Adaptations of Black Tern (Chlidonias niger) Eggs for Water Loss in a Moist Nest

Eggs of Black Terns (Chlidonias niger) are laid in damp, spongy nests 3-5 cm above water line. The daily mass loss of 31 tern eggs in their nests was 70.9 plus or minus 20.3 mg/day (mean plus or minus SD), resulting in a 14.7% loss of mass during the 22-day incubation period. Eggshell water vapor conductance was 28.13 plus or minus 7.52 mg/day/kilopascal (kpa), a value 1.5 times greater than predicted for bird eggs with similar mass and incubation period and 1.4 times greater than predicted for other tern eggs in particular. The increased eggshell conductance, relative to other eggs laid in drier circumstances, is the result of an increased number of pores in the shell. The water vapor conductance per pore of the Black Tern egg is not significantly different from the pore conductance of eggs of 7 other tern species and of eggs of other bird species. This observation is consistent with the argument that all bird eggs may have a similar water vapor conductance per pore (Ar and Rahn 1979). The adaptation of shell conductance to a humid nest microclimate is accomplished by increasing the number of pores in the shell rather than by altering the ratio of pore surface area to pore length. (DBO)