Ornis Hungarica. vol.6. (1996) p.23-41.
The phenomenon of hatching asynchrony in altrical birds and different hypotheses for its evolution
The phenomenon of hatching asynchrony - that is the last nestling hatches at least one day later than its siblings - is widespread in a few altricial orders. Its direct cause is that the female begins to incu- bate the clutch before laying the last egg. Thus the last egg begins to develop and hatches later than the earlier la:d eggs. Consequently great size hierarchy develops among siblings which frequently may lead to the death of the smallest nestling. In this case why did hatching asynchrony evolve and why did not natural selection eliminate it? Up to now several hypotheses have been set up for the explanation of the development of this phenomenon. 16 hypotheses consider hatching asynchrony as an adaptive strategy maintained by natural selec- tion, while according to the Hormonal Hypothesis it is only a physiological epiphenomenon.The most widespread hypothesis is the Adaptive Brood-reduction Hypothesis which supposes that in case of food-shortage parents can rear up the reduced brood at the expense of the death of the small- est nestling in asynchronous broods. The extension of this hypothi;sis is the High Quality Offspring Hypothesis in case when asynchronous broods are as productive as synchronous ones but fledge at least some heavier nestlings whose post-tledging sutvival may be better. If brood reduction always occurs, the Insurance Hypothesis gives the best explanation for the evolution of hatching asyn- chrony. One special case of the former one is the Larder Hypothesis, when the younger sibling serves as food for its older nestmate(s). The Sex Ratio Manipulation Hypothesis may be the most suitable for species with sex-dimorphism, as in this case the larger sex may be more vulnerable for food-shortage and thus hatching asynchrony may skew the sex-ratio in favour of the smaller sex, which needs less energy to rear up. From the 7 hypotheses supposing time-saving, the Hurry-Up Hypothesis considers hatching asyn- chrony advantageous as nestlings hatch earlier and thus meet better food conditions during the breeding season. Earlier incubation also reduces the duration of laying thus the risk of predation on eggs (Egg Protection Hypothesis), the risk of egg-inviability due to inclement weather (Egg Viability Hypothesis), the risk of brood parasitism (Brood Parasitism Hypothesis) and the risk the nest is occupied by other pairs (Limited Breeding Opportunities Hypothesis). The risk of predation on eggs versus on nestlings (Nest Failure Hypothesis) and the risk of predation on females during the laying versus brooding period (Adult Predation Hypothesis) may determine the optimal degree of hatching asynchrony. Four hypotheses suppose the reduction of energy investment. According to the Peak Load Reduction Hypothesis the total energy requirement of the asynchronous brood does not decrease but it is more evenly distributed and thus puts less load on parents than all siblings were of equal age. Similarly parents may bring a great variety of food to different aged nestlings (Dietary Diversity Hypothesis) and thus are not forced to search on only some special food types. The Sibling Rivalry Hypothesis supposes that the energy requirement of the brood decreases as asynchronous hatching creates a stable size hierarchy among nest mates and thus reduces unnecessary sibling rivalry. According to the Sexual Contlict Hypothesis the female can reduce its own energy investment at the male's cost due to hatching asynchrony. Finally the Energetic Constraint Hypothesis does not seek for the reason of asynchronous hatching, only supposes that its realisation has energetic constraints and together with the Individual Optimisation Hypothesis tries to explain the intrapopulation variation of hatching asynchrony.