Conditions in the nests (both hand collected litter nests and automatic roll away nests) are not necessary favourable for hatching eggs. Not only is the bacterial load higher due to dust, litter, manure etc, but also the temperature is often too high. The critical temperature for embryo development (the so-called physiological zero) is approximately 25-26oC, so it is important that the eggs stay at least below that temperature, to prevent pre-incubation.
In hand collected litter nests the eggs are partly buried in the litter, and visited by new hens coming in the nests to produce their eggs. As these birds have a high body temperature and will warm the eggs, it is makes it easy to understand why eggs need to be collected frequently.
But also in automatic (mechanical) roll away nests the temperature can be an issue. In summer time the temperature in the houses can go up far beyond the crictical 25oC for embryo development. In the nests it will normally even be a bit higher than in the house itself, due to limited ventilation in the nest and a high density of birds. With tunnel ventilation we can create air velocity and in this way decrease the appearant temperature for the birds (increase the heat loss), but this has no cooling effect on the eggs. Only with pad cooling we can actively reduce the temperature in the house.
Normally the birds start to produce approximately 1 hour after the lights go on, and about 6 hours the majority of the eggs will be produced, depending on breed, age, feed level, health status etc. This means that when the light period starts at for instance 04.00, at midday the majority of the eggs is produced. As in the morning the temperature of the houses will remain relatively cool, collection of the eggs in the morning will work well.
But if the collection is done in the afternoon, or even worse when a relatively big part of the eggs is produced after the last egg collection because the workers will finish the working day early, the remaining eggs will stay in the nests during the hottest part of the day and the night and will be collected the next morning.
This will not show in a reduction of the number of eggs, as the eggs will be collected anyway, but the hatchability might be influenced due to an increased level of early deads, often recognised as infertiltiy.
A good practice is to regulary check just before the light is turned off how many eggs are remaining on the egg belt. When this percentage is higher than for instance 5%, the collecting hours should be adjusted.