I have heard of a decline in monarch butterflies, some blame wasps as per this link.
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monarchbutterflygarden.net]
Unfortunately, although we used to see them in abundance at our place there seem to be far fewer. We haven't had swan plants growing in recent years but if it would help we would plant lots of them. One garden outlet was giving them away at one time.
We have a bottle brush that continues to flower even through the winter and that is where we mostly see them.
Along with birds in the morning and lots of honey bees in the warmer part of the day.