I have a list of things I want to blog about in this project before I’m done. Some of them I’m not aware of till I see them, but others I’ve been fully aware of since day 1. Pigeons are in the fully-aware category. I originally planned to do a “Pigeon Post.” But when I learned a little bit about Pigeons and how amazing they really are, I just had to declare this Pigeon Week.
Now I know you’re probably thinking, “What? How’s he going to get a whole week out of Pigeons? This sounds lame. I’m so checking out for a week…” Yes, yes, I know- “feathered rats” and all that. But I am telling you, Pigeons are Phenomenally Way Cool, and if you stick with me, by the end of this week you will never look at them the same again. So here we go:
One of the odd things I’ve learned about winter is that it’s actually a really good time to check out birds. On the face of it this makes no sense at all; most birds are gone in the winter. But what I’ve found is that because there are far fewer species of birds about- and far fewer bird-cries in the air- that this somehow makes it easier to really notice the birds that do stick around. Magpies are a great example, as are Scrub Jays and Dark-Eyed Juncos. Pigeons are another. Unlike many of the birds I’ve blogged about, Pigeons aren’t endemic, or even native, to the Western US; they’re everywhere, on all continents except Antarctica, from sea level to 14,000 feet.
When I say “Pigeon” this week, I’ll be talking specifically about the common Rock Dove or Rock Pigeon, Columba livia*. More broadly speaking, “pigeons” and doves- which are really the same thing- are any one of 300+ species within the family Columbidae. Columbids are stout-bodied** birds with short necks, short beaks and a small fleshy protuberance atop the beak called a cere. An example of a native columbid here in Utah is the Mourning Dove, Zenaida macroura***
*An astute reader might ask if I’m talking about wild, domestic or feral C. livia. We’ll get to that in Part 3, but for now assume feral.
**Wikipedia’s term. I was going to say “plump.”
**Which deserves- and hopefully one day will get- a post of its own.
There are so many cool things about Pigeons that it’s hard to know where to begin, so let’s start with something everyone knows: they’re fantastic navigators. We’ve all heard of homing pigeons, and their remarkable ability to return to the home roost from hundreds of miles away, traveling across unfamiliar terrain. Humans have been using pigeons to carry messages for at least 3,000 years* and in doing so have been credited with a critical role in many wars, including both world wars.
*Pigeons have been domesticated for at least 5,000 years, as we’ll talk about in Part 3.
We’ve heard so much about homing pigeons that it’s easy to forget that “homing” isn’t something that suddenly came about when we humans decided to start sending messages around; Pigeons have been regularly using their formidable navigational capabilities for thousands- maybe millions(?)- of years, and in fact use them daily in the wild.
Here’s something cool: we’ve all seen- or maybe see every day- pigeons around cities. Pigeons adapt readily to urban environments, nesting under overpasses or in the eaves of high buildings, as these spots are similar in many ways to the rock ledges and overhangs in which their (truly) wild ancestors nested. For a pigeon, a modern city or town is a wonderfully convenient assemblage of nook-and-cranny filled cliffs, as good as- or better than- any grouping of natural nesting cliffs. But what you maybe didn’t know is that many urban pigeons are commuters- specifically reverse commuters- who each day fly out to suburban or rural areas to forage and then nightly return to their downtown nests. Pigeons navigate for a living.
Tangent: For years I’ve noticed pigeons flying by my office window. But I never really had a handle on where they roosted. Then last week I changed one of my standard running routes, and stumbled upon a big pigeon roost, in the strip mall at 1300 East and 8600 South. And I noticed as well that pigeons really dominate the stretch of 1300 East between 7800 South and 8600 South.
And this in turn has lead to a blazingly-obvious “epiphany” which I somehow never noticed until last week: Birds have neighborhoods throughout developed areas, and in fact certain flocks of certain species dominate specific urban/suburban blocks. I’ve blogged previously about how the local Magpie flock aggressively controls Sunnyside Ave between my house and the zoo. There’s another flock dominating the cottonwoods and willows on Union Creek Road just between I-215 and Ft. Union Blvd, where I exit the freeway for work every day. My standard 5-mile lunch-run loop includes a stretch along 1000 East between 7800 South and 8600 South which is solid Starling territory, although there’s a dense, tightly-controlled, 2-to-3-yard* “cell” of House Sparrows around 8000 South.
*”Yard” in this case as in the yard of a house, not the unit of measurement.
These little “city-states” must change with season and migration throughout the year, and what I’ve realized this past week is that throughout the valley, superimposed upon and around us, is this whole, fantastically complex, dynamic, multi-species, socio-political landscape/map to which 99% of us are completely blind. How did I miss that for so long?
The interesting question is how pigeons navigate, and this question has 2 parts. The first is: how do pigeons navigate mentally? Meaning are they just following a path of memorized cues or landmarks? Or do they construct a mental map of their surroundings and intended destination?
Until the latter half of the 20th century, it was unclear how pigeons homed. One hypothesis was that pigeons simply homed by following the outbound path they had taken away from the roost. Many animals, such as ants, use this type of Hansel & Gretel approach, following landmarks recognized and/or marked (by scent in the case of ants) on the outward journey. But observation quickly ruled out Hansel & Greteling for pigeons on 2 counts. First, pigeons transported to remote locations blindfolded home just fine. And second, pigeons returning home from the same location multiple times don’t follow the same exact path. (Remember this- we’ll come back to it.)
So pigeons seem to use a mental map. But what sort of map? The simplest “map” in animal navigation is a cue gradient map, in which the animal extrapolates from remembered landmarks or cues to determine location and course. These cues may be visual, olfactory (smell) or magnetic. But a simple cue gradient map is limited as to how far it can be extrapolated, and it doesn’t really require a compass. Pigeons navigate hundreds of miles across unfamiliar terrain, and have not just 1, but 2 internal compasses. Most researchers today agree that pigeons construct true spatial navigation maps.
Tangent: In thinking about animal navigation, it occurred to me that we humans use all three methods: simple track-following, cue-gradient maps, and full spatial-mapping. Here are 3 examples.
Until 2 years ago, my parents lived in the house where I grew up. When I visited once or twice a year, I’d return to their house from the airport (by either taxi or rental car) by simply following the same exact set of landmarks: Callahan Tunnel, 93 North, Mystic Valley Parkway, Right at the Armory, Left at the rotary, past the cemetery, the town-line sign, left at Simms’ Corner, up the hill. But 2 years ago my parents moved across town to a townhome, and I had to think just a little bit more about where I was going. The townhome was still in my hometown, so I didn’t have to consult a map or ask my parents for directions, but I had to reference additional memorized “cues” in getting to their home- the train station, the plant nursery, the street where I first rode a friend’s motorcycle, the intersection where a college student broadsided our Toyota station wagon on the way to church. I don’t exactly go past all of these places, but they’re reference points I think about to get me to the condo. So nowadays I use a cue gradient map of sorts to get to my parents’ place.
Last night I flew into San Jose and took a cab up to a hotel in Santa Clara. Before I left home I checked out a map online to get a feel for the route. Though it was dark when I landed, I paid attention to the turns and direction the driver took, tracking the route along unfamiliar streets using the spatial-navigational map I’d constructed prior to take-off.
Nested Tangent: I’ve traveled hundreds (thousands?) of times for work and pleasure in 47 US states, 26 countries and a couple hundred strange cities/towns, and I almost never use a GPS navigation system. I love route-finding; it’s one of the most enjoyable parts of being human. Really, if you can’t find your way around, I think at some point you have to stop and ask yourself: are you really fully human, or a just a good-looking chimpanzee who somehow learned to speak? (Then again I’m one of those weird guys who hates automatic transmissions, so don’t pay me any mind* if you like GPS’s and I’ve offended you.)
*This is my polite way of saying, “Don’t bother leaving an irate comment.” Or do- whatever makes you feel good. I really just want you to be happy. If you’re a GPS-addict and I’ve offended you, then please go ahead and leave a snarky comment. You’ll be filled with a righteous, well-I-told-him-what, sense of well being, and you won’t hurt my feelings at all. I’m actually happy whenever I get comments, even when it’s that Indian flower-delivery place that spams my comments sometimes.
The second part of the question is how pigeons construct and follow these navigational maps. Are they visual, olfactory, magnetic? The evidence seems pretty conclusive that pigeon maps are both visual & magnetic.
The vision of birds is something I’ve covered in several previous posts, so I won’t spend a lot of time on it here, but the short version is that it blows ours away. Bird vision excels not only in distance and clarity but also in color. We humans are trichomatic*, meaning that our retinas have 3 different kinds of cones, or color-wavelength receptors, which are optimized for (relatively speaking) long, medium and shorter frequencies of visible light. Most other mammals are dichromatic or monochromatic, meaning that we see a more colorful- and possibly more detailed**- picture of the world than they do.
*Most of us, anyway. Plenty of people- mostly men- are dichromats or even monochromats, and some number of women may actually be tetrachromatic, as we discussed in this post.
**When I say “possibly”, I mean it. There’s a case to be made that in at least some instances, decreased sensitivity to color enables one to notice detail less clear to those with full color vision. The classic example is colorblind aviators in WWII bomber crews who were supposedly better able to spot camouflaged targets.
But pigeons have 5 different cone types, making them pentachromatic, and what’s more, they have specialized color-tinted oil droplets covering some of their retinal cones which shift wavelengths of light before they reach those cones, thereby optimizing those cones for yet additional wavelengths. In the case of pigeons, it appears that their eyes are therefore effectively at least octochromatic, meaning that they see a world with almost 3 times as many colors as you or I.
At the risk of belaboring the obvious, pigeons- and all flying birds- have yet another visual advantage over us: they’re way high up, and so able to visually map on a scale rarely available to us ground-dwellers.
But really, all of that is review; what I want to focus on in this post is magnetic navigation. For nearly 2 centuries naturalists have suspected the Earth’s magnetic field to be implicated in the navigation of many birds, and in fact it appears that not just birds, but also salmon, turtles and even honeybees* utilize magnetic field clues in route-finding.
*The navigation of honeybees is worth a post in itself, and is way complicated. Though I haven’t really researched it, I stumbled across enough references in researching this post that I suspect it involves elements of all 3 mental navigation models- path-tracking, cue-gradients, and spatial-mapping. Like pigeons, it appears to involve at least 2 compasses- visual/solar and magnetic, and even more interestingly, as honeybee vision (they have apposition compound eyes, which I explained in this post) extends into UV wavelengths, the visual/solar compass is still effective on overcast days.
But before we talk about how pigeons “see” a magnetic field, what exactly is the Earth’s magnetic field?
All About The Earth’s Magnetic Field
Way back when the Earth was molten blob, the heavier elements of the blob- including iron*, nickel and cobalt- sank to the core. These elements are ferromagnetic**, meaning that they can be magnetized- and remain so- by a magnetic field. The core of the Earth is still molten, and these molten ferromagnetic elements are moved around through the core- specifically the outer core- by convection currents, which generates a magnetic field.
*Iron BTW is pretty common in the universe because it’s the final, un-fuse-able end product of really, really big stars, as we saw when we covered Orion and supernovas. Like all heavy elements, Iron is the product of exploded stars. Think about that- all the Iron in your life- you car, tools, maybe your bike- was once stuff in a monster-sized star that blew up.
**An atom of a ferromagnetic element possesses electrons in its outer electron shell, yet the next inner shell is not filled, which means that its electron spin moment is not cancelled.
This convection-generated field extends far out into space, deflecting (to our benefit) most of the cosmic rays headed toward Earth. The field has a direction, with an approximate “North” and “South” pole, sort of (but not really) like a bar magnet.
Not all planets BTW have magnetic fields. Venus has a mainly iron core, like Earth, but no magnetic field. It’s thought that Venus’ extremely slow rotation (roughly as long as its year) is too slow to churn things up and get the necessary convection currents going in the outer core. That’s the thinking anyway. In truth much about the Earth’s magnetic field is a mystery, including its alignment and direction. Most people know that the Earth’s magnetic and geographic poles don’t line up exactly right. Right now the Magnetic North pole is somewhere in Greenland up around 79 degrees North. That’s why you need to know the variance between geographic and magnetic North poles (declination) at your location to use a compass accurately. But what’s weird about the magnetic poles is that they’re moving- fast. Since 1831 (when the North Magnetic Pole was located) it’s moved an average of 6 kilometers per year, meandering about (apparently) aimlessly some 1,000 km. Even weirder, it’s speeding up; in recent years it’s been moving more like 24 km/year.
Weirder still, every once in a while the Earth’s magnetic field flips, or reverses. It’s not clear why this happens- everything from Chaos Theory to asteroid impacts has been suggested- but the field has reversed more than 71 times in the last 171 million years. And bizarrely, the reversals don’t happen at regular intervals. Between 120 million and 83 million years ago, the field never reversed once. But around 42 million years ago, the field reversed 17 times over just 3 million years. Why is the pole moving faster now? Is it getting ready to flip again? We don’t know.
Speaking of “poles”, the term is a bit of a misnomer. There really is no magnetic pole “point” you can go stand or plant a flag on; it’s really just the sum or average of the many, many magnetic dipoles aligned by the core-convection processes. A similar misconception is that a compass points to that magnetic pole, as if there were a big underground magnet up in Greenland yanking on the needle. But that’s not really what’s happening; rather your compass-needle is aligning with the magnetic lines of force at your location.
The magnetic field varies in intensity and inclination- or “dip”- across the globe. The field is 3 times as strong in Antarctica for example as it is in Brazil. And while the magnetic lines of force are oriented almost perpendicularly to the Earth’s surface in Antarctica (~90 degree dip), they’re almost parallel in Brazil.
On a local level, the field varies quite a bit. Natural geologic features including mountain ranges and ore deposits have distinct magnetic signatures, lending to regional magnetic topographies. Many of the loudest/brightest magnetic features are man-made, including bridges, buildings, railways and transmission lines. And at a local level, magnetic field details can fluctuate a bit from day to day. (Remember this too- we’re coming back to it as well.)
Side Note: In fact an ice axe or even a large belt buckle can throw off a handheld magnetic compass by as much as 3 or 4 degrees.
Wow, the Earth’s Magnetic Field is way freaky. What was I talking about again? Oh right- pigeons.
So how do we know pigeons navigate magnetically? Because we can mess them up. When pigeons have bar magnets strapped to their heads, their navigation is totally messed up, but only- get this- on overcast days. That’s right, pigeons can manage just fine with their solar compass even if their magnetic compass is jacked, but when they lose sight of the sun, they appear to rely solely on their magnetic compass.
Interestingly the converse is also true. A pigeon’s solar compass is calibrated by its internal biological clock; in other words it knows where the sun is supposed to be at a given time of day. Clock-shifted pigeons, whose day/night cycle has been shifted by isolation and/or relocation (think jetlag), also experience jacked-up navigational ability, but only on sunny days. On overcast days, the clock-shifted pigeons do just fine, because they’re navigating magnetically.
Extra Detail: BTW, the strapping-a-magnet-to-your-head experiment has been tried in all sorts of interesting variants. One version involved strapping a coil* around the bird’s head that effectively reversed the magnetic field, and sure enough, when released on an overcast day they flew 180 degrees away from home. Other versions have entailed blasting pigeon-heads with magnetic blasting prior to release, which seems also to mess them up, though less dramatically. Similar experiments have been conducted with humans- applying magnets or magnetic fields to their heads, transporting them to unfamiliar locations, and testing their navigation/homing skills, but the results have been mixed.
*Specifically a Hemholtz coil.
The magnetic sense of a pigeon seems to be much more than a simple compass. On a large scale magnetic dip angle provides approximate latitude and the mental navigational map of a pigeon appears to incorporate local, small-scale magnetic features and anomalies. Remember how I mentioned earlier that pigeons don’t always follow the same route home? And then how I mentioned that local magnetic fields fluctuate a bit from day to day? It’s suspected that those fluctuations may account for the route variation. (Wow.)
But how do pigeons sense magnetic fields? For a long time the prime suspects were specialized receptors on either the retina or somewhere along the neural networks (usually the trigeminal nerve) in the face/head. But research in the last decade appears to implicate the beak. The top of a pigeon’s beak contains numerous very small (1-3 micrometers across) nodules of biogenic*magnetite, the iron oxide (specifically Fe3O4) of which lodestones are comprised, and the most magnetic naturally-occurring material.
*Just means of biological origin. So not like the 6-million-dollar man’s eye, or my father-in-law’s knees.
The magnetite-beak connection wasn’t an obvious one. For many years one of the leading alternative explanations to magnetic navigation in pigeons was olfactory navigation. When pigeon beaks were anesthetized, their navigational abilities were impaired, which lent support to the olfactory-navigation hypothesis. But anesthetizing the beak was also anesthetizing the nerves leading to the magnetite nodules, impairing the magnetic sense of the poor* birds.
*And I mean it. When you spend a few days reading about all the annoying and nasty things scientists have done to these poor birds, you really start to feel for them. It’s no wonder they crap while flying over us.
So in the last decade, our understanding of how pigeons appear to utilize magnetic fields appears to be becoming clearer. But none of this tells me what I really want to know: How does a pigeon see magnetic fields and features? What does it look, or feel, like? How does the bird experience a magnetic mental map??
Does a pigeon feel magnetism, as we might feel a breeze on the face, or texture when we run our fingers across a tablecloth? Or is it a slight tug or pull or pressure transmitted to the beak in a more indirect way, like how the “texture” of a rocky trail is transmitted to my hands through the wheel, fork and handlebars of a mountain bike? Or does it smell magnetism, meaning does it come across like a scent of something familiar or welcoming or acrid or spicy?
Or, does it really see magnetism? And while it’s impossible to know, neural networks provide a tantalizing hint. The magnetite receptors in the beak are connected to the underside of the pigeon’s brain by nerves branching off the trigeminal nerve, the main nerve hooking up most everything in the head/face, and one of the dozen main nerve-paths connected directly to the brainstem (as opposed to the spinal cord) in most* land/air-dwelling vertebrates, including us. The trigeminal nerves breaks into many, many pathways in its various connections, but these pathways are grouped into several main branches. The magnetite receptor nerves link into the ophthalmic branch of the trigeminal nerve, the same branch shared by- that’s right- the eyes.
*Specifically there are a dozen in amniotes, which are tetrapod vertebrates with- or descended from creatures with- a terrestrially-adapted egg. So reptiles, mammals and birds. Other vertebrates have different numbers.
So perhaps pigeons really see magnetic direction and topography. Maybe, just maybe, it’s superimposed upon, and integrated with, the already-unbelievably-fantastic octochromatic visual image the bird has of the world. An image more detailed, more comprehensive, more sophisticated and perhaps more beautiful than anything we can ever even imagine.
When released far from home a homing pigeon circles the release point a few times, presumably getting its bearings, before flying off on an initial heading within a degree or so of home. I like to think about the image the bird must see as it circles, the little ah-ha moment it must experience as it aligns its world-view with its mental map and aims for home.
So. Are you starting to see how Pigeons might be way cool yet? Let me tell you what- we are just getting started.
Next Up: Gender, Souls and the Parallel Evolution of Milk.