Abstract
Global warming is likely to alter reproductive success of ground-nesting birds that lay eggs normally left unattended for days or even weeks before actual parental incubation, especially in already warm climates. The native North American bobwhite quail (Colinus virginianus) is such a species, and pre-incubation quail eggs may experience temperatures ≥45°C. Yet, almost nothing is known about embryonic survival after such high pre-incubation temperatures. Freshly laid bobwhite quail eggs were exposed during a 12 day pre-incubation period to one of five thermal regimes: low oscillating temperatures (25–40°C, mean = 28.9°C), high oscillating temperatures (30–45°C, mean = 33.9°C), low constant temperatures (28.85°C), high constant temperatures (mean = 33.9°C), or commercially employed pre-incubation temperatures (20°C). After treatment, eggs were then incubated at a standard 37.5°C to determine subsequent effects on embryonic development rate, survival, water loss, hatching, and embryonic oxygen consumption. Both quantity of heating degree hours during pre-incubation and specific thermal regime (oscillating vs. non-oscillating) profoundly affected important aspects of embryo survival and indices of development and growth Pre-incubation quail eggs showed a remarkable tolerance to constant high temperatures (up to 45°C), surviving for 4.5±0.3 days of subsequent incubation, but high oscillating pre-incubation temperature increased embryo survival (mean survival 12.2±1.8 days) and led to more rapid development than high constant temperature (maximum 38.5°C), even though both groups experienced the same total heating degree-hours. Oxygen consumption was ~200–300 μl O2.egg.min-1 at hatching in all groups, and was not affected by pre-incubation conditions. Oscillating temperatures, which are the norm for pre-incubation quail eggs in their natural habitat, thus enhanced survival at higher temperatures. However, a 5°C increase in pre-incubation temperature, which equates to the predicted long-term increases of 5°C or more, nonetheless reduced hatching rate by approximately 50%. Thus, while pre-incubation bobwhite eggs may be resiliant to moderate oscillating temperature increases, global warming will likely severely impact wild bobwhite quail populations, especially in their strongholds in southern latitudes.
Highlights
Temperature influences the rates of virtually all developmental processes in bird embryos, including metabolism, development, and growth [1,2,3]
Eggs exposed to high-oscillating temperatures (HTOsc) developed to a mean stage of 13.7±0.2, equivalent to 2.2 incubation days at 37.5 ̊C
Eggs exposed to high heat loads (HTOsc and high-constant temperature (HConst)) and low heat loads (LTOsc and LTConst) had equal quantities of heating degree-hours within groups, but they all exhibited differential pre-incubation development and growth rates
Summary
Temperature influences the rates of virtually all developmental processes in bird embryos, including metabolism, development, and growth [1,2,3]. Most studies evaluating temperature effects on avian development use a constant incubation temperature equal to mean daily-temperatures of a particular species’ natural environment [14]. Constant (non-cyclic) incubation temperatures do not fully represent the naturally encountered daily thermal fluctuations that eggs of many precocial, nondomesticated birds. Some species may lay multiple eggs in a clutch (e.g., ducks, gallinaceous birds, and ratites), depositing eggs in their nest at a rate of one egg per day. In such species, true incubation by one or more parents does not begin until the penultimate or ultimate egg is laid (see [2] for review, [16]). For many precocial birds there is an extended pre-incubation period—the focal period of this study—where eggs that are laid first are unattended for the entire pre-incubation period and lack the thermal protection and stabilization provided by an incubating parent
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