Open Access

Scott H. Stoleson, Steven R. Beissinger

• 1. Birds are unique among animals in being able to influence the birthing intervals of their young through the timing of the onset incubation. However, many species hatch their young asynchronously, frequently resulting in reduced survivorship for later-hatched young. This is the Paradox of Hatching Asynchrony. 2. The Brood Reduction Hypothesis provided a resolution to the paradox by suggesting an adaptive function to the offspring mortality that results from asynchrony. Experimental tests have provided little support, and 16 alternative hypotheses have been proposed, but few have been tested. Most experimental tests have not measured important parameters such as parental effort and postfledging survival. Many have lacked adequate controls or sufficient statistical power. 3. We divide the hypotheses for hatching asynchrony into four categories based on the effects of intrinsic or extrinsic factors during a critical period of the nesting cycle which constrains reproductive success. Hatching asynchrony could be simply the consequence of the early onset of incubation during egg-laying, either as a result of physiological constraints on incubation or because parents derive fitness benefits from the protective function of early incubation. During the nestling period, hatching asynchrony could be adaptive if it allowed parents to eliminate one or more nestlings selectively, or increased parental efficiency. Alternatively, parents could manipulate the duration of the different periods of the nesting cycle to maximize benefits. 4. Because the onset of incubation generally determines hatching patterns, we encourage refocusing attention from the search for adaptive hatching patterns during the nestling period to the events surrounding the onset of incubation during egg-laying. Many factors can affect when incubation is begun, including physiology, and interactions with the environment, predators, competitors, and mates. 5. Patterns of the onset of incubation are difficult to determine and to quantify, in part because many birds begin incubating gradually, or at night. In some species, the onset of incubation varies with clutch size, but not in others. 6. The onset of incubation is the principle proximate control of hatching patterns, but other factors, such as egg size, embryonic vocalizations, and time of year may also affect hatching patterns. 7. Synchronous hatching is the primitive condition in birds, and is widespread in the lower, primarily precocial taxa. Most altricial species hatch their eggs asynchronously, although some exhibit synchrony as a secondarily derived trait. Hatching patterns show wide variation within some orders and families. 8. Patterns of the onset of incubation and hatching in a species may reflect the influence of multiplefactors. The relative importance of those factors may depend on the trade-offs associated with the potential benefits of early incubation to the survival of eggs and the potential costs to the survivor of later-hatching young associatedwith nestling size hierarchies. 9. The relative effects of multiple factors can be examined by integrating the results of empirical tests of single factors through modeling. 10. We demonstrated the use of a stochastic model by using empirical data from the House Sparrow. Results revealed the trade-offs inherent in the onset of incubation from differences in egg viability and nestling survivorship. An intermediate onset of incubation produced the greatest fledging success. 11. Other factors may be integrated into such models if they can be measured in terms of their effects on fledging success. Different factors, represented by different hypotheses, vary in how readily they may be modeled.