As with many things in nature
Seems there is a bit of controversy regarding Purple Martins and their appetites. My wife has always believed that they were general insectivores and has gone out of her way to cultivate habitat for them and swallows. We have very few mosquitoes near our house even though it is next to a lovely draw that's full of them in the evening. Each late afternoon and evening we enjoy an aerial show of daring and skill as these guys go on the hunt. After reading these latest posts I did a literature search and it appears that the diet of martins is greatly affected by environment and prey species availability. In other words, your martins and swallows may not eat many mosquitoes if there are is a tree canopy and/or structures in the way and there are larger insects readily available. Ours seem to make a dent in our mosquito population.
Bottom line: As with so many aspects of the natural world, we don't have a freakin' clue as to much of what is really going on. Yet we continue to meddle and act like we do. Thanks for the prod to get me to dive into the literature. Sometimes I get lazy.
I arise in the morning torn between a desire to save the world and a desire to savor it. Makes it hard to plan the day.