Hawks in Berkeley?

You are walking down a trail in a city park right here in the city of Berkeley. As it winds its way between huge boulders and beneath overhanging bushes, your attention is drawn up into a nearby tree by the sound of scratching. There, a squirrel is scrabbling up the trunk of a tree. As it ascends you notice a thick mat of sticks higher in the tree, way up near the tree’s crown. Is it the squirrel’s nest? The squirrel climbs higher, and then moves beneath the lower edge of the stick platform and then up around the side. As the squirrel begins to get on top of the mass of sticks, it is met with the oncoming rush of the beak and talons of a female Cooper’s Hawk. The mass of sticks is not squirrel’s home, it is the Hawk’s. The squirrel is in full retreat now, and the Cooper’s Hawk in full pursuit. The squirrel begins racing in the direction in which it can go the fastest, strait down the tree trunk. The Cooper’s Hawk immediately drops after it, skimming strait down along the bark of the tree. As the hawk gains, the squirrel jumps, letting go of the tree completely and free-falling into the bushes below. It is only then that the hawk gives up the chase and returns to her nest.

What you just witnessed was the defense of one of about a dozen Cooper’s Hawk nests in Berkeley and Albany. Cooper’s Hawks in Berkeley? That’s right. These roughly crow-sized predatory birds get along quite well with humans and the cities humans live in. They eat birds and small mammals which they often capture by ambushing their prey in dense vegetation and then chasing it down in a burst of speed. They are superbly adapted to this style of hunting with short rounded wings which give the speed while not getting hung up on foliage and a long tail which acts and a rudder allowing them to turn and twist through thick undergrowth and not lose their prey.

The ability to live well with humans is unusual for a predator. Most birds of prey are too sensitive to disturbance to live in such close contact with humans or need larger areas of open spaces in which to hunt than are available within such a human-dominated landscape as downtown Berkeley, but Cooper’s Hawks find a way. They are so tolerant in fact, that in Berkeley and Albany, more Cooper’s Hawk nests can be found per square mile than in any other recorded area in North America. They nest in mature trees in almost any kind of habitat from quiet city parks to busy city intersections. And Berkeley in not alone in being an urban area that has nesting Cooper’s Hawks. In California, most urban areas have populations. Outside of the state, populations that have been studied in some detail include urban areas of Arizona, Illinois, and Vancouver, Canada.

Beginning about 15 years ago, a group of dedicated volunteers have been keeping tabs each year on the levels of Cooper’s Hawk activity in Berkeley. The group, the Cooper’s Hawk Intensive Nest Survey (CHINS), has divided the 10.4 square miles of Berkeley into smaller units. Each volunteer takes responsibility for one of these units, and searches it to locate and track the presents of Cooper’s Hawks. Searching begins in late January when the hawks first arrive in the area back from their wintering grounds, and continue all through the breeding season until the adults and a new generation of young Cooper’s Hawks depart to return south in August. In between, many observations are made and recorded such as when the hawks arrive, when nest building begins and ends, how many young hatch and when, how many young fledge and when, what they are eating and how much, and finally when the adults and young disperse.

Some the information that CHINS has found is that while Cooper’s Hawks nest in greater density in Berkeley than elsewhere.Also, they are eating a lower number of species then in more rural areas, and of these fewer species a large portion (~33%) are Rock Pigeon, European Starling, and other species that are not native to California. On this limited diet, the hawks are able to breed just fine with most nests fledging 3-4 young each year.

The CHINS project is a branch of the Golden Gate Raptor Observatory, and both are examples of scientific studies made possible by volunteers from the community working to monitor and record raptor populations. Cooper’s Hawks, and birds of prey as a whole, can function as indicators of ecosystem health and stability. This makes them especially important and useful to study, and the fact that individuals from the community are able and willing act as citizen scientists and devote their time and energies to such an endeavor show both that such work can be done and that there is interest and support that such work be done.

To find out more about the CHINS Contact Allen Fish at (415) 331-0730 and find out how you can help protect your raptorial neighbors.

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Reduction in Gene Flow: Dietary Preferences

Speciation takes place when groups that were part of one species become reproductively isolated from each other.  Once the groups have become reproductively isolated from one another, speciation may result from each population becoming more and more adapted to their local environment (natural selection).  The gradual accumulation of random genetic mutations (a process known as drift) can also contribute to speciation, but at a much slower rate.   In the classic model of speciation, the process was only complete when no gene flow occurred between the divergent groups at all.  However, more recent research has shown that species can maintain their distinctness even when small numbers of hybridizations occur.

One way for two groups to become reproductively isolated from one another is by developing different dietary preferences.  This can happen when groups specialize on different parts of a resource (large versus small seeds, or insects that live on the outer tips of tree branches versus inner foliage near the trunk).  This results in individuals of different groups encountering one another only rarely simply because they are foraging in different habitats or different parts of the same habitat.  Diet-driven habitat isolation is different from the patterns of spatial separation I covered in my last post because it is not an geographical accident or some inherent physiological tolerance that is separating members of the two groups.

Dietary preferences can arise by mutation or in response to competition.  Favorable mutations may allow one group to utilize a whole new type of food which opens new habitats for that group to evolve into.  Such dietary innovations can lead to the evolution of different morphological features that further aid in the use the new resource.  These different morphologies can in turn lead to further reproductive isolation, and this process can become a self-reinforcing cycle.  This cycle is also supported by hybrids frequently having morphologies that are intermediate between the two groups.  These intermediate morphologies will likely be inferior to either of the groups and will result in hybrids leaving fewer, or no, offspring.  Natural selection will then favor adaptations to avoid hybrid matings because these matings will be wasted reproductive effort.  Competition can lead to differing dietary preferences by favoring the individuals in a population that are best suited to utilizing the extremes of a resource.  This can occur when competition for food resources is high.  Then if one group is able to utilize one end of a continuum and another group is able to utilize the other end, these populations may become favored because they avoid much of the competition.  Again, hybrids may do poorly because of intermediate morphologies that have evolved in response to the extreme ends of the food continuum, and also because they may have to deal with stronger competition from the other group.

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