Nest Microclimate Research Articles

Incubation Water Loss in Eggs of Heermann's and Western Gulls

We have studied various factors which govern the constant rate of water loss of gull eggs (Larus heermanni and L. occidentalis livens) during natural incubation in the relatively warm and dry climate of the Gulf of California, Mexico. The water vapor conductance of the eggshell in the former species is fixed by approximately 5,700 pores averaging 8.6 and 270 μm in radius and length, respectively. The constant daily rate of water loss by diffusion across these pores (300 mg H₂O · day⁻¹) is therefore controlled by maintenance of a water vapor pressure gradient across the eggshell averaging 26 torr. The vapor pressure in the air spaces of the egg is determined by the egg temperature, but the vapor pressure in the nest's microclimate is controlled by parental behavior serving to ventilate the nest air. The resultant transport of the accumulating water vapor from the nest to the atmosphere allows maintenance of a relatively constant nest humidity.

Humidity in the Avian Nest and Egg Water Loss during Incubation

An egg diffusion hygrometer is described for measurement of absolute humidity in the nest air immediately surrounding bird eggs during natural incubation. The empty eggshell of the species under study is filled with a drying agent and calibrated by observing its weight change in time upon exposure to known water vapor tensions. Such an egg is then placed in the nest for several days and its rate of gain in weight recorded, from which the mean vapor pressure during exposure can be calculated. The vapor pressure in the nest microclimate of eight species varied from 18 to 26 torr. An indirect method for predicting nest air vapor pressure without the use of a hygrometer is also presented, and for 10 other species yields values similar to those of the diffusion hygrometer. The vapor pressure of nest air is always greater than that of ambient air; from the difference and the known rate of water loss of the egg, nest ventilation can be calculated.

Effects of the microclimate on parasitic nest mites of the dusky footed wood rat, Neotoma fuscipes Baird.

Comparisons of the macroclimate with the microclimate within the house and nest of Neotoma Fuscipes Baird in the Berkeley hills, California, were investigated. An analysis was made of the suitability of the nest microclimate for 2 species of nest mites, Haemogamasus ambulans (Thorell), sensu Keegan 1951, and Haemogamasus liponyssoides hesperus Radovsky. The fluctuations in mite population levels and the species composition were determined from a series of nests collected throughout the year and compared with collections made at the same site during a previous year. The microclimate of the central feeding chamber of the wood rat house had many characteristics commonly associated with burrows of subterranean rodents. In summer the diurnal maximum temperatures were lower and the nocturnal minimum temperatures were higher than those of the macroenvironment. In winter a similar cycle occurred, but nocturnal temperatures of micro- and macroenvironment were almost identical. The average relative humidity of the microenvironment varied from 10–20% above that of the macroenvironment during the 6 seasonal periods studied. At no time did the maximum temperature of the microenvironment approach the unfavorable zone for H. ambulans (30°C). During 79% of the observed time in 6 seasonal observation periods (428 hr observed), the relative humidity of the microenvironment was within the favorable range (above 80%) for H. ambulans . By contrast an optimal relative humidity for H. liponyssoides hesperus was maintained only 24.8% of the time. Of 9 species of parasitic mites collected from 22 wood rat nests, H. ambulans was the only 1 found in all nests and accounted for almost 60% of all parasitic mites collected. Haemolaelaps fahrenholzi Berlese was present in 73% of the nests but was seldom present in large numbers. H. liponyssoides hesperus was very rarely encountered. Other parasitic mites taken were Haemogamasus pontiger (Berlese), Hirstionyssus breviseta Strandtmann & Morlan, Hirstionyssus bisetosus Allred, Hirstionyssus neotomae Eads & High tower, Hirstionyssus occidentalis (Ewing) and Ornithonyssus bacoti (Hirst). Nest population levels of H. ambulans rose during the wet season to a peak in early spring and were moderate to low in the dry summer season. The delay in onset of heavy rains in 1957–1958 was accompanied by a delayed increase in population levels of H. ambulans as compared with the 1964–1965 season.