Good one, we wish!
There have been lots of studies, including unofficial ones like the pointy-ness verses more rounded eggs being male verse female. No one has found a way to determine that.. or manipulate the gender of poultry via temperature, humidity or any other means that I know of.
In my experience... changing the temperature changes the rate of development and thus speeds up or slows down hatch relatively if still kept within viable parameters. Humidity does not change the speed of growth but it does change the overall growth potential. Too high humidity can allow the embryo to grow to big (air cell too small) and not be able to turn into position for hatching, possibly even drowning at pip.
Using the same flock eggs.. if they are incubated under the same basic conditions but temperature fluctuations have occurred it essentially spreads your hatch farther apart. Some may be early (like day 18) while others could be late (day 24). The idea to get them all close as possible on day 21 and have the BEST chance of viability for all of them.
Alternatively, it can even cause other issues... such as hatching without eyes... has been seen in birds that got temps too high during a certain time frame.
However... depending on when the temp things happen... sometimes a late short term temp spike could actually give the bird better heat tolerance later in life.
In other words... life is SOOO fantastically complicated and amazing!
My favorite hatching resource, aside from BYC of course...
https://www.hubbardbreeders.com/media/incubation_guideen__053407700_1525_26062017.pdf
It doesn't warn you about chicken math, and just how addicting hatching can be though.
Would LOVE to see different information... but so far,all indicators are that temperature effecting embryo gender is species specific and not relative to fowl.
