If changes in bird populations are one barometer of the state of the natural environment then it would behoove us to better understand what factors influences the decline, stability, or rise of these populations. Although there have been many academic studies on this topic, here I draw on the work of Ian Newton who categorizes these into three major influences: behavior and density regulation, natural limiting factors, and human impacts.
The first category, regulation, includes the social dominance of each bird within a species whereby dominant individuals tend to have more resources, making them better able to survive. Bird populations are also limited by habitat and density influences. These include not only food availability but also the size of the territory claimed by each bird or nesting pairs. Increasingly, habitats become fragmented as a result of human activity. Thus, large flocks are forced to spread out to find other suitable environments, if they exist at all. Unfortunately, fragmented flocks often tend to suffer population decline since they no longer have the advantages of the large flock that aid survival such as keeping predators at bay.
The second category, limiting factors, focuses on food-supply. This not only influences the very young, which are more likely to die without adequate food, but also adults since a lack of food will reduce the number of newborns in the current or subsequent breeding seasons. Appropriate nest sites are also important, since they offer the most protection to the young of each respective specie. Predators (and parasites) are another critical influence on population size–fewer predators/parasites, fewer deaths. Again, losses here can affect adults, the young, or both. Weather, particularly large storms, can disrupt migration and breeding cycles. A good example here is superstorm Sandy in 2012 that occurred during the fall migration along the Atlantic flyway. We will not know of any adverse impacts by Sandy on birds until the results of the 2013 bird counts are in.
Then there is inter-specific competition, meaning that with limited resources species with identical needs cannot live together indefinitely in the same area. A good example of this are the Gulls on islands off the coast of Maine that displaced the Puffins, driving them nearly to extinction. The gulls prospered as a result of the Waterbird Protection Act, augmented by greater food supplies inadvertently supplied by fisherman, and human-made landfills.
The third category, human impacts, considers the impact of hunting and pest control, pesticides and pollutants, and extinction. Hunting in and of itself is not a problem so long as there are regulations limiting the game that can be taken, and when. However, pesticides and pollutants are a different matter. Even low-levels of pollutants can adversely affect bird populations over time. Pesticides, no matter how safe to animals and humans are designed to change the environment-either by eliminating plants or insects. Since these are food sources for area birds, pesticides have an adverse effect on birds even if these pesticides do not harm birds directly.
Another important factor is that each of these impacts do not act alone, that is, independently of the others. In most cases two or more of these factors act in concert, often resulting in adverse population effects that can be greater than either of the factors independently, increasing the likelihood of extinction. Quoting from Newton, ” Nest-sites, water, cover, predators or disease organisms can all restrict species to only parts of an area where food is available, while predators and parasites can reduce rates of food-intake and body condition, thereby enhancing the effects of food shortage. Alternatively, certain factors, such as increased food-supply or reduced predation, might enable species to increase to high density, which facilitate the spread of disease, . . .” (p. 375).
Extinction is certainly not a new phenomenon. What is new, however, is that although humans have always played some role in animal extinctions, the current extinction is far more human-driven than in the past. As Newton notes, “In the past 400 years alone, human action has eliminated 127 of the approximately 9672 modern species of birds, on average about one every four years” (p. 449). Another difference is that unlike the past when human impact on extinction was limited to over-hunting or the destruction of local habitats, with global warming our impact has dramatically increased as we are changing the entire earth environment, threatening all species to varying extents, including ourselves.