I think I found the answer, they do use their own eggs for energy when they don't spawn. Which is maybe why they don't need to feed so much. I found this on a forem online by a guy called crappiepappy. I think this is what has been happening to Bartlett
The fish don't "decide" when "they're ready" to spawn ... water temps decide it for them. Absorption of the eggs can & does occur when the water temps get too high before the fish have a chance at spawning (or finish spawning). This has been observed at Fish Hatcheries, according to biologist Dr Hal Schramm (Crappie Now article Apr 2013).
So technically it's not JUST "by the time the fish are ready to spawn" ... but, before/during/after attempts have been made & conditions have thwarted those attempts long enough for the water temps to get so high that the eggs wouldn't survive ... that the fish would absorb the eggs. Their survival instinct tells them that it's better to put that protein back into their own body, than to make futile attempts at dropping them in water that's too warm for much of a chance for survival of the offspring.
Now, they will keep trying ... and when conditions change quickly, it can devastate the spawn. It's just those times where things happen in just such a way that the water gets too hot, too quick, and the fish don't all respond to the narrow window of opportunity to spawn, that some may not spawn (or drop ALL their eggs during that period). Those eggs will be absorbed back into the fish, so as to not waste their food value (protein).
That potential ... for conditions to not be normal or optimal for spawning ... is one reason why most spawns take place over a period of several weeks, rather than just a few days. So many things can interrupt the spawn, so not all of them try to spawn at the same time or at the exact same water temp. The more interruptions, the more chance the water temps have of getting past the safe range, and keeping the fish from spawning or finishing their spawning ... those fish will absorb whatever eggs remain.