Back in 2000, I published a paper on the way circadian clock controls the time of day when the eggs are laid in Japanese quail. Several years later, I wrote a blog post about that paper, trying to explain in lay terms what I did, why I did it, what I found, and how it fits into the broader context of this line of research. The paper was a physiology paper, and my blog post also focused on the physiological aspects of it.
But then, I wrote (back in March 2006 – eons ago in Web-time) an additional blog post on one of my old blogs (reposted on this one here, here and here) in which I followed further, thinking about the data in more ecological and evolutionary terms, and proposing hypotheses following from the data that can only be tested in other species out in the wild. As you can see if you click on the links, this post did not receive much commentary.
Then, about a year ago, I received an e-mail out of the blue, from a researcher at the Cornell Ornithology Lab, essentially offering to test one of the hypotheses I outlined in that post. My first reaction was “sure, go ahead, I am happy someone wants to do this, but please cite the blog post as the origin of the hypothesis”… The response was along the lines of “no, no, no – we are thinking about working WITH you on testing this hypothesis”. Wow! Sure, of course, I’m game!
They already had preliminary data which they sent to me to take a look. They are coming from an ecological tradition and are very familiar with the ecological literature, some of which they sent to me to read. On the other hand, I am coming from a physiological tradition and am very familiar with that literature, some of which I sent to them to read.
A month or so later, one of them, Caren Cooper, came down to Chapel Hill. We met and, over coffee, spent a couple of hours staring at the data and discussed what it all means. Then we got started at writing the paper.
And now, the paper is out: Caren B. Cooper, Margaret A. Voss, and Bora Zivkovic, Extended Laying Interval of Ultimate Eggs of the Eastern Bluebird, The Condor Nov 2009: Vol. 111, Issue 4, pg(s) 752-755 doi: 10.1525/cond.2009.090061
In this paper – which is really a preliminary pilot study (who knows, we may yet get a grant to do more) – Caren and Margaret set up video cameras on a bunch of nests of Eastern Bluebirds (Sialia sialis). From the tapes they got times when the eggs were laid. The times were approximate. But the analysis gave us exactly the same result when we used the times when the nest was obviously empty before the bird sat on it to lay the egg, the times when the bird first got up to reveal the egg to the camera, and the mid-point between those two times.
I am not aware of anyone ever looking at timing of egg-laying in wild birds out in the field. There is a huge literature on timing of laying in quail and chicken (and some in turkeys) in the laboratory, but none I am aware of in wild birds. Most researchers, when asked when their species lays eggs are surprised at the question and answer something along the lines of “no idea, but we find the eggs when we come to check the nests in the morning, so perhaps over night, or at dawn?” So, this paper is a first in this domain.
What we have shown is that bluebirds, just like chicken and quail, have an S-shaped pattern of egg-laying patterns (see my older post for theory and graphic visualization).
The question is: how does a bird “know” when to stop laying? When is enough enough? When is the clutch (all of the eggs laid in one breeding attempt) complete? Most of ecological literature is focused on energetics: are birds getting hungry, have they depleted some important source of energy, etc.
But the circadian field looks for internal mechanisms. Running a circadian clock takes very little energy. Even when the animals are extremely hungry, the clock keeps ticking with no changes in frequency (if anything, the amplitude gets bigger, implying even more work!). Even when an animal gets very sick and is dying, at the time when many bodily functions start ceasing, the clock works until the very end. Being produced by a molecular feedback loop in which some reactions use and others release energy, and all of this happening in just a small number of brain cells, the clock is very energy efficient and does not require the organism to be healthy and well fed.
What is important in regard to circadian regulation of egg-laying is to understand that female birds have not one, but two circadian clocks. Let’s call one of them A and the other one B. Clock A is located in the brain (or retinae or pineal or some combination, depending on the species) and is sensitive to light: it readily entrains to a light-dark cycle. No matter what the intrinsic frequency of the clock may be (as uncovered in constant darkness conditions), it is forced to a frequency of exactly 24 hours by the entraining power of the day/night cycle.
Clock B, on the other hand, is intimately tied to reproduction. It is a result of an interplay between the clock in the brain and neuro-endocrine signals between the brain and the ovary (which may itself house its own part of the clock). Brain clock sends hormonal signals to the ovary. Those signals entrain the ovarian rhythms AND result in ovulation. Ovulation itself produces hormones that signal to the brain clock and entrain it. This feedback loop is in itself The Clock. This clock is light-blind and its intrinsic frequency is not 24 hours – it is around 26-27 hours in both quail and chicken, and almost two days long in turkeys.
These two clocks, A and B, interact with each other. Let’s imagine a hypothetical scenario in which clocks A and B are very tightly coupled. The external light-dark cycles that all the birds in the wild are constantly exposed to entrain the clock A to the exactly 24 hours period. Clock B, being tightly coupled to Clock A is then also forced to oscillate with a period of exactly 24 hours. What would that mean to the bird? She would be laying one egg per day, always at exactly the same time of day, every single day of her life: in spring, summer, fall and winter. She’d spend all her resources on making big yolky eggs every day. She would be sitting on a huge pile of eggs throughout her life. She would not be able even to move short-distance to a better nesting ground, let alone prepare and undergo a long-distance migration. Her eggs would be also hatching at the rate of one per day. Thus, she would have progeny of a variety of ages at all times, each age having different requirements for care or abilities to follow the mother around. Some hatchlings would freeze to death in winter, or starve to death at time when the food is scarce. Others would die from predation at times when they are highly visible (in the snow) or just because there are so many of them they cannot all hide under a bush.
An opposite scenario: clocks A and B do not interact with each other at all. In this case, A would be entrained to the 24 hour cycle of night and day. Clock B, being light-blind, would freerun with its own endogenous frequency, i.e., with a period of roughly 26-27 hours. Again, the poor bird would be laying one egg per day all of her life. The only difference is that the eggs would not be laid always at the same time of day, but scattered all over the 24-hour cycle. Both scenarios are obviously maladaptive to the bird.
But, oscillator theory provides a third scenario in which clocks A and B are only loosely coupled. There are phase-relationships between the two clocks when they are coupled: A entrains B. There are phase-relationships when the two are at odds: A inhibits B (and thus no ovulation happens). The phase-relationships are dependent on daylength: when the days are short in winter A inhibits B and no eggs are laid. When the days are very long in the middle of the summer (or in constant light) all phases are permissive to ovulation and the clock B can freerun with its own period of 26-27 hours.
But the interesting phenomenon happens in-between, once the length of the day gets just a little bit longer in spring, in normal breeding season. There is only a narrow zone of phase-relationships in which the two clocks are coupled – outside of that zone, ovulation is inhibited. Thus the clock A starts ticking at the beginning of that zone (e.g., at dawn in some species, at around noon in quail) and starts freerunning through it until it “phase-locks” with the clock A and, for a while, appears to be running with the period of 24 hours. But underneath, the pulses of hormones are gradually shifting later and later, just a little bit each day. Finally, these hormonal influences allow the clock B to again break free from the clock A, freerun some more until it gets out of the permissive phase – the feedback loop is broken and the ovulations stops. The clutch is over.
The resulting pattern is S-shaped: early in the clutch eggs are laid a little bit later each day, the middle of the clutch appears entrained to the 24-hour cycle, and the last egg or two again are laid later until the egg-laying stops completely. In quail, which was bred for centuries for egg-production, the selection affected the strength of coupling between the two clocks. Thus, in photoperiods (daylengths) that are just barely longer than the ‘critical photoperiod’ (the minimal daylength needed to provide any permissive phases at all, thus the first daylength in spring at which the bird can start laying), quail will have S-shaped patterns but the middle portion, the “straight one” that is entrained, is artificially long – I have seen clutches lasting for two months and consisting of 60 eggs!
Birds out in the wild, where natural selection is likely to produce an optimal clutch-size (not a maximal one that humans prefer), may or may not use the same mechanism to determine how and when the clutch starts and ends. So, what we did was see if Bluebirds also show the S-shaped pattern that would suggest they do. And they do:
The first egg in the clutch is laid earlier than the subsequent eggs. All the eggs in the middle (1-6 of them, not 30 – we collapsed them all into one “time-point” in the graph) are laid at about the same time, indicating entrainment of B by A (i.e., to the light-dark cycle). The second-to-last egg may be laid a little later, and the very last egg is laid much later. These results suggest that quail is not a weird unique animal, or that Galliformes (chicken-like birds) are different from other kinds, e.g.., Passeriformes (songbirds). The mechanism is likely the same – not dependent on external factors like food and energy, but a result of a fine-honed system of interactions between two circadian clocks.
Of course, this is just a first observational study, but the results are encouraging. Next steps would be to: a) improve the temporal precision of measurements by, perhaps, installing thermo-couples in the nests (there is a huge but short-lasting body temperature spike exactly at the time of lay), b) increase the sample size, c) compare the bluebirds living in three very different latitudes where both the weather conditions and photoperiodic changes are different to see how the natural selection shaped their responses, and d) do a comparative study of a few more species belonging to other groups. We’ll see if we’ll try to submit a grant proposal in the future.
Unfortunately, this paper is not Open Access. I wanted to send it to PLoS ONE, which I think is the best journal in the world and IS the future of publishing. But it was important for Caren and Margaret to publish in a journal that their peers consider important, and Condor is a fine little journal for this. So I agreed to go along with it.
Also, the listing of the original blog post in the List Of References, to my dismay, disappeared between the Provisional PDF and Final PDF versions. It is now linked to inline in the text, placing it down to the level of the dreaded “personal communication”, once again foiling our attempts to give serious science blogging some respect. Ah well….
Interestingly, I did not know when the paper came out. Apparently, it was published back in November. I learned about it a couple of days ago when I got a first reprint request from a researcher in Russia!
But hey, I am happy. I got a paper published. And now I am using my blog and social networks to promote it… 😉
Cooper, C., Voss, M., & Zivkovic, B. (2009). Extended Laying Interval of Ultimate Eggs of the Eastern Bluebird The Condor, 111 (4), 752-755 DOI: 10.1525/cond.2009.090061