During incubation, metabolic heat is produced as a by-product of the growth of the embryo. This metabolic heat needs to be removed adequately to keep the temperature of the embryo at the right level.

During the last decades, the growth rate of broiler chickens as well as the feed efficiency has improved dramatically due to efficient genetic selection. As this genetic change will be present already during the embryonic stage, people have wondered if the genetic progress both in growth and feed efficiency has resulted in a change in metabolism in the embryo as well. As many people have the impression that the modern embryo needs more cooling and therefor perhaps produces more metabolic heat than the embryos in the past, the question rises if the efficiency and the growth rate of the modern embryo is different than the embryo in the past.

This question cannot be answered easily without an extensive study and comparison between randomly selected lines and modern broiler lines, which to my knowledge is not available. However, there are a couple of factors that give an insight in the possible answer. 

First of all, the heat production of an embryo is the result of the "inefficiency" of the organism to convert energy and protein into body tissue. The more heat is produced per gram of nutrient converted, the less efficient the organism is using its nutrients. This means that a genetic improvent in feed efficiency should actually lead to a reduction of the heat production, and not to an increase.

The ratio between uptake of Oxygen and production of Carbon dioxide (the so-called RQ value) is determined by the type of nutrient that is used for development, for instance fat versus protein. If we compare this RQ value from today with the RQ values published in 30 or 40 years ago, we do not see any difference, indicating that the source of nutrients has not changed. Also the chick yield, the percentage of chick weight hatching from the original egg weight is still the same, about 66-67% on average. It seems that the way embryos are using nutrients has not changed over the years. And as an embryo can only use the nutrients that are available in the egg, it suggests that the heat production has not changed.

But if we look at the growth of the embryo we do see a change between present and past.

In the following table I compared some results in embryo weight at three moments in incubation between a publication of Romanoff originating from 1939 and a recent publication (Nangsuay et al., 2015).

The egg weigths from 1936 and 1960 are estimated based on the reported chick weights.

If the chick yield at hatching has not changed but the embryo is relatively bigger at 11, 14 and 18 days, the conclusion should be that (at least part of) the difference in heat production is because the embryos are nowadays growing faster and become bigger at an earlier moment, but slow down at the end to hatch with the same body weight. So in other words, the differences in embryo weights between past and present suggest that the pattern of embryonic development seems to have shifted forward, but the limitations for growth at the end (perhaps because the egg shell doesnt allow more exchange of oxygen and carbon dioxide and therefore the embryo cannot keep up the higher developmental speed) make them stay in the "plateau phase" relatively long, and perhaps longer than in the past. And as more growth and bigger embryos will result in more heat production. 

As the embryo at the end will have more or less the same body weight as before, the heat production at the end will be identical as in the past, but perhaps the pattern of heat produciton has shifted.