Selection for temperature regulation may drive the evolution of egg color in birds

A new paper in BioRχiv, submitted but not yet accepted, suggests that in some parts of the world the color of bird eggs may reflect selection for temperature regulation. In short: in colder northern climes, eggs may be darker because it helps them absorb sunlight and stay warm, a crucial factor in keeping embryos warm enough to develop. The paper is below (click on screenshot, and the free pdf is here):

I will try to be brief. The authors measured reflectance and color of eggs from 634 species of birds representing 32 of the 36 living orders. They also collected biogeographic data on where the birds nest and what kind of nest they made in terms of exposure: open nests, “cup” nests in dense foliage, or cavity nests.

As I noted above, the results show that egg color is correlated with latitude (e.g., temperature), but only in the north where nesting birds experience cold temperatures. Northern-nesting birds tend to have brownish eggs rather than bluer or whitish eggs, while birds that live in more southerly climes tend to have a variety of colors, implying that in those areas temperature is not as important as other factors (camouflage, etc.). In other words, the worldwide correlation between latitude and egg color comes entirely from northern birds having darker eggs. Outside the colder areas, there’s no correlation between latitude and egg color, implying that there are other factors influencing color. (I’m sure you can think of some.)

Overall the correlation between latitude and color was highly significant, but you can see from the plot below that this was  entirely dependent on darker colors at higher (e.g., more northern) latitudes. Here’s the plot showing egg coloration with latitude (three colors are represented in the mix: brown, blue-green, and whitish):

(From paper): Figure 2. An equal Earth projection of the global distribution of avian eggshell colour depicted in a (a) bivariate plot illustrating continuous variation in blue-green to brown eggshell and darkto light eggshell colours, in units of standard deviation from their means. We depict colour variation using the colours of the gray catbird Dumetella carolinesis and the peregrine falcon Falco peregrinus to represent blue-green and brown colour, respectively.

Here are the correlations of color and luminosity with latitude (you can see how they collected the data by reading the original paper). As you see from the “bends” at the right side of the plots, the correlations come from higher (more northerly latitudes) alone: 40 degrees and higher):

(From paper): Both avian perceived (b) colour (R 2 = 0.83, λ = 0.94, AICc = −40,475; latitude: z = −0.06, p = 0.95; latitude2 : z = 4.67, p < 0.0001) and (c) luminance (R2 = 0.88, λ = 0.94, AICc = −30,736; latitude: z = 2.08, p = 0.04; latitude2  : z =−9.26, p < 0.0001) vary non-linearly across latitude, such that dark brown eggs are more likely at northerly latitudes.

You can see the same correlation with color and luminosity in the blue-colored plot here for “cold Köppen regions” (not sure what these are) versus the absence of a correlation in warmer regions (reddish areas):

(From paper): Figure 3. Variation in avian perceived (a,c) colour and (b-d) luminance in (a,b) cold Köppen climate regions (blue dots) compared to (c,d) other ecoregions (pink dots). The central inset depicts those climate regions on the Earth. See figure 1 and methods for further details.

Now if this is really due to temperature, you can make two further predictions:

a). If egg color really does reflect selection to warm up and retain heat for incubation, you should be able to demonstrate that in the lab by using eggs of different colors from a single species and exposing them to light.

b). In areas where there is a correlation between egg color and temperature, more exposed nests that receive more sunshine should be on average darker than those from nests in dense vegetation, which experience less sunlight and thus less selection for capturing sunlight, and that eggs in cavities, which experience very little sunlight, should be even lighter.

The authors demonstrated that a) is the case by using dark brown, light brown, blue-green, and white eggs obtained from commercial chicken breeds. They exposed them to direct sunlight after a control period of acclimation at lower temperature, and measured both internal temperatures and external temperatures through reflectance. Color made a highly significant difference in both rate of heating up and retention of temperature after light-based heating (p < 0.0001).

Here’s a graph showing the retention of temperature in dark brown (“db”), light brown (“lb”), blue-green (“bg”) and white (“w”) eggs after heated up (top lines, left scale) and the difference in temperature from the white egg (bottom eggs, right scale).  You can see that dark brown eggs, more prevalent further north, retain heat substantially longer than do lighter eggs. This, of course, will help with the incubation of chicks, especially when the mother or father aren’t on the nest incubating them:

As for b), the correlations of egg color with nest site was highest for exposed eggs (ground nesters), weaker (but still significant) for cup-nesting birds who incubate in foliage, and even weaker (but still significant) in cavity-nesting birds. Presumably all these nests might be exposed to some light, and so might still select for darker colors in more northern areas:

There may often be conflicting selection on egg color, as dark eggs in warmer climes may be more cryptic, but they also may heat up too much, injuring or killing the incubated chick. In the north there is not as much danger of overheating, so eggshell color is free to darker (and more cryptic). In more southern areas, color may be freer to vary for other reasons. As the authors note:

In cold climates, the ability to maintain temperature for longer periods of time afforded by darker coloration is particularly important. This is not to say that species laying exposed eggs will leave their eggs unattended for longer, but instead, when unattended, dark eggs would have greater heat retention over comparable time periods. Eggshell pigmentation thus can confer an additional advantage over the chill tolerance found in some species. By contrast, in warmer climates dark eggs might be more costly because they heat relatively quickly (e.g., nearly twice as fast as white eggs). In these environments, species are subjected to competing selection pressures and while eggs may have greater luminance (less pigmentation) in these warmer climes the colour  is unlikely to be selected for thermoregulation. Instead, in these environments eggs are likely impacted by a range of other selective pressures: solar filtration, anti-microbial defence , signalling of mate quality, and egg recognition. Additionally, crypsis and eggshell strength are known to influence egg coloration, and are likely important selective pressures globally. This interpretation is supported by our data. Egg colour was increasingly variable nearer the equator, indicating other selective pressures (e.g., ecological or behavioural) are acting on eggshell coloration.

Other information, such as how the authors phylogenetically corrected the color and nest-site data, can be found in the paper.

So, the upshot is that we have a tentative explanation for why birds that incubate in more northerly areas have darker eggs, and some experimental data that supports the explanation. As always, this idea makes other predictions that can be tested (for example, tropical eggs might be lighter colored if there are fewer egg predators around).

24 Comments

  1. Dominic
    Posted March 6, 2019 at 9:48 am | Permalink

    https://en.wikipedia.org/wiki/K%C3%B6ppen_climate_classification

    I’d like some connection with size…

  2. Posted March 6, 2019 at 10:41 am | Permalink

    I confess I haven’t read the paper…

    So are we talking only about the colors as they appear to human eyes, in the “visible” (to us) color spectrum? Some materials appear black to us but reflect infrared. Taking this into consideration might affect the results of this study — like, perhaps some eggs that are dark for better camouflage don’t necessarily heat up too much in direct sunlight.

    • Posted March 6, 2019 at 10:46 am | Permalink

      As I recall, they used colors that would be visible to birds, but of course not all egg predators are birds!

      • ThyroidPlanet
        Posted March 6, 2019 at 6:55 pm | Permalink

        These are all completely naïve questions:

        Does this mean all birds are assumed to have the same color vision pigments? I guess that’s a fair assumption but I don’t know.

        And what about insects that (I think) see ultraviolet like bees? What do the egg spectra in UV look like, and what happens the to the correlation with latitude (i didn’t read the original paper).

        What if predator vision needs only be black and white? Like (I think) dogs?

        Etc. along that line of reasoning about color.

        • Torbjörn Larsson
          Posted March 7, 2019 at 12:11 pm | Permalink

          I would not bet on same color vision, bird diversity is twice that of mammals and mammals have evolved many different opsins.

          That said the color vision in birds seems sophisticated, here I see a claim that the color filtering process (which mammals largely lack) may have more of an adaptive pressure than the opsins: https://en.wikipedia.org/wiki/Bird_vision . (And yes, a discussion on differences in opsins as well.)

          • ThyroidPlanet
            Posted March 7, 2019 at 6:07 pm | Permalink

            That’s an interesting article- UV light activity is discussed. I didn’t know birds use oil droplets of carotenoids to filter light.

    • Peter N
      Posted March 6, 2019 at 10:46 am | Permalink

      I pose the above question based on something I noticed long ago. As I always say, everything I know, I learned through recording classical music…

      While recording a performance of the opera Madama Butterfly, watching the stage via a black-and-white video system, I noticed something interesting. The sea captain’s trousers, which were navy blue wool with a satin band of the same color, appeared with a bright white strip on the video monitor. The wool absorbed infrared but satin material strongly reflected it.

  3. Posted March 6, 2019 at 10:55 am | Permalink

    “…dark eggs would have greater heat retention over comparable time periods.”

    Odd. I seem to recall from my high school physics that dark objects emit as well as absorb heat faster. Is there an egg shell thickness difference?

    • Martin X
      Posted March 6, 2019 at 12:53 pm | Permalink

      Yes, but I think it matters whether you’re trying to keep heat in or absorb it from the environment.

      An auto radiator works best when black because it’s primarily an emitter of heat.

      • Posted March 6, 2019 at 7:08 pm | Permalink

        Would not a warm egg be an emitter of heat?

        • ThyroidPlanet
          Posted March 6, 2019 at 7:18 pm | Permalink

          I wonder if the apparent coloration is part of a spectral camouflage for precisely the egg’s heat, which predators can pick up by sending heat – I’m thinking snakes.

    • Posted March 7, 2019 at 12:50 pm | Permalink

      But in the steady state, the dark object is nevertheless warmer. I remember a simple experiment, done by a scientist ages ago, that I reproduced as a child – put on snow pieces of cloth of different colors and then see how the darker the color, the deeper the piece has sunk as a result of melting the snow beneath.

      Some propose to use lighter colors for roofs, alleys and roads to reflect light (anti-warming).

  4. ThyroidPlanet
    Posted March 6, 2019 at 11:28 am | Permalink

    Sub

  5. ThyroidPlanet
    Posted March 7, 2019 at 5:59 am | Permalink

    More thoughts :

    I wonder if parent birds spend more time foraging in the north, and more time incubating in the south.

    What is the ideal temperature for incubation, and how might that vary across species? Perhaps the nesting sites are locally well insulated in the southern areas, in some way by vegetation…

    I am getting stuck viewing the problem in north/south terms, when only the color shows correlation along those dimensions.

    I don’t think this would be so interesting when I first heard about it, but I keep thinking about it.

    • ThyroidPlanet
      Posted March 7, 2019 at 6:02 am | Permalink

      And do all the species studied produce the same levels and identities of pigments in the shells? I.e. what if brown pigment is simply not in the genes of birds that incubate eggs in the south?

      Oh and what does the north/south categorization mean? It’s only where they _raise_ the young, not anything at all to do with migration?

    • ThyroidPlanet
      Posted March 7, 2019 at 6:05 am | Permalink

      I think my comment disappeared –

      North/south is only where the young are raised – nothing to do with migration?

      Are all pigments equally available to all studied species? For instance What if the southern species simply do not have genes for brown pigment (I suspect I’ll find this in the details).

    • Dominic
      Posted March 11, 2019 at 6:13 am | Permalink

      Do not forget they have more daylight in the northern summer, so more time to forage.

      • ThyroidPlanet
        Posted March 11, 2019 at 6:23 am | Permalink

        You mean the tilt gives more daylight? I can understand that – never thought of that before – interesting!

  6. W.Benson
    Posted March 7, 2019 at 9:06 am | Permalink

    My comments:
    1. Birds (I read), generally the female, typically incubate their eggs and otherwise attend their nests 50%-75% of the time. That leaves little time for sun heat up.
    2. Nesting-bird videos on youtube show almost all small birds (including many ground-nesters) build their nests in the shade. Typical descriptions of nest locations are “bushes or low trees, usually just inside canopy”, “bushes, …, and dense vegetation”, and “well hidden under ivy … under thick herbage in bushes or hedges.” Only very exceptionally could direct sunlight be expected to significantly increase egg temperature.
    3. The study apparently does not control for phylogeny. Many the far-north birds could have brown eggs because they share the character through evolutionary affinity.
    4. Darker brown eggs could be favored in northern climes because of the prevalence of mammalian (color blind) predators active over extended twilight conditions.
    5. The authors use latitude for their egg correlations whereas actual field temperature during incubation would be the appropriate parameter. Must remember that in summer, in the far north, the sun neither sets not remains for long in any sector of the sky.
    5. Data mapped in the paper suggest that patterns may differ between North America and Asia, whereas the mechanism proposed by the authors gives no reason why this might occur.
    I would like to see simultaneous video and egg-temperature recordings of natural nests of one of the bird species used in this study to confirm that the postulated phenomenon — darker eggs heat up more, hatch sooner, and give fitter progeny — actually occurs in the real world.

    • Torbjörn Larsson
      Posted March 7, 2019 at 12:00 pm | Permalink

      My limited experience from northern Swedish mountains is that the ground nests (that I saw) were placed in the open. Youtube vs hiking?

      This could tie in with “Presumably all these nests might be exposed to some light, and so might still select for darker colors in more northern areas”, since *everything* during daytime and cloud breaks in the northern spring is exposed to light. Spots of high lying snow are highly reflective, and you have to go deep into shadow to feel the cold.

    • W.Benson
      Posted March 7, 2019 at 4:14 pm | Permalink

      A simple experiment that actually tested the speculative conclusion — that at high latitudes chicks from brown eggs develop faster and better, and are ultimately fitter — is worth a thousand correlations.

  7. Torbjörn Larsson
    Posted March 7, 2019 at 12:17 pm | Permalink

    Thanks, I learned something new today!

    Also, “The Köppen climate classification is one of the most widely used climate classification systems.” [ https://en.wikipedia.org/wiki/K%C3%B6ppen_climate_classification ]

    Seems to me that the map “cold Köppen regions” correspond roughly to the linked map of Köppen-Geiger polar and continental climate groups.

  8. Posted March 7, 2019 at 12:55 pm | Permalink

    Thank you for the interesting article! There is another, on a related subject, for those who are interested in egg color:

    “Dinosaur egg colour had a single evolutionary origin.”
    https://www.ncbi.nlm.nih.gov/pubmed/30464264


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