Questioning Robin's Eggs

My sojourns out-of-doors often generate questions that lead to protracted stays indoors pursuing the answers.  How counterproductive.  This post is the result of such a stay inside.

‘Tis the season for encountering robin’s eggs, well, broken shells mostly.  Such encounters trigger two questions for me.

• Why are robin's eggs blue or blue-green?  Let me rephrase that one.  What purpose does their bright, beautiful color serve?
• Where’s the nest from which these eggs came?  Scanning trees and bushes in the very immediate area has never been fruitful for me.

Clearly, I'm not alone in this.  For instance, in a recent Urban Jungle article in the Washington Post titled The Bluer A Robin Egg The Better (May 1, 2012), Patterson Clark skates lightly over these same two issues.

(The picture of an American robin was taken by Bruce Finnan and is used with his permission.  The pictures of an egg are mine.)

Why This Color?

It is to Charles Darwin and the theory of evolution that we must attribute the asking of “why” questions in natural history and their serious, scientific exploration.  This question regarding the color of birds’ eggs is no exception.  As biologists Philina A. English and Robert Montgomerie have written:

The adaptive significance of avian egg colors has fascinated naturalists ever since natural selection was first used to explain the characteristics of wild species.  (Robin’s Egg Blue:  Does Egg Color Influence Male Parental Care?, Behavioral Ecology and Sociobiology, Volume 65, 2011.)

Naturalist Alfred Russel Wallace, whose brief paper of 1858 on the role of natural selection in evolution memorably forced Charles Darwin’s hand, posited in a much later book that the myriad colors of birds’ eggs were all, or nearly all, selected for their camouflaging properties.  Even those dramatic, vivid colors, Wallace argued, could be, in a biological sense, cryptic.

The colours of birds’ eggs have long been a difficulty on the theory of adaptive coloration, because, in so many cases it has not been easy to see what can be the use of the particular colours, which are often so bright and conspicuous that they seem intended to attract attention rather than to be concealed.  A more careful consideration of the subject in all its bearings shows, however, that there too, in a great number of cases, we have examples of protective coloration.  (Darwinism:  An Exposition of the Theory of Natural Selection With Some of Its Applications, 1889.)

He quickly dispensed with any notion that “the beautiful blue or greenish eggs of the hedge-sparrow, the song-thrush, the blackbird, and the lesser redpole” might actually draw unwanted attention the eggs.  When viewed from a distance in nests in their natural surroundings, he asserted, they “harmonise very well with the colours around them.”

Then he employed a clever debating ploy to buttress his position, though this stratagem may be one possibly grounded in the actual workings of evolution.  In essence, Wallace argued that, if the colors that some birds’ eggs exhibit today might be somewhat counterproductive and ineffective as camouflage, that’s only because we are catching the process of natural selection in midstream – it’s always a work in progress.  He noted that “changes that occur in the conditions of existence of birds must sometimes render the concealment less perfect than it may once have been.”  But, he continued, birds are capable of responding to threats arising from this quarter, through a change in egg color, the nest, or parental care.

Wallace’s position on egg colors has its contemporary adherents.  For instance, ornithologist Yoshika Oniki, in Why Robin Eggs Are Blue and Birds Build Nests:  Statistical Tests for Amazonian Birds, (Ornithological Monographs, No. 36, 1985) argued that “[t]he main function of egg color in the birds I studied seems to be protection against predation.”  This even applied to the blue-green of robin’s eggs – “normally a protective coloration in situations of contrasting light on green foliage.”  Though biologist Frank Götmark concluded that the color blue in song thrush eggs is most likely now a neutral trait, that is, neither serving to protect the eggs from predation or making them vulnerable to it, he aligned himself with part of Wallace’s account:

It is possible that blue eggs evolved in the song thrush (or in the ancestor of today’s Turdus species) in a habitat type different from the one studied here, where blue eggs did provide camouflage.  (Blue Eggs Do Not Reduce Nest Predation in the Song Thrush, Turdus philomelos, Behavioral Ecology and Sociobiology, Volume 30, 1992.)

Nevertheless, much of the current thought on the role of coloring in birds’ eggs and, specifically, with regard to blue or blue-green eggs, has moved markedly away from a sole focus on the camouflaging role and embraces a richer, more complex array of possible explanations.  Among the other general hypotheses for eggshell coloring and patterns are the following:

• They reflect the maternal addition of substances that strengthen the eggshells.
• They protect the developing embryo from the effects of sunlight (e.g., acting as temperature controls or shields against UV rays).
• They better enable parents to distinguish their eggs from those added to the nest by parasite birds.

Some doubt that it’s a matter of choosing among the various proffered explanations.  As biologist Phillip Cassey has noted, “[A] single hypothesis is unlikely to explain the wide variety of eggshell colours and patterns observed across avian lineages.”  (Cassey, et al., Eggshell Colour Does Not Predict Measures of Maternal Investment in Eggs of Turdus Thrushes, Naturwissenschaften, Volume 95, 2008.)

For blue-green eggs, such as those at hand from the American robin (Turdus migratorius), the debate so far has been particularly protracted, vigorous, and, to my mind, inconclusive.  (By the way, turdus doesn’t mean what my scatological view of the world would have it.  Rather, it is from the Latin turd (pause) which means “a thrush.”)

In their analysis, English and Montgomerie (cited above) noted that “blue-green pigmentation has long defied explanation.”  A situation they have sought to remedy.  They identified biliverdin, a bile pigment (it appears in bruises on our skin), as the main agent responsible for the blue-green egg color.  Biliverdin is an antioxidant and important for maternal health.  Its infusion into eggs comes at a cost to the female and, so is likely to provide some countervailing benefit.  English and Montgomerie suggested three possible reasons for this expenditure of biliverdin by those species with blue or blue-green eggs:

• The pigment might function to shield the embryos from deleterious effects of sunlight, but they stress this doesn’t preclude the pigment providing other services for which it was selected (for a description of the findings from a recent analysis of this possibility, see Brandon Keim’s piece titled Debate Over Purpose of Bird-Egg Coloration Continues, in Wired Science, September 22, 2011).
• Laying eggs that are particularly noticeable may “blackmail” the male partner into a more active role with regard to egg incubation or feeding the female while she incubates the eggs, but English and Montgomerie note that this would apply only to those species where males actually perform these functions.  (I really like the Machiavellian nature of this hypothesis.)
• A female’s significant infusion of biliverdin, resulting in more vivid eggs, may signal to her male partner that she is exceptionally health and that her developing offspring will be robust.  This signal, it is argued, will generate greater attention by the male to the survival of the hatchlings.

It is the last of these hypotheses – the sexually selected egg color (SSEC) hypothesis – that English and Montgomerie explore in their article, Robin’s Egg Blue:  Does Egg Color Influence Male Parental Care? (see link above).  They are not the originators of this hypothesis, but are among the most prominent voices in its support.

I found their particular study fascinating, both for the methods they used to test the hypothesis and how they dealt with the challenging results they obtained.  In the study, they monitored American robins’ nests, some of which they manipulated by replacing some of the eggs soon after they were laid with pale or particularly vivid artificial eggs.  As soon as the real eggs hatched, the artificial eggs in the experimental nests were replaced with unrelated robin hatchlings.  They then monitored the parental behavior, with a focus on male provisioning of the nestlings.

After analyzing the results, they concluded:

In this study, we provide a clear, experimental demonstration that male American robins adjust their nestling provisioning rates in response to the vividness of the blue-green color of their mate’s eggs.

No doubt being expressed there.  A strikingly robust conclusion.

Unfortunately, immediately after making that statement, the authors spend the remainder of the paper backtracking because the data they generated only weakly support that assertion.  The strongest evidence in support of that bold  conclusion was that “[m]ale robins who saw more vividly colored artificial eggs in their nest fed 3-day–old nestlings almost twice as often as those who saw pale eggs, controlling for other variables.”

But there’s the rub – the critical response by the males was correlated with nests containing the vividly colored artificial eggs.  In fact, the control group of nests, those that were not manipulated, held eggs exhibiting some variation in color intensity (though not of the extremes used for the artificial eggs) and male investment responses to those natural color variations were only very weakly correlated to color.  Further, even for the nests with the artificially vivid blue eggs, the effect on male investment disappeared by the sixth day after the babies broke out of the eggs.

The authors worked to defend the SSEC hypothesis from their own findings, expressing how “puzzling” they found the results in the control group of nests.  Not surprisingly, their descriptions of the possible explanations for these results did not preclude sexual selection on the basis of egg color from playing a role.  Ultimately, though, as they closed the paper, they wrote,

While a weak correlation between male provisioning rates and egg color support the SSEC hypothesis, other adaptive (and non-adaptive) explanations for the blue-green color of American robin egg [sic] cannot be ruled out.  For example, blue-green pigmentation due to biliverdin might sometimes be cryptic, may provide some protection from solar radiation, or could help strengthen the eggshell.  (Cited references omitted.)

These findings might not be surprising to Frank Götmark (cited above) who concluded that the blue color was probably a neutral trait.  Further, the results for the control nests are consistent with those of Cassey (also cited above) who found that the blue-green color in the eggs of the species they studied had only a weak correlation with their various measures of maternal investment in the eggs.  As a result, they argued, color variation would be “unlikely” to function as a signal to males of the quality of the female and the eggs.  As I understand that analysis, in their eyes, so much for the SSEC hypothesis.

Well, we’re left with several working hypotheses without any single one, in my reading of the literature, rising to the top.  Given how uneasy I am with bold, take-no-prisoners statements, I really like Frank Götmark’s conclusion:

Thus, there are no simple explanations.  If we assume there is a cost of producing blue pigment, it is likely that blue eggs are in some way adaptive.

More research is in order.


Where From?

Ah, this one’s apparently a lot easier to answer.  Ornithologist Joseph C. Howell provided a response in his Ph.D. dissertation, which was published in an abbreviated version in as Notes on the Nesting Habits of the American Robin (Turdus migratorius L.) (American Midland Naturalist, Volume 28, Number 3, November 1942).  Presumably this was not the first time it was described.

The shells from which young Robins have hatched are removed from the nest almost as soon as the young are free.  The fate of the shells is not always the same.

Often, he observed, the females ate the shells.  Though I don’t know about the broader prevalence of the eating of the empty shell, the policing of the nest to remove the eggshells soon after hatching is not unique to robins.  Of course, in our Darwinian world, this raises the question, why?  In his Urban Jungle article (cited above), Patterson Clark asserted that this action was prompted by the white insides of the broken egg which might attract predators.  He linked to an essay on empty shells that appeared originally in The Birder’s Handbook by Paul Ehrlich, et al. (1988).

And questions beget questions.  For the robin parents, is the white interior of a broken eggshell more problematic than the blue exterior?  Does it make a difference where the eggshells are dropped?  Might there be some premium on dropping those conspicuous eggshells in the territory of some other nesting robin pair or some other competing species?  Is it only the female who removes the eggs as various accounts would have it?  How common is it for the female to eat the eggshells?  Might she be doing it to recover some of the resources she invested in the eggs?

I think it’s time to go outside and watch some robins.

Later Edit

Amazing what a hobgoblin occupied my mind as soon as I uploaded this post.  I've been grappling with what quite possibly should have been the first question considered here - Is "robin's eggs" grammatically correct when I'm using that phrase to describe the eggs of robins in general, not the eggs of a specific robin?  Or should I have written "robin eggs" or, perhaps, "robins' eggs"?  (Should that last question mark have gone inside the end quotes?  Sigh.)  Upon reflection, I believe I should have opted for the phrase "robin eggs" if only to be consistent with my usage of "shark teeth" elsewhere in this blog.  As a middle school teacher once told me when I noted she'd missed an error on one of my test papers (yes, I was that kind of student), "We progress, we don't digress."  So, if error this is, I will leave it and move on to something else.