Sparrowrama8

In mammals, light signals mediated by the retina are required to entrain the suprachiasmatic nucleus (SCN), the body's master clock, to light-dark cycles. <a href="https://youtu.be/fkgwfTnOQZQ" target="_blank">This video</a> summarizes the light input pathway in mammals. But what's the light input situation in non-mammalian vertebrates, like birds? [[Proceed->Untitled Passage]] to perform your very own experiments with sparrows to explore this topic, following in the footsteps of Michael Menaker and other circadian biologists!

You're thinking of testing the role of a sparrow's eyes in input pathways to its circadian clock. To do this, you separate wild-type sparrows into 4 groups, and plan an experiment. <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie.gif" style="width:25%;height:auto;"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie2brown.gif" style="width:25%;height:auto;"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie3black.gif" style="width:25%;height:auto;"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie4reddish.gif" style="width:25%;height:auto;"> [[Continue->Untitled Passage 1]]

First is your control: an unmodified bird kept in light-dark (LD) conditions. <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"> <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie.gif" style="width:35%;auto;"> [[Continue->Untitled Passage 2]]

Second, another unmodified bird, this one in constant dark conditions (DD). <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"> <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie2brown.gif" style="width:35%;auto;"> [[Continue->Untitled Passage 3]]

This is where the interesting variables will begin. Bird #3 is blindfolded and in LD conditions. <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"> <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie3bblack.gif" style="width:35%;auto;"> [[Continue->Untitled Passage 4]] ***DISCLAIMER***<img class="flimg" src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepsit.jpg"><p><i>In the original experiments by Michael Menaker that we're simulating here, the birds weren't actually blindfolded. Their eyes were surgically removed in a procedure called "enucleation," which was approved as meeting ethical guidelines regarding the use of animals in experiments.</i></p>

The last sparrow is blindfolded in constant dark conditions (DD). <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"> <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie4breddish.gif" style="width:35%;auto;"> [[Continue->Untitled Passage 5]]

Given the importance of light for the circadian system, what would you predict about the entrainment (or lack of entrainment) in each of these 4 cases? [[Continue->Untitled Passage 6]]

Hypothesis for Bird #1? <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"> <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie.gif" style="width:35%;height:auto;"> Sight: normal Environment: LD [[Entrain?->Untitled Passage 7]] or [[Free Run?->Untitled Passage 18]]

Hypothesis for Bird #2? <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"> <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie2brown.gif" style="width:35%;height:auto;"> Sight: normal Environment: DD [[Entrain?->Untitled Passage 8]] or [[Free Run?->Untitled Passage 9]]

Hmm. You said that a sighted bird in LD would entrain just like a sighted bird in LD. Why do you think this bird could entrain without an LD cycle? <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"> <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie2brown.gif" style="width:35%;height:35%;"> Sight: normal Environment: <big>''DD''</big> Perhaps you're thinking that the bird could entrain to non-photic cues (like temperature). Temperature and other non-photic cues are indeed very important. But if this is a well-designed experiment, you've already controlled for all those factors by holding them constant, since you're using light as your //Zeitgeber//. (Consider reviewing the <a href="http://ccb.ucsd.edu/the-bioclock-studio/education-resources/basics/part3.html#cfe" target="_blank">criteria for entrainment</a> before proceeding with the simulation.) For Bird #2, light is held constant along with all other known environmental cues. So you have no reason to think there is any cue to which the bird could entrain. The well-motivated hypothesis is that it will [[free-run->Untitled Passage 9]].

Hypothesis for Bird #3? <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"> <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie3bblack.gif" style="width:35%;height:auto;"> Sight: Blindfolded Environment: LD [[Entrain?->Untitled Passage 20]] or [[Free Run?->Untitled Passage 11]]

The pineal gland! A bird brain and a human brain aren’t very different from each other structurally. <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/pineal.jpg" style="width:70%;height:auto;"> However, a bird’s pineal gland is at least as important to its biological rhythms as the SCN. Their pineal gland has its own circadian rhythmicity, can be directly entrained by light //in vivo// (as seen in this experiment) and //in vitro//, and secretes melatonin to synchronize other peripheral oscillators. It is also very close to the surface of the skull, where it can receive direct light input. A human’s pineal gland is also very important. It rhythmically secretes melatonin, which helps entrain peripheral oscillators, and it can even modulate the SCN. But the strength of its own circadian rhythmicity is nowhere near the robustness of a bird's pineal gland. <a href="http://www.sciencedirect.com/science/article/pii/0006899374901887">Experiments in non-human mammals</a> suggest that rhythms in the mammalian pineal gland may depend upon the SCN. And the pineal gland in mammals also does not express any known photoreceptors: if you put mammalian pineal tissue in a dish, you cannot entrain it to an LD cycle. So in short, birds have important circadian photoreceptors in the pineal gland while humans do not. In fact, besides the pineal gland, birds have been shown to have additional deep-brain photoreceptors that are also an important part of their biological time-keeping mechanisms. [[Finish->Untitled Passage 13]]

Hypothesis for Bird #4? <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"> <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie4breddish.gif" style="width:35%;height:auto;"> Sight: blindfolded Environment: DD [[Entrain?->Untitled Passage 36]] or [[Free Run?->Untitled Passage 15]]

Congratulations! You're an experienced avian circadian biologist! <b>CREDITS:</b> <b>Creative Director</b>: Stephanie Leon <b>Chief Technical Officer</b>: Stephanie Leon <b>Lead Designer</b>: Stephanie Leon <b>Artists</b>: Nathan Carmichael & Ben Sheredos <b>Executive Producer</b>: Susan S. Golden <b>Assistant Producers</b>: Karen J. Tonsfeldt & Ben Sheredos <b>Testers</b>:<ul><li>Susan S. Golden<li>Michael R. Gorman<li>Rahil Hamza<li>Elizabeth M. Harrison<li>Stephanie Leon<li>Michael Menaker<li>Ben Sheredos<li>Karen J. Tonsfeldt<ul> <i>All the experiments we discussed were actually performed by Michael Menaker and colleagues. Their original, landmark research papers are freely available online! The <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC224688/" target="_blank">original 1968 study</a> suggested the existence of non-retinal photoreceptors in sparrows. A trio of follow-up papers were then published in quick succession, showing <a href="https://www.ncbi.nlm.nih.gov/pubmed/5238974" target="_blank">entrainment of the circadian clock</a> in 1968, effects on <a href="https://www.ncbi.nlm.nih.gov/pubmed/5241517" target="_blank">reproductive physiology</a> in 1968, and finally a paper in 1970 showing that these reproductive physiological changes are in fact controlled completely by the newly-found <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC283206/" target="_blank">non-retinal photoreceptors in the brain</a>.</i>

Your hypotheses: <table id="birdgrid"><tr><th>Bird</th><th>Entrain</th><th>Free Run</th></tr><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie.gif"></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/entrainsparrow.jpg"></td><td></td></tr><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie2brown.gif"></td><td></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/freerunsparrow.jpg"></td></tr><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie3bblack.gif"></td><td></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/freerunsparrow.jpg"></td> </tr><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie4breddish.gif"></td><td></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/freerunsparrow.jpg"></td></tr></table> [[Now let's get on with the experiment!->Untitled Passage 17]] [[I'm having second thoughts... I want to REDO my predictions!->Untitled Passage 6]] [[Wait, what are all those squiggly black marks?->Untitled Passage 14]]

Now let's run the experiment, and see what actually happens in each case! [[Proceed->Untitled Passage 19]]

Hmm, that doesn't seem right. You said that a sighted bird would free run in an LD cycle. Click here for a hint (click-replace: "Click here for a hint")[You may need to review the definitions of "Freerunning" and "Entrainment" before proceeding. Try reading <a href="http://ccb.ucsd.edu/the-bioclock-studio/education-resources/basics/part1.html" target="_blank">Part I</a> of the BioClock Studio's Introduction to Chronobiology. After reviewing, choose the option that pertains to your bird's environment.] Bird #1? <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"> <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie.gif" style="width:35%;height:35%;"> Sight: normal Environment: <big>''LD''</big> [[Entrain?->Untitled Passage 7]] or [[Free Run?->Untitled Passage 18]]

Results: <table id="birdgrid"><tr><th>Bird</th><th>Entrain</th><th>Free Run</th></tr><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie.gif"></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/entrainsparrow.jpg"></td><td></td></tr><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie2brown.gif"></td><td></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/freerunsparrow.jpg"></td></tr><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie3bblack.gif"></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/entrainsparrow.jpg"></td><td></td> </tr><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie4breddish.gif"></td><td></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/freerunsparrow.jpg"></td></tr></table> Bird #4 predictably fails to entrain: not only is it blindfolded, but there are no entraining cues. But notice how bird #3, which is blindfolded and held in LD conditions, manages to entrain. Why do you think this is? [[A) Bird senses a temperature change->Untitled Passage 41]] [[B) Light got through the blindfold->Untitled Passage 23]] [[C) Light stimulates the optic nerve directly->Untitled Passage 40]] [[D) Photoreceptors may exist outside of the eyes->Untitled Passage 22]]

Hm, you predicted that a blindfolded bird would entrain to an LD cycle. Why did you choose this option? [[A) My hand slipped. I don't really think it will entrain. Let me try that prediction again->Untitled Passage 9]] [[B) Organisms entrain to LD cycles. This bird is in an LD cycle. So it should entrain->Untitled Passage 35]] [[C) The bird could entrain to non-photic cues, like temperature->Untitled Passage 34]] [[D) I am a scientist from the future and already know how this will turn out. I'm sticking with my prediction that this bird will entrain->Untitled Passage 21]]

Let's go to bird #4 then, you cheater. [[Proceed->Untitled Passage 12]]

Hmmm, so you think the most likely explanation is that the sparrows have photoreceptors outside of their eyes? You discuss this hypothesis with a more experienced colleague, who happens to remember reading about <a href="http://rdcu.be/uvKV/" target="_blank">an older experiment in ducks</a>. It seems that blinded ducks (especially their reproductive systems) are still physiologically responsive to light. Could something similar be true of your sparrows? This is too good a hypothesis to pass up. Let's test this possibility with a new set of birds! [[Proceed->Untitled Passage 24]]

<b>B) Light got through the blindfold</b> <blockquote>Remember, in the real experiments we're discussing, the bird's eyes were enucleated, and the photosensitive retina was destroyed. (We're trying to keep things upbeat by saying they were blindfolded). So light couldn't have any effect through the eyes. </blockquote> [[Yeah...->Untitled Passage 19]]

You need a new bird: one that is blindfolded, and also has the feathers plucked from its head, leaving it bald. Your control will be a bird that is only blindfolded (with head feathers intact). [(Wait, why would I pluck the feathers?)]<why|(click-replace:?why)[Birds' feathers prevent a lot of light from reaching the scalp. If you remove them, the amount of light hitting the scalp will increase by an order of magnitude!] <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg" style="width:35%;height:auto;"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg" style="width:35%;height:auto;"> <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie3blackbald.gif" style="width:35%;height:auto;"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie4breddish.gif" style="width:35%;height:auto;"> For your experiment, you'll put both into the same LD cycle, then reduce the intensity of light, trying to see if there is any condition where the bald bird entrains but the other bird doesn't. If this is so, then you'll have pretty strong evidence for thinking that light is affecting non-retinal photoreceptors in the brain. Do you think that the bald bird will continue to entrain, or will it free run? Why? Write your reasoning down and click to [[see the results of this experiment...->Untitled Passage 25]]

<b>C) Light stimulates the optic nerve directly</b> <blockquote>You're on the right track: light must be having an effect that doesn't rely on the retina. But again, in the actual experiment the birds were enucleated, and all neurons in the eyes were destroyed. The optic nerve is just a bundle of axons from neurons in the retina, so as a side-effect of enucleation, the optic tract degenerates. So light couldn't stimulate the optic nerve, since there was no optic nerve.</blockquote> [[Return->Untitled Passage 19]]

<b>A) Bird senses a temperature change</b> <blockquote>Birds //can// sense a temperature change, and if you were using incandescent light bulbs, then they might produce a lot of waste heat. In reality, researchers would have to rule out the possibility of a tiny unintended temperature cycle -- perhaps by using LEDs, or moving the light source farther away. Let's assume that yours was a well-designed experiment: light was the only variable and temperature was held constant.</blockquote> [[Return->Untitled Passage 19]]

Somehow we need to test whether the non-retinal photoreceptors in our bald bird are really inside the brain, or just on the skin. How could we do that? <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie3blackbald.gif" style="width:35%;height:auto;"> [[A) Check if light is being detected by the skin: remove the skin and see if it still entrains.->Untitled Passage 28]] [[B) Check if light is being detected by the skin: paint over the skin on the skull, and see if it still entrains.->Untitled Passage 29]] [[C) Check if light is being detected by the brain: give the bald bird a sort of under-skin tattoo, injecting opaque ink under the skin, and see if it still entrains.->Untitled Passage 30]]

A) Put the feathers back in, and see if it still entrains. Once the feathers are plucked out or shaved off, they don't really go back in. Besides, the feathers would block light to //both// the brain and the skin. So this wouldn't really help us distinguish the location of photoreceptors. [[Oh...->Untitled Passage 26]]

<b>A) Remove the skin and see if it still entrains.</b> Technically that could work, but it's pretty hard on the bird. Can you think of anything less extreme and less invasive? [[Hm, maybe.->Untitled Passage 26]]

<b>B) Paint over the skin on the skull and see if it still entrains.</b> Hm, if you paint over the skin then light won't be able to hit either the brain or the skin. You'd be back to the equivalent of a sparrow that's basically just blindfolded with all its head feathers, like this one: <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie4breddish.gif" style="width:35%;height:auto;"> You've already determined that there are non-retinal photoreceptors that are probably in either the skin or the brain. The question is how to tell where exactly they are. [[Hm, I guess that's true...->Untitled Passage 26]]

<b>C) Give the bird a sort of under-skin tattoo, injecting opaque ink under the skin, and see if it still entrains.</b> That's a pretty non-invasive way to check! Light will still hit the skin, but with opaque ink under the skin, light won't be able to get into the brain. [[Prep the experiment!->Untitled Passage 31]]

You give your bald bird a tattoo to block any light getting into the brain. Then you put it back into the dim LD cycle that it just entrained to: <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"> <img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie3blackbaldink.gif" style="width:35%;height:auto;"> If the sparrow now fails to entrain, you've got very strong evidence for thinking there are non-retinal photoreceptors in the brain that are enabling it to entrain. If it continues to entrain, you've got very strong evidence for thinking the non-retinal photoreceptors are in the skin on top of the head. [[See the results!->Untitled Passage 32]]

Hold on, there. You predicted that a //blindfolded// sparrow in //constant lighting conditions// would entrain? Perhaps you're thinking that the bird could entrain to non-photic cues (like temperature). Temperature and other non-photic cues are indeed very important. But if this is a well-designed experiment, you've already controlled for all those factors by holding them constant, since you're trying to determine the effect of light as a //Zeitgeber//. (Consider reviewing the <a href="http://ccb.ucsd.edu/the-bioclock-studio/education-resources/basics/part3.html#cfe" target="_blank">criteria for entrainment before proceeding.</a>) For Bird #4, light is held constant along with all other known environmental cues. So you have no reason to think there is any cue to which the bird could entrain. The well-motivated hypothesis is that it will [[free-run->Untitled Passage 33]].

Your hypotheses: <table id="birdgrid"><tr><th>Bird</th><th>Entrain</th><th>Free Run</th></tr><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie.gif"></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/entrainsparrow.jpg"></td><td></td></tr><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie2brown.gif"></td><td></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/freerunsparrow.jpg"></td></tr><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie3bblack.gif"></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ldbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/entrainsparrow.jpg"</td><td></td></tr><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/cheepie4breddish.gif"></td><td></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/ddbar.jpg"><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/freerunsparrow.jpg"></td></tr></table> [[Now let's get on with the experiment!->Untitled Passage 17]] [[I'm having second thoughts... I want to REDO my predictions!->Untitled Passage 6]] [[Wait, what are all those squiggly black marks?->Untitled Passage 37]]

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<b>C) The bird could entrain to non-photic cues, like temperature</b> Temperature and other non-photic cues are indeed very important. But if this is a well-designed experiment, you've already controlled for all those factors by holding them constant, since you're using light as your //Zeitgeber//. (Consider reviewing the <a href="http://ccb.ucsd.edu/the-bioclock-studio/education-resources/basics/part3.html#cfe" target="_blank">criteria for entrainment</a> before proceeding.) Since all other environmental cues are held constant, and since this blindfolded bird cannot detect the LD cycle, the well-motivated hypothesis is that it will [[free-run->Untitled Passage 11]].

<b>B) Organisms entrain to LD cycles. This bird is in an LD cycle. So it should entrain</b> For intact organisms, that is a very good prediction. But the key manipulation you've performed in this experiment is to prevent this bird from detecting LD cycles through its eyes. If you expect that this bird cannot detect the LD cycle through its eyes, and if you expect that the eyes are the pathway to the clock as they are in mammals, then the well-motivated hypothesis is that it will [[free-run->Untitled Passage 11]].

<b>Wait, what are all those squiggly black marks?</b> You mean these? <table id="birdgrid"><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/entrainsparrow.jpg"></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/freerunsparrow.jpg"></td></tr></table> Those are actograms. If they look unfamiliar, you'll definitely need to review a bit before proceeding with the experiment. Try watching our video on <a href="https://youtu.be/jkdiis6o5dc" target="_blank">Understanding the Actogram</a>. Once you're feeling more comfortable, you can [[return to your hypotheses->Untitled Passage 15]], or [[start again from the beginning->Start]].

<b>Wait, what are all those squiggly black marks?</b> You mean these? <table id="birdgrid"><tr><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/entrainsparrow.jpg"></td><td><img src="http://ccb.ucsd.edu/_images/bioclock/sparrows/freerunsparrow.jpg"></td></tr></table> Those are actograms. If they look unfamiliar, you'll definitely need to review a bit before proceeding with the experiment. Try watching our video on <a href="https://youtu.be/jkdiis6o5dc" target="_blank">Understanding the Actogram</a>. Once you're feeling more comfortable, you can [[return to your hypotheses->Untitled Passage 33]], or [[start again from the beginning->Start]].