Introduction Page 11


Long-term research results

 

The graph to the right contains bird abundance from 1969-1998. This graph shows the average number of birds per 10 hectares in a section of the HBEF.

What does the graph suggest about the number of birds present in the HBEF? Does it appear that bird abundance has increased or decreased since 1969? What could be some reasons for this apparent trend? Do you think that all species are following this trend, or could some be following a different trend?

As it turns out, if you were to examine trends for individual species (as opposed to all the birds species found in the HBEF, shown in this graph), you would find that some show strong declines, others are stable, and the rest show increases in abundance. That is, not all species are doing the same thing.

The most probable cause for the species that have declined is that the forest has changed over time, has become less suitable for those species, and they have moved elsewhere. Different bird species prefer different types of habitats. For example, some occur primarily in open areas, others in very young forests, and others in older, more mature forests.

American Redstart

Therefore, as forests age, some bird species leave, while others come in and become abundant. This is what we are mostly seeing at the HBEF, and is the best explanation for population declines of Least Flycatchers and American Redstarts.

It is also possible for some species that conditions on their winter grounds in the tropics or at places along their migratory routes may have been altered, resulting in higher mortality and thus population decline. This is possible for a species like the Wood thrush.

Avian ecologists (scientists who study the ecology of birds) at the HBEF have been studying these and other patterns for over 30 years. In fact, it is only because scientists have been asking these types of questions that we have such an excellent record of bird abundance. These scientists, their students, and their technicians have gone out every summer since 1969 to monitor populations of over 30 bird species.

This involves waking up every morning well before dawn and traveling to the far reaches of the HBEF to listen for, look for, and ultimately count all the different birds seen in certain areas. This work involves a tremendous amount of effort, but it can be a lot of fun.

Using these data, scientists are learning more about what sorts of natural or human influences affect bird populations, and why. This is an excellent example of one of the many benefits of long-term monitoring.

To take a quick tour of other long-term results, click here. You will learn about:

          • Snowpack depths
          • Forest floor lead concentrations
          • Tree biomass, density, and basal area
          • Precipitation amounts

    There are many components of the forest at the HBEF that researchers and technicians monitor daily, weekly, monthly, or annually over the long-term. You can read about these on the next page.


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