Thursday, December 22, 2011


A weak but negative relationship exists between primary forest productivity and the number of bird species that inhabit forests.

A stronger, positive winter relationship exists between temperature (greater elevation and latitude equals lower average temperature) and the number of species that inhabit forests.

The U. S. Geological Survey is collaborating with Bird Conservation Research, Inc. as we continue to analyze the year-round distributions of southern New England’s forest birds. USGS is providing technical assistance in the field of satellite imagery.

We have already demonstrated that a significant winter relationship exists between forest bird diversity and winter energy availability (see above) in that our region’s birds move to warmer, coastal forests in winter. This is likely because birds can live near the coast for fewer calories/day. In winter, when food is limited, reducing energy costs translates into a greater probability of survival. Other of our analyses demonstrate that particularly our wintering permanent resident species congregate toward the coast.

Despite such a relationship, summer average temperatures (as measured by elevation and latitude) exhibits little relation to where birds are found. This is likely because during summer, energy in the form of forest productivity becomes available. This productivity is ultimately what produces the insects, fruits and seeds that birds rely on for food.  

Measuring the productivity of forests is possible with satellite imagery. Satellites routinely make a measurement called the normalized difference vegetative index, or NDVI, which is a measure of the relative greenness of forests. Research has shown that forest greenness serves as a measure of primary productivity.  NDVI data are gathered by the U. S. Geological Survey, so we have teamed with them to seek relationships between their data and our large scale surveys of bird distributions. 

To date, our findings have not been consistent with expected trends.  As noted, theory predicts that NDVI should relate positively to the number of bird species. Instead, our data show a weak negative relationship between NDVI and species (see above). The reason for this pattern is the subject of ongoing investigations.


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