A trade - off between overheating and camouflage on 1 shorebird eggshell colouration

25 In ground-nesting birds egg colour and appearance may have evolved due to opposite 26 selection pressures. Pigmentation and spottiness make the eggs darker and have been 27 suggested to improve camouflage. However darker and more spotted eggs may reach 28 higher temperatures when not attended by adults and receiving direct sunlight, which 29 may be lethal for embryos. Some authors suggested that this trade-off may not exist 30 because eggshell pigments mainly reflect in the infrared region of the solar spectrum, 31 but have not considered that wavelengths in the visible part of the spectrum may also 32 contribute to overheating. To test the occurrence of a trade-off between camouflage and 33 overheating of eggs, we took digital images to analyse colour and camouflage in 93 34 nests of four shorebird species (two stilts and two plovers) in two regions (tropical and 35 mediterranean sites). We predicted that these species (closely related) may have evolved 36 different eggshell designs depending on solar radiation, which is supposed to be 37 stronger in the Tropics. To record egg temperatures, we placed Japanese quail eggs in 38 natural nests of shorebirds, and registered temperatures using a datalogger. We found 39 that darker and more spotted eggs reached higher temperatures than lighter ones, and 40 that after controlling for environmental temperatures, eggs overheated more in the 41 Tropics, likely because of a more intense solar radiation. We also found that tropical 42 shorebirds’ eggshells have darker spots and lighter backgrounds. Overall, darker eggs 43 were better camouflaged. Taken together, our results show that the benefits of 44 increasing pigmentation of eggshell backgrounds and spottiness for a better camouflage 45 are counteracted by the increased risks of overheating when eggs remain exposed to 46 direct solar radiation. 47 48