birds nest looking sad

[sansouci01]

I've had a birds nest coral for about 4 months. Looks very healthy. Very green. Lately the branches have started to look a little bare. Nothing has changed so far as water chemistry and lighting. alk is 9.2..cal is 420 no nitrates. mag is a little low at 1100. Could this be the problem?

 

[Sniperhank]

Hard to see, but looks like what happens with all birds nest. Shading from new top branches kill off the lower ones.