The conversion of forests to cropland, pasture and urban sprawl, the downgrading of virgin stands to second growth, and the conversion of mixed forests to pine monocultures The inescapable implication of this for conservation is that there is only a limited amount of time left in which to slow human population growth and to institute other fundamental changes in the countries of this hemisphere or many of our migratory birds will be little more than memories. After this has happened, there is really not much that can be done to salvage winter habitat for migratory birds. As man's development of the planet continues, available habitat for animals and plants is being carved up into smaller pieces. The fragmentation of ecosystems separates populations genetically from each other, and if a particular habitat is not large enough, remnant populations contained within them are doomed due to genetic inbreeding. If there are not enough large areas, chance occurrences such as an extremely harsh winter, floods, localized disease, etc., can drive remaining populations to the brink of extinction.
Some species are dependent on environmental policy in more than one place.
In some cases other birds have been responsible for the demise of some bird species. Kirtland's warbler, already compressed into a small remaining jack-pine country in Michigan, was subjected to nest parasitism by the brown headed cowbird laying its eggs in their nests. The baby cowbirds push the Kirtland's own young from the nest and are then raised by these hapless birds. The European starling has spread across the United States since its introduction in the early 1900's, depriving many of our resident and less aggressive birds of habitat. In the words of Pogo, 'We have met the enemy and he is us'.