RT Dissertation/Thesis
T1 Trade-off	between the risk of overheating and	 camouflage on eggs of ground-nesting birds
T2 Compromiso evolutivo entre el riesgo de sobrecalentamiento y el camuflaje en huevos de aves que nidifican en el suelo
A1 Gómez Esteban, Jesús
K1 Anidación
K1 Aves
K1 Temperatura
K1 Camuflaje en la Naturaleza
AB Many Charadriiform birds (shorebirds and allies) nest on the ground in sites with novegetative cover, which receive direct solar radiation. When incubating adults depart fromnests, the eggs may reach critical temperatures for embryos if they receive direct solarradiation. Nest predation is an important cause of breeding failures, so that to have wellcamouflaged nests may be critical to reduce the risk of predation while the nests areunattended. Eggshell pigmentation and spottiness have been suggested to improvecamouflage, but by making the eggs darker they may increase the rates of overheating wheneggs receive direct solar radiation. So, this may lead to a trade-off between both factors(camouflage and egg temperature) on eggshell coloration. The aim of this thesis was to showwhether there is such a trade-off, and to analyse whether ground-nesting birds mitigate theadverse thermal conditions of their nesting sites in hot environments by means of biophysicalmechanisms (e.g., egg colour and spottiness, nest materials with favourable thermalproperties), as well as by behavioural mechanisms (e.g., risk-taking by incubating birds inrelation to nest camouflage, choice of sites where eggs are better camouflaged).First, we show that darker eggs of shorebirds (plovers Charadrius spp. and stiltsHimantopus spp.) were better camouflaged when the nests were not attended by adults, butsuffered quicker overheating than light-coloured eggs due to direct solar radiation, and alsothat, after controlling for environmental temperatures, eggs overheated more in the Tropics,likely because of a more intense solar radiation, than in the Mediterranean region. Thus, thebenefits of increasing pigmentation and spottiness to improve camouflage are counteractedby the increased risks of overheating when eggs remain exposed to direct solar radiation.We expected a similar trade-off at a specific level across latitudinal ranges, as athigher latitudes solar radiation is less intense that at lower latitudes. Indeed, we found thateggshell reflectance in the Kentish plover (Charadrius alexandrinus) is primarily determinedby latitudinal variations in solar radiation, with eggshells of lower reflectance (darker) foundin higher latitudes. However, in the southernmost localities (nearer to the equator), wheresolar radiation is very intense, eggshells are of dark coloration in spite of high ambienttemperatures, likely to protect embryos from UV radiation. Therefore, differential effects ofsolar radiation on functions of coloration of eggshells may shape latitudinal variations in eggappearance in the Kentish plover.Then, we analysed whether eggshell coloration and spottiness are related to pigmentcontents in the Kentish plover. As expected, we found that protoporphyrin was moreabundant than biliverdin in eggshells. However, eggshell coloration was not related topigment concentrations, which may be due to a different allocation of pigments eitherbetween eggshell¿s background and spots, or between different eggshell¿s layers.Interestingly, there was a positive relationship between the fractal dimension of eggshellspottiness and the amount of protoporphyrin. Likely, the fractal dimension of spottiness maybe related to the mechanical function of protoporphyrin (e.g. by strengthening eggshells tointerfere with a potential fractal structure of fractures), and/or it may have a functional role if it affects the resulting egg camouflage. If this is generalised among species, the fractaldimension of spottiness could be used as a proxy of protoporphyrin content in spotted avianeggshells.In addition to eggshell coloration, the use of nest materials may be anotherbiophysical mechanism with which to counteract the adverse effects of high ambienttemperatures on egg overheating. Kentish plovers added into their nests lighter materials(pebbles) than those available around nest sites. Because light materials are thermallyreflective, the microclimate of nests was likely improved. However, light materials worsenednest camouflage. We showed experimentally that rates of egg heating on lighter materialswere lower than on darker materials. Yet, in a field experiment the plovers removed most ofthe experimental materials independently of their thermal properties, so that egg camouflagereturned to the original values within a week of the experimental treatments. Although thethermal environment may affect the choice of nest materials by the plovers, at our study sitesit was not too stressful as to determine the acceptance of the lightest experimental materials.Beyond those biophysical mechanisms, ground-nesting birds may use behaviouralstrategies to enhance egg camouflage and mitigate the risk of overheating when eggs are notattended. By applying predator visual models to images of nests, we analysed whethermicrohabitat and nest material selection by three ground-nesting birds (pied avocetRecurvirostra avosetta, Kentish plover, and little tern Sternula albifrons) facilitatedcamouflage. Plovers and avocets selected microhabitats and nest materials that matched theirindividual egg appearance. In contrast, the lighter and less spotted eggs of the terns did not aseffectively match nest microhabitats and nesting material, despite choosing lighter substrates,and their eggs were an inferior match to their background than eggs of the other two species.The paler eggs and poor camouflage of tern eggs could reflect a trade-off between thermalprotection and camouflage, because terns breed later in the season than the other two species,when there is a greater risk of egg overheating. Thus, by simulating predator vision systemswe show that wild birds select their laying substrates at an individual level according to theireggshell appearance. The apparent need to respond to other selective drivers that maycompromise embryo survival, such as the risk of overheating, may explain why thecamouflage of little tern eggs was worse than that of the other species.Leaving eggs unattended during predator disturbance may expose embryos topotentially harmful factors, to which parent birds should respond when making decisionsabout when to leave or return to their nest. In an experimental study, we showed that diurnalchanges in flushing behaviour of incubating little terns from nests during predator approachwere affected by egg camouflage, the terns allowing closer approaches to individual nests inmidday, when the eggs appeared better camouflaged, than in the morning. Return times to thenests were affected by ambient temperature, with the terns shortening such times at highambient temperatures (i.e., in midday), thus diminishing the risk of egg overheating. As awhole, our results show that the decisions of the birds on when to leave or return to theirnests depended on shifting payoffs, as a consequence of diurnal variations in both the thermalrisks incurred by embryos and egg camouflage.In conclusion, our observational and experimental studies allowed us to understandtrade-offs among factors that affect embryo performance in the evolution of coloration andpatterning of eggshells of ground-nesting birds. This may be useful to predict the responsesof ground-nesting birds under a scenario of global warming.
YR 2018
FD 2018
LK http://hdl.handle.net/10433/6273
UL http://hdl.handle.net/10433/6273
LA en
NO Programa de Doctorado en Medio Ambiente y Sociedad
NO Línea de Investigación: Biodiversidad y Biología de la Conservación
NO Clave Programa: DAM
NO Código Línea: 83
NO Universidad Pablo de Olavide de Sevilla. Departamento de Biología Molecular e Ingeniería Bioquímica
DS RIO
RD May 24, 2026