Impacts of ambient temperature and clutch size on incubation behaviour onset in a female‐only incubator songbird. Diez-Méndez, D., Sanz, J.J. & Barba, E. 2021 IBIS. DOI: 10.1111/ibi.12937 VIEW

Songbirds that breed in temperate habitats are commonly assumed to begin incubation when the last egg of their clutch has been laid. Starting incubation while still laying eggs is costly, particularly in species where the female is the only member of the pair that incubates. Furthermore, delaying incubation after clutch completion might increase the risk of egg predation and also affect the viability of the eggs. Why then would laying females alter the start of their incubation?

Hatching asynchrony and the onset of incubation

The onset of incubation is tightly linked to hatching asynchrony, that is, the time lapse between the first and the last egg hatching within a nest. The length of this period is crucial because if it is too long, early and thus larger hatchlings would outcompete the late and smaller ones, dying within a few days (Stoleson & Beissinger 1995). Hatching asynchrony is the consequence of starting incubation before the entire clutch is laid, because embryos of the first laid eggs would begin to develop while the last eggs are not yet in the nest. Many and contradictory theories have been formulated to explain the asynchrony of hatching and the onset of incubation (Stoleson & Beissinger 1995), but we focused our work on the idea that Great Tit females advance or delay the onset of incubation depending on the ambient temperature during the egg laying period.

Figure 1 Great Tit hatchlings and eggs © Sara Guillamón Prats.

Waiting for the caterpillars

Great Tit hatchlings grow from 1 g to 18-20 g during the first 15 days of their life. Breeding females attempt to synchronize the hatching date of their clutch with the peak of caterpillar availability to provide enough food resources for this impressive growth. The peak is highly influenced by ambient temperature, because at higher temperatures caterpillars could hatch earlier and develop faster, ending in an earlier abundance peak (Perrins 1991). With increasing temperatures, Great Tit females would need to start laying eggs earlier if caterpillars hatch earlier, but that might be insufficient and, if temperatures continue to rise during the egg laying period, caterpillars would develop faster, and thus females would need to readjust the expected hatching date of their clutch by starting incubation while still laying eggs.

Incubating day and night

What do we mean when we talk about the onset of incubation? Many ornithologists could relate to the experience of finding a female incubating on the nest or checking the clutch and feeling the eggs warm. If one follows a strict nest checking, it is possible to find the first day that the onset of incubation occurs, and around 11-15 days later the eggs begin to hatch. Incubation, however, starts earlier, most of the time unnoticed during standard fieldwork (Haftorn 1981). Since females are the only incubators, they have to balance their time between incubation and foraging, or other self-maintenance activities, off the nest. That is why daylight incubation is defined as intermittent incubation, while at night the females remain continuously incubating on the nest (Deeming 2002). Females usually incubate the first night after egg laying begins, just a few minutes or hours per night (nocturnal partial incubation), until they cover the whole night a few days later (nocturnal full incubation). Around the day that nocturnal full incubation starts, females initiate diurnal incubation in the evenings (diurnal partial incubation), extending it into the mornings within a few days. We could consider that diurnal full incubation is reached when females incubate for at least 50% of the daytime, which could relate to the onset of incubation detected in the field.

Fake eggs and incubation behaviour

In order to record detailed incubation behaviour, we mainly used fake eggs with a temperature sensor inside, connected to a data logger. By placing one fake egg at every nest cup just after egg laying began, we were able to record the temperature that the eggs experienced. From there, incubation behaviour can be extrapolated because females maintain a nearly constant egg temperature while incubating, therefore sudden temperature drops followed by an increase indicating when females leave the nest and then return to it. At the same time, we were measuring ambient temperature and collecting other important parameters for incubation behaviour, such as clutch size. Don’t hesitate to read our paper for the full story.

Figure 2 Two experimental nests where we added a fake egg (indicated by the white arrows) to record ambient temperatures for later incubation behaviour analyses © David Diez Méndez.

Ambient temperature and incubation behaviour

Considering the four mentioned periods of incubation behaviour, we found that Great Tit females only advanced the onset of diurnal partial incubation with increasing ambient temperatures. This behaviour dragged the entire incubation period, advancing the hatching date, which would theoretically allow females to keep track of the caterpillars. The earlier onset of incubation behaviour led, as expected, to longer hatching periods. Females were able to adjust the onset of their incubation but at the cost of increasing the differences in size between the first and the last hatchlings. This is a negative effect of warmer than expected springs. However, we did not find any effect of temperature on nocturnal incubation. Why was nocturnal incubation not affected by ambient temperature, especially when the onset of diurnal partial incubation and nocturnal full incubation are related? Is nocturnal and diurnal incubation differently regulated? These are questions that will need new exciting experiments to be understood.


Deeming, D.C. 2002. Behaviour patterns during incubation. In Behaviour, environment and evolution (D. C. Deeming, ed), pp. 63–87. Oxford University Press, Oxford.
Haftorn, S. 1981. Incubation during the egg-laying period in relation to clutch-size and other aspects of reproduction in the Great Tit Parus major. Ornis Scandinavica 12: 169–185. VIEW
Perrins, C.M. 1991. Tits and their caterpillar food supply. IBIS 133: 49–54. VIEW
Stoleson, S.H. & Beissinger, S.R. 1995. Hatching asynchrony and the onset of incubation in birds, revisited. When is the critical period? In Current Ornithology (D. M. Power, ed), pp. 191–270. Plenum Press, New York. VIEW

Image credit

Top right: A Great tit Parus major female surprised while incubating © David Diez Méndez.