Final answer:
The most significant decrease in carrying capacity for Bohemian waxwings occurs when humans bulldoze the forest for development, causing direct habitat destruction which is a severe threat to the species.
Step-by-step explanation:
The carrying capacity for Bohemian waxwings living in a tall coniferous forest would decrease most significantly when humans bulldoze the forest to build houses. Habitat destruction has a profound impact on the ecosystem, leading to a loss of shelter and food resources for the birds. While waxwings, like other temperate-climate species, can sometimes adapt to harsh weather conditions like cold winters, and though interspecific competition for resources and natural events can also influence carrying capacity, the direct destruction of their habitat is likely the most severe threat.
Likewise, historical evidence has shown that sustained habitat destruction and human-driven landscape alteration have had significant effects on wildlife population dynamics. Therefore, the direct removal of the forest ecosystem for development poses the greatest risk to the waxwings carrying capacity because it eliminates essential components of their habitat.