Readers’ wildlife photos (with extra science)

Reader and professor Bruce Lyon  at the University of California at Santa Cruz (America’s most beautifully-situated university) sent a science-y post about his latest work with sparrows. His notes are indented:

These photos on golden-crowned sparrows (Zonotrichia atricapilla) follow up an early batch that Jerry kindly posted here.  Along with two of my former graduate students, Alexis Chaine and Dai Shizuka, I have been studying the winter ecology of these migratory sparrows for 14 years at the UCSC Arboretum. The previous post focused on color banding to follow individual birds and crown plumage variation, which the birds use as badges of status to settle contests over food. Today I will focus on the intriguing winter social behaviors we discovered in these birds, including singing in winter.

Below: Sparrows in the Mist. Sometimes it is foggy in Santa Cruz, which makes the search for our sparrows seem exotic. The location right on campus is not so exotic but it does mean that lots of undergraduates can volunteer on the project and get experience working on bird field project. The project also involves people from the community at large: the two women below have been assisting us for so many years that they are now an essential part of the project.

I recently had a bit of fun with the Sparrows in the Mist theme for a talk I gave last year. Of course, the phrase ‘in the mist’ was made famous in the movie about Dian Fossey’s work on gorilla social behavior in East Africa. Apart from the fact that we sometimes get misty conditions in the Arboretum, we discovered that the sparrows have much more complex winter social lives than anybody suspected— and, at least at a superficial level, some aspects of sparrow social organization are reminiscent of primate social organization!

The original Gorillas in the Mist movie poster:

Coming soon to a theater near you, Sparrows in the Mist:

The details of winter social organization of migratory songbirds is poorly understood. A casual visitor to the Arboretum would quickly notice that the sparrows spend much of the day foraging in small flocks. However, are these flocks stable so that the same birds always flock together? Or, are flocks just random collections of individuals from the population that happen to join in a flock for a meal? Below, a small flock of sparrows, two of which are banded:

Below: Another small feeding flock:

By studying color-banded individuals in a huge number of flock observations, we learned that the winter flocks are dynamic, often changing membership in the course of an hour or so. We then used ‘social network analysis’ methods to see if there were patterns to the flock dynamics. Dai Shizuka, now a professor at the University of Nebraska, did all of the network analysis and has become an expert in this area.

Below is an example of a social network from one year of our study. Each dot is an individual sparrow and the lines connecting pairs of dots represent flocks where the two birds were seen together in the same flock at least once. Line thickness indicates how often two birds flocked together, taking into account the total number of flocks they were seen in. Note that many birds are never seen together in a flock (e.g. pairs of dots not collected by a line). There are methods for determining whether social affiliations are random or whether there are clear social groups and couple of interesting patterns emerged from these types of analysis.

(1) The sparrows live in clear social communities. The individuals are color coded to show the four different communities. Flocks are not random gatherings but instead are mostly subsets of birds from the same communities.

(2) The communities are spatially distinct—different communities use different parts of the Arboretum, as seen on the right in the map of our study site that shows where the different colored communities occurred. Additionally, about 50% of the birds return after migrating to and from their breeding areas, and these birds almost invariably return to their same communities and socialize with any of their same buddies from the previous season that also survived. And they do this above and beyond levels predicted just by the fact that birds in the same communities tend to use the same areas. These birds are forming lifelong winter partnerships—best friends forever.

These findings indicate a surprising level of social complexity in the lives of these seemingly modest little sparrows—basic aspects of their ‘fission-fusion’ societies are similar to those described for some primates, including chimpanzees. However, unlike chimpanzees, we found no evidence that kinship plays any role in structuring the communities.

Whether our findings are unusual is unclear because so few species have been studied in the same level of detail. What is unusual though is the species’ diet—the sparrows eat a lot of grass, and they consume nectar as well. Herbivores (plant eaters) are usually large animals so it is a bit odd to have such a small animal eating grass. Perhaps their social organization is connected to their diet—a vegetarian diet may allow for high densities, which in turn would permit the formation of the communities we observed.

Below: Herbivores of the world unite—two sparrows and a rabbit all eating grass.

We are also studying winter song. The sparrows sing a lot in the fall—you know that fall has arrived on the California coast when you start hearing the golden-crowns singing. It is somewhat unusual for migratory birds to sing on their wintering grounds so we have been interested in figuring out who sings (not all individuals sing) and why. One possibility is that the singers are young males learning to sing—in a few species it is known that yearling males begin to practice their new songs in the fall. We rejected this hypothesis because it is mostly older birds that sing, not yearlings. Also we found that both males and females sing in winter, which is interesting because females do not sing in summer. Many birds sing to defend territories. Because the sparrows live in social groups, classic territory defense does not explain the singing, but defense of a group territory is a possibility. Song does appear related to social dominance because we found that singers are more socially dominant than would be expected based only on their plumage coloration.

One type of song interaction is particularly bizarre—two birds will sometimes have song duels that can last for hours. We call these events ‘tethering’ because two birds remain very close to each other while moving about, singing back and forth (typically <1 m), as if joined by an invisible tether.

Here is a video of two birds in a tether interaction:

We are not yet sure what these song duels are all about but we have a few possible explanations we are currently testing. We suspect that the singing might be connected to their interesting social organization, but that is mostly a guess at this point.

Finally, very recent advances in tracking technology allow us for the first time to figure out specifically where our birds breed, as well as their migration routes to and from the breeding areas. Just this year GPS tags became small enough to attach to a sparrow-sized birds (officially tags should not exceed 3% of the body mass).

Below: a GPS tag, weighing about 1 gram:

Below: The tags are attached to the bird with a backpack harness system with slightly stretchy loops that go around the legs. The tag itself lays concealed under the bird’s back feathers and the only evidence that the bird is tagged is a little antenna sticking out above the tail, as can be seen in the photo:

The tags can be programmed to take up to 80 very accurate GPS readings (<10 m) on times and dates of our choosing. It was fun trying to figure out the best dates to use for tracking migration routes and determine breeding areas. What is really astonishing is that with a 10-meter accuracy one could in principle get an estimate of breeding territory size (assuming the birds stick closely to their territories). The only downside is that we have to recover the tags to get the data— the tags get GPS information from satellites but do not themselves transmit any information to satellites. Based on our long-term survival rates, we can expect to get half of the tags we deployed back next fall. Here’s hoping that the birds have a great summer vacation!