Could anyone explain to me why these birds cruise at diffeerent altitudes on different days? It used to be said that, if they were flying low, it was a sign that rain was to be expected. Do they catch different insects at different altitudes or are the insects they prefer to be found at differnt altitudes in different weather conditions?
I think you've basically answered your own question , they go where the insects happen to be. Swifts , being the biggest strongest fastest of the three do tend to go highest of all in search of food.
Galatas is right. But sometimes you see generalisations in print about these habits. For instance, "swifts feed at high altitude". The fact is swifts will feed at any altitude that yields the best crop of flies. If for instance you see swifts swallows and martins feeding together over open water or a river, they will often be at the same height, fairly low, because that's where the flies are. I have often seen swifts feeding below rooftop level in suburban areas too.