Iridescence as Camouflage

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Multiple signaling in a variable environment: expression of song and color traits as a function of ambient sound and light

Many animals communicate using more than one signal, and several hypotheses exist to explain the evolution of multiple signals. However, these hypotheses typically assume static selection pressures, and previous work has not addressed how spatial and temporal environmental variation can shape variation in signaling systems. In particular, environmental variability, such as ambient lighting or noise, may affect efficacy (e.g., detectability/perception by receivers) of signals. To examine how signal expression varies intraspecifically as a function of habitat characteristics, we evaluated relationships between spatial environmental variation and song and plumage color expression in a tropical songbird, the Red‐throated Ant‐tanager (Habia fuscicauda) in Panama. We recorded male ant‐tanager song, plucked feathers to measure coloration, and quantified the acoustic and light environments from each male's territory. In addition, we took several morphological measurements from each male to assess the potential information content of song and plumage color. We found that males with redder and more saturated crown plumage occurred on darker territories, and males that sang shorter and lower frequency songs occurred on noisier territories. We also found that more colorful males tended to sing longer and lower frequency songs. Finally, we found that song and color correlated similarly with male morphology (e.g., tarsus length, body mass). Altogether, these results indicate that spatial variation in the environment is related to male coloration and song, and that males might be optimizing color and song expression for their particular territorial environment.     

Latitudinal variation in biophysical characteristics of avian eggshells to cope with differential effects of solar radiation

Solar radiation is an important driver of animal coloration, not only because of the effects of coloration on body temperature but also because coloration may protect from the deleterious effects of UV radiation. Indeed, dark coloration may protect from UV, but may increase the risk of overheating. In addition, the effect of coloration on thermoregulation should change with egg size, as smaller eggs have higher surface‐volume ratios and greater convective coefficients than larger eggs, so that small eggs can dissipate heat quickly. We tested whether the reflectance of eggshells, egg spottiness, and egg size of the ground‐nesting Kentish plover Charadrius alexandrinus is affected by maximum ambient temperature and solar radiation at breeding sites. We measured reflectance, both in the UV and human visible spectrum, spottiness, and egg size in photographs from a museum collection of plover eggshells. Eggshells of lower reflectance (darker) were found at higher latitudes. However, in southern localities where solar radiation is very high, eggshells are also of dark coloration. Eggshell coloration had no significant relationship with ambient temperature. Spotiness was site‐specific. Small eggs tended to be light‐colored. Thermal constraints may drive the observed spatial variation in eggshell coloration, which may be lighter in lower latitudes to diminish the risk of overheating as a result of higher levels of solar radiation. However, in southern localities with very high levels of UV radiation, eggshells are of dark coloration likely to protect embryos from more intense UV radiation. Egg size exhibited variation in relation to coloration, likely through the effect of surface area‐to‐volume ratios on overheating and cooling rates of eggs. Therefore, differential effects of solar radiation on functions of coloration and size of eggshells may shape latitudinal variations in egg appearance in the Kentish plover.     

Do the fluorescent red eyes of the marine fish Tripterygion delaisi stand out? In situ and in vivo measurements at two depths

Since the discovery of red fluorescence in fish, much effort has been invested to elucidate its potential functions, one of them being signaling. This implies that the combination of red fluorescence and reflection should generate a visible contrast against the background. Here, we present in vivo iris radiance measurements of Tripterygion delaisi under natural light conditions at 5 and 20 m depth. We also measured substrate radiance of shaded and exposed foraging sites at those depths. To assess the visual contrast of the red iris against these substrates, we used the receptor noise model for chromatic contrasts and Michelson contrast for achromatic calculations. At 20 m depth, T. delaisi iris radiance generated strong achromatic contrasts against substrate radiance, regardless of exposure, and despite substrate fluorescence. Given that downwelling light above 600 nm is negligible at this depth, we can attribute this effect to iris fluorescence. Contrasts were weaker in 5 m. Yet, the pooled radiance caused by red reflection and fluorescence still exceeded substrate radiance for all substrates under shaded conditions and all but Jania rubens and Padina pavonia under exposed conditions. Due to the negative effects of anesthesia on iris fluorescence, these estimates are conservative. We conclude that the requirements to create visual brightness contrasts are fulfilled for a wide range of conditions in the natural environment of T. delaisi.     

Black-and-white plumage in male pied flycatchers (Ficedula hypoleuca) reduces the risk of predation from sparrowhawks (Accipiter nisus) during the breeding season

Bright colors in birds might signal that they are undesirableas prey (aposematic), an idea that has been difficult to test.When stuffed pied flycatchers Ficedula hypoleuca are exposedto migrating sparrowhawks Accipiter nisus in spring or in autumn,the hawks attack cryptic females more often than bright males.To achieve better statistical control and to assess whethermale plumage also reduces predation risk in the breeding seasonand in the nesting habitat of the pied flycatcher, I placedpairs of male and female flycatcher mounts in similar positionsnear 22 nests of sparrowhawks. The hawks attacked mainly femalemounts, verifying that the preference is real. The sparrowhawkscaught at least 19 live pied flycatchers; 12 young, 5 adultmales, 1 adult female, and 1 female or young. Hawks that caughtan adult male seemed to prefer attacking female mounts. I discussthree interpretations of these results, suggesting that black-and-whitemale flycatchers may benefit from being a novel and aberrantprey, at least early in the breeding season.     

Diapause induction and coloration in the Senegalese grasshopper, Oedaleus senegalensis

Abstract. Embryonic diapause induction in the Senegalese grasshopper, Oedaleus senegalensis Krauss (Orthoptera: Acrididae), is influenced both by the photoperiod and the temperature experienced by females. High temperatures (40°C) and long photoperiods (LD 14:10h), which characterize the beginning of the rainy season in the Sahel, cause non-diapausing eggs to be laid. Lower temperatures (25°C) and shorter photoperiods (LD 12:12h), which occur at the end of the rains, result in the production of diapausing eggs. At 30°C and constant photoperiods, O. senegalensis exhibited a long-day-short-day response with critical photoperiods of c. 13 h and c. 20 h, only the former value being of ecological significance. The photoperiodically sensitive stages to diapause induction in females occurred from the fifth stadium onwards. Temperature also affected the coloration of both nymphs and adults. Dark-black and pale-white individuals were produced by low (25°C) and high (40°C) temperatures respectively, whereas an intermediate temperature (30°C) produced individuals which were greyish brown. These results are discussed in relation to the ecology of O. senegalensis.     

Do bright colors at nests incur a cost due to predation?

Summary Birds show much interspecific variation in the coloration and brightness of their plumage. I examine the hypothesis that selection due to predation on incubating birds and their nest contents can explain part of this diversity. First, I argue that rather than using absolute rates of nest predation to make predictions about the costs of conspicuous colours, we should measure experimentally whether increases in plumage conspicuousness elevate rates of nest predation. Second, I present experimental data investigating the cost of red and brown colour at ground and tree nests. These data provide the first evidence that bright colours do attract predators to nests and that, in addition, this cost varies according to the nesting site. Natural selection seems to most strongly oppose the evolution of conspicuous colours in ground-nesting birds.