Different temperature and cooling patterns at the blunt and sharp egg poles reflect the arrangement of eggs in an avian clutch.

Miroslav E Šálek,Markéta Zárybnická

doi:10.1371/journal.pone.0117728

Abstract

Incubation is an energetically demanding process during which birds apply heat to their eggs to ensure embryonic development. Parent behaviours such as egg turning and exchanging the outer and central eggs in the nest cup affect the amount of heat lost to the environment from individual eggs. Little is known, however, about whether and how egg surface temperature and cooling rates vary among the different areas of an egg and how the arrangement of eggs within the clutch influences heat loss. We performed laboratory (using Japanese quail eggs) and field (with northern lapwing eggs) experiments using infrared imaging to assess the temperature and cooling patterns of heated eggs and clutches. We found that (i) the sharp poles of individual quail eggs warmed to a higher egg surface temperature than did the blunt poles, resulting in faster cooling at the sharp poles compared to the blunt poles; (ii) both quail and lapwing clutches with the sharp poles oriented towards the clutch centre (arranged clutches) maintained higher temperatures over the central part of the clutch than occurred in those clutches where most of the sharp egg poles were oriented towards the exterior (scattered clutches); and (iii) the arranged clutches of both quail and lapwing showed slower cooling rates at both the inner and outer clutch positions than did the respective parts of scattered clutches. Our results demonstrate that egg surface temperature and cooling rates differ between the sharp and blunt poles of the egg and that the orientation of individual eggs within the nest cup can significantly affect cooling of the clutch as a whole. We suggest that birds can arrange their eggs within the nest cup to optimise thermoregulation of the clutch.

Highlights

Egg incubation in birds is a complex process that involves the maintenance of a steady egg temperature within the nest to ensure embryonic development [1]
The images created with the thermal imager were analysed, and the eggshell surface temperatures at the given times were plotted on a set of nine points located at three areas of each egg: sharp pole, equator and blunt pole (Fig. 1A)
The more curved, sharp pole with its relatively large surface area per unit egg volume at the tapered portion of the egg probably suffers from greater heat loss during cooling than does the less curved and more bulky blunt pole

Summary

Introduction

Egg incubation in birds is a complex process that involves the maintenance of a steady egg temperature within the nest to ensure embryonic development [1]. The amount of energy transferred to a developing embryo is mediated by the extent of heat flow from the brood patch of an incubating parent to the eggs, which is achieved through contact between the skin and the PLOS ONE | DOI:10.1371/journal.pone.0117728. Excessive heat loss from the egg to the environment is an important issue for bird parents striving to limit energy expenditures during the incubation period [2]. Several behavioural adaptations aimed at maintaining an optimal incubation temperature have been described. Shortening both the length and frequency of incubation breaks, which accelerates embryo development and reduces the incubation period, are among the behavioural patterns birds used to optimise the incubation process [7]

Methods

Results

Conclusion