Why would a bird want to eat those pellets? Have you though of putting up a camera to see what kind of creature snitches your cakes?
My wife's theory is that it went for the insects that are attracted to the fertilizer. Doesn't make sense to me though.
A camera would be interesting (also for other purposes), but I don't have one.
Might have been a Jackdaw, got loads of 'm here. Maybe it was just curious (as it has happened only once up till now).