The old-time advice is that turning is necessary for distributing the nutrients and preventing malpositions. Some say twice a day proves to be enough and we get a decent hatch.
It's been observed with time-lapse cameras that a hen fidgeted and turned her fresh batch of eggs an unbelievable 300-something times in a 24 hour period. Using artificial eggs that record movement, shows them being turned about 50 times a day, an average of every 35 minutes. But these eggs are artificial and lifeless. You might wonder why I am making this distinction?

My take on the hen's instinctive egg-turning is that her hot brood patch gives her discomfort, equivalent to menopausal hot flushes, and to soothe that discomfort, she constantly brings the cold bottoms of the eggs up against her skin and brings the cold outer eggs in, all instinctively. I propose that it may also be behind the reason we see her picking her breast feathers as she prepares to go broody and her patch heats up. And as the eggs gain metabolic heat from about Day 10 onwards, the intervals between her turning them become
increasingly longer. Note that these are only my original opinions and observations.

In addition to the mechanical egg-turning, there are many internal manoeuvres when the embryo repositions itself as it develops. The mechanical turning helps these and prevents later problems with malpositions. The primitive embryo first lies across the egg, on the short axis and head to the right. It then turns 180 degrees and comes to still lie across the egg and head to the left. Once the embryonic sac is full of fluid the embryo floats about but by Day 10 it needs to be close to the air cell end. If not, we get the pip-at-the-wrong-end malposition. It settles in on the long axis and with the head to the right and growth is exponential such that if it's head is at the small end, there is no room for it to reverse itself. Its back starts off against the yolk and as it grows, all the albumen is displaced to the small end.

Small wriggles continue and about Day 14 there is a large turn made which brings its back against the shell, head to the left pointing at the air cell. I call this an air-seeking turn. Displaced air-cells, those low in the egg, trick it into a mid-egg-pip malposition. The head remains between the legs until Day 17 when the beak slides under the right wing and from now turning has no effect on outcome. Studies show that turning past Day 14 also have negligible effect on outcome.

Most cases of repeat malpositions are resolved by increasing the turning frequency. In the case of damaged air-cells and in the case of eggs which are more elliptical/spherical in shape than they are oval, malpositions are resolved by placing the eggs upright from Day 14 onwards until the very end.

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