Reader Bruce Lyon, who happens to be a professor of ecology and evolution at the University of California at Santa Cruz (America’s most beautiful campus), sent a group of photos illustrating a research project, so we can haz some pictures and some science as well. His notes are indented:
Golden-crowned Sparrows (Zonotrichia atricapilla) are migratory songbirds that breed in the north (Alaska, BC) and winter from southern BC to Baja California. For the past 14 years, my students and I have been studying a population of sparrows that winter right on our campus, on the University of California Santa Cruz Arboretum. The key feature of the study is we band each sparrow with its own unique color band combination—like an avian social security number. This allows us to recognize, census and observe known individuals in the field without having to catch them to read band numbers. Color banding for individual recognition, first adopted in the 1930s, revolutionized the study of birds because biologists could focus on studying individual variation. Margaret Morse Nice’s famous song sparrow study was one of the first of these studies. More recently, color banding was an essential part of Peter and Rosemary Grant’s study of natural selection in Darwin’s finches. Our focus with the sparrows is more modest—we want to understand aspects of their winter social behavior, including plumage signaling, winter singing and patterns of social organization.
Below: The habitat at the UCSC Arboretum. The sparrows feed on the ground close to bushes so they can dive for cover whenever a raptor shows up. A pair of bird-eating Cooper’s hawks lives in the Arboretum, so this happens often.
Below: A color banded sparrow: red over green on the left leg, yellow over metal on the right. Only this bird has this specific combination and order of bands. By law, all banded birds must also receive a metal band (stamped with a unique number) provide by the Bird Banding Laboratory of the US Geological Survey (USGS).
Below: Another banded bird—violet over orange on the left, violet over metal on the right. The bands we use are not real bird bands but a kid’s toy. Official color bands are expensive ($0.25 per band) but ornithologist Geoff Hill discovered a cheap work-around: the plastic Perler beads kids melt to make artwork make great bird bands, at least for birds with the right leg size. They are a thousand times cheaper than real bands.
We use Potter traps to catch the birds for banding and measurement, and we build our own traps. We bait the traps with millet seed and when a bird goes in for the ‘free’ food, it steps on a treadle which releases a bar holding a trap door open, and the door then snaps shut due to gravity. No such thing as a free lunch.
Below: a couple of Potter traps waiting for customers.
Below: A sparrow in the hand is worth two in the bush, because it shows nicely why they are called golden-crowned sparrows.
Below: A color-banded sparrow visits a Grevillea flower (native to Australia) to drink nectar. These sparrows also eat a lot of leaves and grass, an unusual diet for a small songbird. This diet often leaves a mark on us during banding—the birds donate gifts of bright green poo that invariably lands on our clothing. One interesting finding from our banding study is that the same individual birds return year after year to their same patch of the Arboretum. The survival rate across years is something like 50% and our oldest birds have been about 5 or 6 years old.
Below: Bird banders have long known that golden-crowned sparrows show tremendous variation in their crown plumage. Some birds have brown rather than black stripes, and there is also variation in the size of the stripes (some birds even lack stripes altogether). The color and amount of gold also varies. Some of this variation reflects age (yearlings are always dull) but some birds remain dull for life. This plumage variation is also specific to winter (and grown specifically for winter in a fall molt); breeding birds show much less variation.
Below: A bird with a boldly colored crown bows to show its crown patch to another bird just out of view. These crown displays suggested that the crown patches might have a signaling function, and we showed that the size and color of the crown patches are signals used to determine social dominance in contests over food. These signals of fighting ability are called ‘badges of status’ and they have been previously observed a wide diversity of animals: wasps, lizards, fish, birds and mammals.
Below: An interaction over food with a clear winner. The departing wimpier bird had been feeding at a little seed pile when a dominant bird then showed up and displaced it. Observing lots of interactions like this showed us that the boldness of the black and gold crown patches is a good predictor of dominance. A badge of status signaling system is thought to be favored by natural selection because it allows the birds to figure out who would win a fight over a resource without actually having to engage in a risky fight. A key question remains: how is this signaling system evolutionarily stable? Put another way, why don’t subordinates simply cheat by looking studly? We don’t know the answer yet but possibilities include: punishment (dominants discover and punish cheaters) or physiological costs to producing the bold signals that are too expensive for wimps.
Below: A sparrow gets an experimental black unibrow. We needed to do experiments to be sure that the birds actually use the plumage to settle contests, and not some other feature. For example birds with bold signals tend to be a bit larger so perhaps they use size to assess each other. Experimentally altering the plumage features allows us to decouple plumage effects from all other features. We did trials with two birds that were matched by plumage and then randomly chose one to make studlier. We also had to trap the two birds in each trail from different areas to make sure that they had not previously met—we needed to be sure that they do no have previous information about each other that could influence the outcome. The results of the experiments were impressive: simply by making birds appear more studly with a Sharpie increased their chances of being the socially dominant bird from 50% (the random expectation) to 95%! At the end of the experiments we removed the Sharpie markings with alcohol and set the bird free.
Below: A bird gets a gold comb-over with paint. Yuge patch! Best plumage ever! This bird is going to win fights bigly! As with the Sharpie, this treatment increased the probability of being dominant from 50% (random expectation) to 95%. The treatment also increased a bird’s tendency to want to build walls around the seed piles.
Wow, photos and learning at the same time.
Great, very interesting information!
Oh,and the commentary on the last photo was yugely entertaining, the best ever…
Beats a comb-over.
Very, very cool! I love it when we get a narrative along with the pics, and this one was especially interesting. Thanks.
Did they make the pigeons pay for the walls?
I am face-palmingly embarrassed to admit that even with the prior Trump reference, I didn’t get the joke in the article until I read your comment. I was thinking literally, “Wait.. they build walls? With twigs or something? Why? How does a wall deter animals that can fly? Why have I never heard of this bird behaviour before??? Explain this!”
You were not alone in your ignorance and now humiliation. 😎
Color me embarrassed as well.
So did I. You do not expect such a joke in a scientific article, and the post reads like one.
By the way, it is perplexing to me how aggressively pigeons chase away collar-doves and sparrows from the food, even when there is clearly enough for everybody. Because this behavior is all on the ground, a wall separating different species might help! As a compromise, I have put bread pieces in 2-3 different places.
Looks like Lyon has a good setup to test for fitness effects of patch development. You could alter winter patch size to test whether altered birds (patch-size increased or decreased in low and high dominance birds, respectively) survived less between years than (treated) control birds. If natural selection is at work, birds with the wrong badge size should pay some kind of demographic price.
Also, what happens if you artificially train yearlings to be dominant or submissive? As 2-year-olds do they grow head patches that correspond to their training regime?
Absolutely fascinating. I sure know about Perler beads, having raised 3 human chicks. Since they become very pliable with modest heat, I wonder if one could use heat and an expander to get them to stretch to larger size for larger bird legs.
Mark—this is exactly what we do! I thought the post was getting too long so I left this detail out. Two of the bird species we study have legs that are too big for Perler beads out of the box. To expand the inside diameter of the beads we use a nail with the correct leg diameter, put the beads in boiling water, slip them over the nail, cool them and then put a slice in the side with a razor so that the band can be opened and put on a leg (little shoehorn type device is used to open the band and slide it on to the bird’s leg). We are thinking of writing a very short note for a bird journal to alert others to this option.
This post was definitely not too long! The illustrated notes were fascinating. Thanks.
Love everything about this post! Makes me want to abandon retirement, move to Santa Cruz and help with the song bird program.
Me too!
We actually do have community members helping out. Two of our longest term ‘volunteers’ are not scientists and one is a retiree. They have invested so much in the project that they are now coauthoring some of the papers with us and, for a couple of years, they collected most of the data.
Fascinating study. It is wonderful how close observation and simple experiments reveal the intricacies of bird behaviour.
As someone who has been fascinated watching the intelligent behaviour of crows using tools, I find myself wondering if crows would be taken in by the simple Potter traps. Or, having been caught, could work out how to get out again.
Has anyone ever studied how banding might affect mate selection? Maybe those pretty anklets would be a huge turn-on.
Excellent question. Yes, they can affect mate choice in some species. A famous study of captive Zebra Finches by Nancy Burley showed this effect: female finches find red bands attractive and green bands ugly. There have been several other studies but I cannot recall how many found effects.
I graduated from UC Santa Cruz in the 1970s. Truly a beautiful place.
What about the effects the coloured bands might have on the birds? They are colourful and would be noticed by other birds.
Yes, I wondered about this as well. Could they make the wearers more vulnerable to predation?
Very interesting.
I wonder:
1) If a dark brow and yellow cap indicate dominance and allow first rights to food, why don’t those traits become pervasive?
2) Do those traits make the birds dominant or do dominant birds develop those traits?
Great questions. The first point is what I was sort of getting at with the issue of cheating, but cheating implies that the birds don’t have the goods to back up the signal. The most general explanation is that some birds don’t have the ability to back up the signal and for these individuals it is better to tell the truth and signal their wimpiness honestly (or perhaps their wimpiness prevents them from growing studly feathers). Young birds have dull signals and this makes sense—they do not have the experience to back up an aggressive signal. The birds that remain with a dull plumage throughout life are more puzzling. One possibility is that they had a rough childhood that affected their size or fighting ability for live (these early life effects have been shown for some birds but for traits other than plumage). Alternatively, it is possible that there are advantages to being a subordinate such that what evolution favors is a population with a mixture of types.
For your second question, it seems very likely that dominant birds grow signals to show their dominance, not that the signals alone make them dominant.
It’s a very interesting topic – dominance. Thank you.