Give it another month (could be sparrows still feeding young and they believe it or not are a 'red listed' species!)
then you need something to fit that gap - a suitably cut bit of 2x3 might slide in their nicely, bit of paint - jobs a good'n
'red listed species' and Sparrows are on that list and could still be feeding young in early august
As suggested, a bit of wood lodged into the gap, maybe with some sealant around it, will stop birds, and insects getting in again.
But how did the gap arise? Is the plastic cracking up and this bit just dropped out, or was the gap carelessly left by the original installer?
If the former you may need to inspect all the plastic for further cracks.
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