Change is one of the constants in life, and as the Massachusetts landscape has been altered by development in recent decades, the bird population has been transformed dramatically.



With the loss of farmland and the gain in forestland in the state since 1970, many birds that nest in meadows, such as eastern meadowlarks, are vanishing in the commonwealth, while many of those that nest in forests, such as Cooper's hawks, have been on the rise, according to a recent report, "State of the Birds," by the Massachusetts Audubon Society.



"However, we're getting a lot right in Massachusetts," said Joan M. Walsh, one of the principal authors of the report.



"We have nearly as much protected land as developed land in the state. And nearly 70 percent of our bird species are stable or increasing. Yes, there's no doubt there are challenges ahead, but I think we can come out on the good side of them," she said.



The report was based on more than four decades of data from three periodic birding surveys in which nearly 300 species have been followed, including wintering birds that do not nest in the state.



The report notes that the populations of a third or more of the species studied are declining, some precipitously. Habitat loss was the single biggest factor in the declines.



Roughly 400,000 acres of cropland and pastures could be found in the state in 1971, but nearly 150,000 of those acres now have other uses, according to the report. About 75,000 acres were developed over the period, but 71,400 acres reverted naturally to forests.



The bird populations of Western Massachusetts have not been as affected as those of other regions because the change in habitat, such as development, has not been as pronounced as in eastern and central Massachusetts, according to Walsh.



The course of climate change will likely determine future changes to the state's bird population, the report suggests.



If year-round temperatures continue to warm in the Northeast as they have (about a half-degree warmer per decade on average since 1970), for instance, birds with a more southern range will move in to the state while others, which prefer colder conditions, will move out.



In a letter introducing the report, famed Harvard biologist E.O. Wilson, a two-time Pulitzer Prize winner, said one of the "unmistakable markers" of climate change already is that "many historically southern species are now permanent residents."



For instance, the Carolina wren and red-bellied woodpecker, primarily Southern birds, are becoming more common in Massachusetts.



By 2100, according to the report, "Winters in the Northeast U.S. may be 5 to 12 degrees Fahrenheit warmer and summers 3 to 14 degrees Fahrenheit warmer, giving Boston a climate similar to that of Baltimore, Maryland, or Charleston (S.C.)."



The report also notes that anticipated changes in vegetation and insects, due to warming temperatures, will affect birds dependent on them.



"Future projections suggest that the oak-pine forests that predominate in central and eastern Massachusetts will completely cover all of Massachusetts in the future," the report said.



"The northern hardwood forests - dominated by sugar maple, American beech and yellow birch, common in central and Western Massachusetts - are also predicted to disappear," it said.