Here is a photo of one of my brood frames. They all seem to be like this on all my hives, with the comb not extending to the bottom. The cells curve around as they approach the bottom bar. The foundation I initially put on has been chewed away in that area.
Does anyone know why the bees do this?
Thanks.
They’re just not inclined to go that extra 10% are they!.. 
I guess the obvious thing would be ventilation. Remind us of your set up, reduced entrance? Top vents?
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Hi Dan, I believe that the bees do this so that they have the room to circulate air around the hive during the process of air conditioning the hive.
I think that a bigger bee space beneath the frame could eliminate the need for the bees to leave that gap above the bottom bar.
That is the good thing about wax foundation vs plastic foundation, the bees are able to tailor make their own air flow gaps.
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Our bees sometimes do that, and we run “foundationless” hives. Sometimes they leave spaces along the sides too. I’ve also read that bees communicate through vibrations, so with (naturally drawn) wax frames there is no interference from plastic foundation.
Nothing to worry about, they are just remodeling to suit their needs.
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I believe the honey supers, as a contrast to the brood, are often combed right to the bottom, with just some “air” holes. I have a feeling that they still do this on the bottom of brood frames with two deep brood boxes. I wonder if another reason they do this so they have somewhere to make swarm cells if they need them at some future stage?
I wouldn’t discount this theory, but just from my limited experience and observations, they still did it on my deep frames that I set up in two ideal supers (so there just happened to be extra space at the bottom under the frames if you can picture what I mean).
Very interesting point regarding plastic foundation @JeffH .
If you have a solid floor they are more likely draw right down to the bottom
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That’s very interesting @Dee. I’ve converted the floors to solids after a season and a half of trying screened. Based on your observation there, it seems the bees need to leave a lot more open space for air to move, the more open the hive is to the air?
Here’s a shot from my deadout last year, just thought the hole smack in the middle of this brood frame was interesting. There were several other frames like that, with the holes lining up.
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I think it’s just warmer.
I’m taking another look at that frame. The bees still have a LOT of cells left to raise workers in. Without counting them, you’d have to guess around 5,000 cells.
If the bees wanted to, there is plenty of room with those gaps for the workers to build drone comb & raise drones. Another argument for wax vs plastic.
Hi Skeggley-
Sorry I omitted to answer this. I have mainly standard entrances and solid bottoms and no extra ventilation at the moment.
In the past I have had a variety of ventilation arrangements, ranging from from two holes at the front and back, to no holes at all. I seems to make no difference at all to the bees reducing the comb depth on their brood frames.
The frame this photo was taken from (I think) is a hive that has the “ideal” 15cm2 opening, and no top ventilation and a solid floor.
I had a hive last year that was all “ideal supers” (so the brood is in the ideals - quite common here), and all of the brood areas on the ideals had the same reduced comb depth. That hive had a solid bottom and ventilation holes at the top. If you can imagine brood in ideals, and then the comb not extending down on the frames, they had created heaps of extra room in the hive. I can only guess ventilation too, but am not absolutely convinced we really know.
Hey @skeggley, I think he needs a salted rat! What do you think?? 
For everybody who doesn’t know what we are talking about, that is what it was called when Skeggs asked for one of these in Australia:
http://www.brushymountainbeefarm.com/8-Frame-Slatted-Rack
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The two holes I mention " in the front and back" were in the migratory lid.
Good point Dawn, I suppose when those frames are in position they mimic a salted rat, which are still not available here. I really need to make one up and try it.
@Dan2, so why use ideals for brood?
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Probably to avoid of the extra inconvenience of having different sized frames in the hive.
Hi Dan, it recently occurred to me that one way to eliminate the large gaps above the bottom bar of the brood frames is to allow the bees to draw the comb out in the honey super first. The bees will do a better job of building new comb in the honey super than what they sometimes build in the brood box.
After you gently harvest the honey, you’ll finish up with a beautiful frame of fully drawn (top to bottom) new comb ready to place into the brood. I’m doing a bit of that myself lately instead of fresh foundation.
I find that bees can be very picky with fresh foundation in the brood, especially if any wire is exposed. The bees will sometimes chew holes around it.
Another reason for the large gap above the bottom bar is probably because the bees start building from the top down. Once they reach the lower level of the bee mass, they don’t build any lower, because that would leave that brood exposed. Therefore they just chew the unused foundation away, leaving a convenient gap.
By using fully drawn combs in the brood, they may use the whole frame for brood, especially if more than one are used side by side.
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Thanks Jeff…
A beaut explanation.
I just need to cycle out those deep brood frames and use exclusively ideals. it is a difficult and long process actually.
You’re welcome Dan, thank YOU.