen was measured at ambient$P_{{\text{H}}_2 {\text{O}}}$ ranging from 1.3 to 25.0 mmHg. Weight loss of eggs was recorded in clutches of varying size. Nest$P_{{\text{H}}_2 {\text{O}}}$ and ambient$P_{{\text{H}}_2 {\text{O}}}$ were also measured in wild incubating willow ptarmigan nests in dry and wet habitats.

Nest$P_{{\text{H}}_2 {\text{O}}}$ increased linearly with ambient$P_{{\text{H}}_2 {\text{O}}}$ in a way predictable on the assumption that the water vapour conductance ($G_{{\text{H}}_2 {\text{O}}}$) of brood patch skin, plumage and eggs were constant and independent of ambient$P_{{\text{H}}_2 {\text{O}}}$. Nest$P_{{\text{H}}_2 {\text{O}}}$ was also dependent of clutch size. Egg dehydration was quantitatively predicted from measured values and the laws of diffusion.

Our findings confirm earlier conclusions that the adult bird does not actively regulate nest$P_{{\text{H}}_2 {\text{O}}}$ at varying ambient$P_{{\text{H}}_2 {\text{O}}}$. Birds can presumably achieve appropriate egg dehydration by a strategy combining nest site, nest construction, egg shell conductance and incubation behaviour which meets the requirements of their breeding climate.