Final answer:
The largest number of birds are killed by windows and cats, not windmills or savage bears. Making windows visible to birds and managing cat populations can protect birds.
Step-by-step explanation:
The largest number of birds are killed each year by windows and cats. While not the options provided, these two factors have a significant impact on bird populations. Studies have shown that glass windows can be deadly for birds as they often cannot perceive the transparent barriers, leading to collisions. Domestic and feral cats are natural predators and are responsible for a significant number of bird deaths due to their instinctive hunting behaviors. It is important to note that while windmills (or wind turbines as they are more commonly known) do contribute to some bird deaths, the number is relatively small when compared to the fatalities caused by windows and cats. Measures to protect bird populations involve making windows more visible to birds and managing cat populations, especially keeping pets indoors or supervised when outside.