This isn't original - there do seem to be a few models out there, but they cost about £50 each. But has anyone tried to build their own?
The idea is to have 2 concentric tubes. The inner one is a standard birdseed holder, with an access hole for the _little_ burdies. The outer one is somehow spring loaded and has the perch attached to it. When a large bird lands on the perch its weight pulls the outer sleeve down to block the seed access hole. Same principle if a skwrrl tries climbing down the outer sleeve from above - they're too heavy and it closes.
If a small, light bird lands on the perch, it's not heavy enough to draw the outer sleeve down to cover the food hole, so it can chomp away.
The key seems to be the spring mechanism. Ideally, you'd want its sensitivity to be adjustable (or the oouter sleeve to have some adjustable weights attached). That way you can set the max. weight of bird that you want to allow to feed.
So, has anyone tried this? What obvious or non-obvious drawbacks are there? How will it hold up to weather (I can imaging the outer sleeve getting frozen "up" in frosty conditions). What's agood, reliable, long-lasting spring mechanism to withstand outdoor use?