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Birds and Landscape Changes in Northeastern Forests

The past four centuries have brought to the landscape of the Northeast a series of changes of a magnitude and rapidity that has few precedents on Earth. Before European settlement, this region was a mosaic of open old-growth forests, shifting agriculture, and fire-maintained grasslands and savannahs. Following European contact, disease decimated Native American populations, and much of the unsettled interior became wooded. By the mid-1800's agriculture, the demand for wooden fencing, charcoal, tanning, and fuel for households, and iron and lime industries created a landscape that was devoid of all but scattered trees. This century has seen the return of forests throughout much of the region.

   

Such sweeping changes bring with them changes in bird communities. Clear preferences by many bird species for forested environments should mean that their populations wax and wane with changes in the composition, age, and distribution of forested lands. Given these predilections, it should be possible to use information on changes in bird populations as one gauge of the effects humans have had on the landscape.

   

Birds are connected to their environment in a direct and uncompromising manner. They have no buffer against regional changes in food, cover, predators, or landscapes. Marketing factors and government social welfare programs cannot compensate them if their environment deteriorates. If a site changes in such a way that the locale lacks what it takes to support their needs, they must leave or die. Consequently, the distribution, abundance, and changes in bird populations are a direct statement of the quality and suitability of a region to support birds.

   

Two large data sets are available for investigating changes in bird populations in the Northeast: the Christmas Bird Count, begun in 1900 (Butcher 1990), and the U.S. Geological Survey's North American Breeding Bird Survey, begun in 1966 (Peterjohn 1994; Price et al. 1995). Data from the early years of the Christmas Bird Count have not yet been converted to electronic format, but Christmas Bird Count data from 1959 to 1988 and Breeding Bird Survey data from 1966 to 1994 are available. Trends in bird populations from these data provide the means to examine the relative welfare of guilds of forest birds (Robbins et al. 1989).

   

The table presents a state-by-state breakdown of the numbers of bird species increasing and decreasing, organized by several categories or guilds (Droege and Sauer 1989). Results for birds inhabiting mature forests show different patterns. Summer patterns present a mix of regions where increases and decreases predominate in different conditions, whereas winter populations show a pattern of uniform increases (Fig. 1a, b). Bird species composition shifts in forests with the seasons--some birds fly to the tropics, others shift farther south in the United States, and some are permanent residents.

Table. Summary, by season, state, and guild, of increasing and decreasing numbers of species of birds based on data from the North American Breeding Bird Survey (1966­1994) and Christmas Bird Counts (1959­1988).
Breeding season
State Mature forest
 
Scrub
 
Neotropical migrant
Permanent resident
Short-distance migrant
  Decrease Increase Decrease Increase Decrease Increase Decrease Increase Decrease Increase
Maine 11 19 9 6 6 12 1 2 4 5
New Hampshire 11 17 8 3 10 7 0 2 1 8
Vermont 11 8 9 3 6 4 2 1 3 3
Massachusetts 10 10 9 2 8 5 1 1 1 4
Connecticut 12 5 10 4 9 3 1 1 2 4
New York 14 20 7 6 11 7 0 3 3 0
Pennsylvania 14 17 11 4 7 10 3 2 4 9
New Jersey 9 4 13 2 7 2 1 0 1 4
Maryland 8 18 8 2 8 9 0 4 0 1
West Virginia 12 7 9 4 11 3 1 2 0 4
Virginia 12 13 9 1 9 7 1 4 1 3
 

 

Wintering season
State Mature forest
Scrub
Permanent resident
Short-distance migrant
  Decrease Increase Decrease Increase Decrease Increase Decrease Increase
Maine 6 14 6 3 1 4 2 8
New Hampshire 5 10 3 2 1 3 2 6
Vermont 1 9 0 4 0 3 0 6
Massachusetts 10 15 7 8 2 3 5 9
Connecticut 10 13 6 6 2 3 7 7
New York 10 18 8 5 2 4 5 10
Pennsylvania 6 20 7 7 1 5 2 13
New Jersey 10 16 10 5 2 3 4 11
Maryland 7 13 9 3 0 3 6 9
West Virginia 0 15 9 3 3 2 3 12
Virginia 5 21 10 4 2 5 2 14
 
       

If the overall results are broken down along lines of residency status, Neotropical migrants (Fig. 1c) are declining in more states than they are increasing. Short- distance migrants and permanent residents show the opposite pattern (Fig. 1d,e,f,g). Winter patterns for short-distance migrants and permanent residents both show increasing numbers of species in all states. Clearly the difference between the initial summer and winter results comes from the greater number of declines in Neotropical migrants.

 


Fig. 1a-i. Regions of overall increase or decrease for wintering and breeding birds (by state). States colored yellow have more species declining. States colored blue have more species increasing. Breakdowns are given for a, b) the collective sum of all forest birds, c) Neotropical migrants (there are no wintering estimates), d, e) short-distance migrants, f, g) permanent residents, and h, i) scrub-inhabiting birds.

The patterns and causes of changes in Neotropical migrants are a matter of much recent concern and speculation (James et al. 1996). The complexity of their migration, the diversity in their winter and summer life histories, and their great range in geographic and habitat locations in the winter make it difficult to separate out many competing hypotheses as to the causes of these patterns. Data (Table) indicate, though, that populations of Neotropical migrants, as a guild, are experiencing greater negative changes than their less- traveled, forest-dwelling neighbors.

   

Excluding the complex case of forest-dwelling Neotropical migrants, a clear pattern is evident of increases in forest-dwelling birds during the past 30 years. These patterns coincide with patterns of forest change revealed in the U.S. Forest Service Forest Inventory Program. Forest acreage, particularly of mature forests, has increased throughout the Northeast during this time, though increases in acreage have recently plateaued or declined slightly in some regions (Powell et al. 1994). There has also been a shift from pioneer plant species and those encouraged by regular fires, such as Virginia pine, black locust, and oaks, toward species that regenerate quickly following forest cutting, such as red maple, sugar maple, and yellow-poplar (U.S. Forest Service 1995).

   

Such patterns of forest change are the indirect result of the exploitation of rich prairie agricultural lands, the loss of local markets for agricultural goods through decreased transportation costs, and subsequent declines in farming of the rocky uplands of much of the Northeast. Over the past 125 years these lands have slowly returned to forest cover. The fact that this process may have stabilized and that the rate of such change has declined is illustrated by the decline of bird species inhabiting early successional forests (scrub; see Table).

   

Both summer and winter data sets reveal that far greater numbers of scrub-nesting species are decreasing rather than increasing (Fig. 1h,i). Furthermore, the species showing most of the increases are species widely adapted to mechanized agriculture and suburban habitats, such as the mourning dove and the northern cardinal. Land-use patterns in the Northeast are likely to further decrease the number of successional habitats present. Already, Bewick's wren, Bachman's sparrow, and the lark sparrow, all inhabitants of sparsely wooded scrub areas and widely distributed until the middle of this century, have declined to the point that they are locally extirpated or that only a handful of individuals remain. Other species may soon follow.

   

Changing bird populations reflect the Northeast's changing landscape. Species differ in their responses to landscape change, each having unique preferences for the habitat architecture of their surroundings. By tracking changes in bird populations and other species, we can make statements and develop hypotheses regarding the health and future of our environment. Such measures cannot be found through inspection of our gross national product or consumer price indices. We must listen to what these changes tell us about our effects on the lands we share. By distancing ourselves from such information, we risk making the present round of changes irreversible.

   
  Author
Sam Droege
U.S. Geological Survey
Biological Resources Division
Patuxent Wildlife Research Center
12100 Beech Forest Road
Laurel, Maryland 20708

References


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