There was a thread on here a while ago, and it was about crops that were like that. They'd empty out, but the birds gorged all the time that the crop always was HUGE.
About the protien though... Add some taste of the wild cat food. it runs about 36% protien. If it's too big for the serema to eat, if you have a bean grinder, it'll break it down enough. I use the TOTW in the hot days of summer so the birds get enough protien when they really don't want to eat...
I used to feed that to my cat, but she always got hairballs on it, so had to switch back to Wellness which is also 36%. It is their Salmon formula and has Salmon, Salmon Meal and Whitefish as the top three ingrediants - think that should be okay for them?