Signs of Winter 6: Birds in Urban Ecosystems

Photo by D. Sillman

Photo by D. Sillman

I have talked about biodiversity before. In fact, a couple of years ago I included a biodiversity quiz in one of my weekly blogs. For the past twenty years Deborah and I have been studying and describing the biodiversity of Western Pennsylvania especially as it relates to human uses of our area’s ecosystems. The Nature Trail at Penn State New Kensington (and its on-line counterpart, The Virtual Nature Trail), our 2010 Baker Trail hike, and our more recent wildflower and bluebird studies in Harrison Hills Park all have a common biodiversity theme.

Why is biodiversity important? What is its purpose, what is its “use?” There are many answers to that question. Diverse ecosystems tolerate disturbance and stress better than simple ecosystems. An ecosystem with a rich biodiversity will house the vital pollinating species that we need not only for our wildflowers but also for our food crops. A diverse ecosystem will recycle nutrients, generate oxygen, absorb carbon dioxide, and clean up our water and air. A diverse ecosystem is full of species that have unique genes and proteins that may be the tools that we need for our ultimate survival on this rapidly changing planet.

Most of all, though, a diverse ecosystem fills us with joy and awe and connects us to the continuity of life and living on Earth.

The meaning of “biodiversity” can be expressed in many ways, but I especially like this very broad definition: “the variety of life in a habitat, ecosystem, or the world.”

Photo (London) by Kloniwotski, Flickr

Photo (London) by Kloniwotski, Flickr

More than half of the Earth’s 7.3 billion people live in cities or towns. These urban and suburban ecosystems seem the antitheses of biodiversity. The great structural and energetic and goal-directional similarities of a city anywhere in the world would seem to drive these ecosystems to the lowest common denominator of plant and animal species diversity. One standard hypothesis often applied to urban ecosystems looks at the similarities of their physical habitats and uses and infers that they must favor a limited set of species that are hardy and extremely generalized in their food and habitat preferences. Cities, according to this hypothesis, should all have very similar biotic communities. This idea, though, was not supported by recently collected data.

Looking at the plants and bird species of 110 cities around the world a study published in the Proceedings of the Royal Society B in February of 2014 emphasized that each urban area retained a unique set of endemic (“native”) plant and bird species. These species combined with a number of shared, “urban-tolerant” species generated unique biodiversity profiles for each urban area. Cities, of course, had fewer plant and animal species than surrounding “natural” areas, but cities were shown to support nearly 20% of the world’s bird species and 5% of the world’s plant species. Urban areas were far from the “biological deserts” that they have often been hypothesized to be.

Photo by D. Sillman

Photo by D. Sillman

Birds in urban ecosystems have always been important to me. When I was in second grade I was given a Golden Guide to the Birds (and I still have it!). I frequently sat out in the backyard of my suburban community in northern Ohio with a folding pair of opera glasses (my “binoculars”) and tried to identify every bird that I saw. It was far too easy: pigeons, house sparrows, starlings, and blue jays with an occasional robin to liven up the mix. The bird I really wanted to see was a scarlet tanager (red was at that time my favorite color), but I had to wait almost 30 years before I finally saw one out on the Penn State New Kensington Nature Trail!

When we moved to Houston (more suburbs!) the species changed a bit: blue jays (aren’t they everywhere?), grackles, and purple martins were very common. I am sure that there were pigeons and house sparrows, too, but my mind was on cars, girls, music and basketball for quite a few years and I didn’t really get back to birds until I took an ornithology class in my junior year at Texas Tech.

Bahia Blanca (Photo by K. Langlors, Wikimedia Commons)

Bahia Blanca (Photo by K. Langlors, Wikimedia Commons)

Martina Carrete, an ecologist at the Pablo de Olavide University in Seville, Spain, studies the behavioral and physiological characteristics of bird species that colonize urban habitats. In 2011 she published a paper describing the flight initiation distances (FID) of various bird species that lived both in the Argentinian coastal city of Bahia Blanca and also in the surrounding rural areas. The FID of a bird is basically measured by seeing how closely you can approach an individual before it flies away. Dr. Carrete and her team found that the average FID’s of populations of rural bird species were not reliable predictors as to whether that species would also be found in the city. What was a very strong predictor, though, was the variance in a species’ average FID values. In other words, if a species had some individuals with very low FID values (i.e. showed less fear of approaching humans) regardless of the population’s overall, average FID, those individuals would be likely to be able to survive in a human crowded, urban environment. If a species, though, did not have a sufficiently broad range of FID values that included some human-tolerant individuals, that species would not be able to colonize densely crowded human environments. Carret found that about half of the bird species in the area around Bahia Blanca had broad enough ranges of FID values to colonize urban habitats.

Further, Dr. Carrete’s team found that an individual bird’s FID remained the same throughout its life. Birds that were afraid of humans (i.e. that had high FID values) stayed afraid of humans, and birds not afraid of humans remained that way. These observations suggest a possible hardwired, genetic predisposition of some individuals in a population to urban existence.

Photo by A. Vernon. Wikimedia Commons

Photo by A. Vernon. Wikimedia Commons

A more recent paper by Dr. Carrete looked into the levels of stress hormones in burrowing owls (Athene cunicularia) in and around Bahia Blanca. Burrowing owls are found in both rural and urban habitats, and their personal records of stress hormone levels are easily determined from discarded (or directly harvested) feathers. After measuring their stress hormone levels, the birds were then observed for several years in order to determine their individual survival rates. The results of this study were quite interesting: rural and urban burrowing owls had the same average levels of stress hormones, and birds with intermediate stress hormone levels were more likely to survive in the urban environment. This second finding suggested that high levels of stress hormones could be physiologically harmful to an individual and that low levels of stress hormones might make the bird less reactive to potential environmental dangers and thus lead to its early demise in complex urban ecosystems.

A second study on these burrowing owls looked at blood levels of stress hormones during capture and over the minutes and hours after capture. Carret’s team found that urban burrowing owls spiked stress hormones upon capture but very quickly returned to normal stress hormone levels. Rural burrowing owls, though, spiked stress hormone upon capture and maintained high levels for a much longer time. These data also suggest a physiological (genetic) foundation for urban dwelling individuals.

So birds that we see in the city may be individuals particularly adapted to the stresses of an urban ecosystem. Reduced fear of humans (people are, after all. the common factor in all cities) and the ability to react to and recover from stresses appropriately allow these individuals to utilize (and enrich!) this rapidly growing section of our biosphere.

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