Last winter was mild enough to allow most bird species to forage naturally, which explains why the proportion of birds visiting feeders was much lower than usual.
Conclusion
Last year's mild winter is responsible for this year's larger-than-usual bird population.
Evidence
The mild winter allowed many species to stay in their summer range all winter, rather than migrating (and a bunch of them never make it home when they migrate).
The mild winter allowed them forage naturally, rather than visit bird feeders.
Evaluation
There is causality in the evidence and the conclusion. We know that mild winter caused two things, from the evidence: forage naturally rather than visit birdfeeders, and stay local instead of migrating.
In order to say that the mild winter caused a bigger than average bird population, we want to argue that either / both of the effects we are told about in the evidence would result in a bigger than average bird population.
We know that when birds migrate over the winter, there's a typical rate of attrition (attrition normally means people dropping out or dropping dead). So if 1000 birds head south, maybe only 800 come back. But how does that compare to when birds say local over the winter? Does that lead to more of them surviving / dying?
We know that because local birds were able to forage, they didn't have to use birdfeeders. But what does that have to do with a larger than average bird population? Do more / fewer birds survive when they're foraging naturally vs. using birdfeeders?
Goal
We either want to hear that not migrating results in a net gain for bird population, or that foraging naturally rather than using birdfeeders results in a net gain for bird population.