Sunbirds

Over the past 10 years we have noticed a real decline in the numbers of certain sunbird species in our garden in Karen, and visiting our feeder. I can remember there being a constant stream of birds to the feeder a decade ago; amethyst, variable and collared being the most numerous, with bronze, scarlet chested, northern double collared and green headed less numerous but always present.

Now we only have occasional visits from amethysts and rarely see bronze or scarlet chested individuals and I haven't seen the bossy green headed for over a year. Variables are still numerous (maybe even more so in-fact - could this be due to less competition from other species??) and in fact there are pairs nesting everywhere in the garden. Collared are also still present regularly. The surrounding habitat has not changed much and our garden is full of flowering shrubs and aloes. We did change to glucose instead of sugar in the water a few years ago so maybe that could be contributing?? Or is it just a natural trough in the population?