Effect of Selection for Successful Reproduction on Hatching Synchrony and Asynchrony

Abstract

-Hypotheses regarding the evolution of asynchronous hatching in birds are evaluated with respect to the Murray-Nolan clutch-size equation, which distinguishes between the number of broods successfully reared per female during a breeding season (2:1 Pi) and within-brood survivorship (which affects Z. Xx). Asynchronous hatching is best interpreted as an adaptation that reduces the probability of whole-brood loss to predation, inclement weather, or other cause of whole-brood mortality (i.e. it increases a female's probability of rearing at least one young to nest-leaving from a clutch, and increases 1 P). This hypothesis constitutes a different form of the hypotheses of Tyrvainen, Hussell, and Clark and Wilson. The equations of Clark and Wilson and of Hussell erred in assuming a stepped function of the probability of daily survivorship (dj) during the nesting cycle. Instead, I assume a smooth curve and argue that if d, < 1 and if sn = II d,, then asynchronous hatching results in increasing s, and 2;1 Pi of the clutch-size equation. Alternative hypotheses explaining the evolution of asynchronous hatching seem better interpreted as explanations for increasing within-brood success, increasing the number of young to nest-leaving (kn) of successful broods, and increasing w Ax, of the clutch-size equation, or as unselected consequences of selection for reducing whole-brood loss. Received 26 July 1993, accepted 11 January 1994. BIRDS LAY no more than one egg per day, and incubation may begin at any time during the laying sequence. If incubation begins before the last egg is laid, hatching may be asynchronous. Several hypotheses have been proposed to explain the evolution of asynchronous hatching. They include the notions that asynchronous hatching may: (1) result in rapid brood reduction when feeding conditions are poor (Lack 1947, 1954, Ricklefs 1965, Pijanowski 1992); (2) reduce the probability of whole brood loss (Tyrvainen 1969, Hussell 1972, Clark and Wilson 1981); (3) reduce the peak feeding load on adults (Hussell 1972, 1985a); (4) result from laying insurance eggs after incubation has begun (Dorward 1962, Nisbet and Cohen 1975, Stinson 1979); (5) shorten the time during which a clutch is vulnerable to brood parasitism (Wiley and Wiley 1980, Lombardo et al. 1989); (6) reduce sibling rivalry (Hahn 1981); (7) reduce the time that adults are exposed to predation (Magrath 1988); (8) result in greater paternal contribution to the brood (Slagsvold and Lifjeld 1989); or (9) reduce the loss of early laid eggs to inviability (Veiga 1992). Mead and Morton (1985), however, suggested that asynchronous hatching is not adaptive at all but an incidental consequence of the hormonal mechanism controlling the transition from egg laying to incubation. From my clutch-size theory (Murray 1979, 1991a), I infer that asynchronous hatching increases the probability of successful reproduction of parents by reducing the probability of whole-brood loss. Thus, the clutch-size theory lends support to the hypothesis of Tyrvainen (1969), Hussell (1972), and Clark and Wilson (1981), although my argument is different from theirs. Empirical data gleaned from the literature, however, indicate that hatching is more synchronous than expected from the theory (Clark and Wilson 1981, Slagsvold 1986, Magrath 1990). Furthermore, studies on the Boat-tailed Grackle (Quiscalus major; Bancroft 1985), Least Flycatcher (Empidonax minimus; Briskie and Sealy 1989), and Yellow Warbler (Dendroica petechia; Hebert and Sealy 1993), specifically undertaken to test the hypothesis, confirm that hatching is more synchronous than expected from the formulations of Clark and Wilson (1981) and Hussell (1985a, b). Thus, the nest-failure hypothesis seems to have little empirical support. In this paper I examine the nest-failure hypothesis and some other hypotheses on asynchronous hatching in light of my clutch-size theory. THE HUSSELL EQUATION Tyrvainen (1969), Hussell (1972), and Clark and Wilson (1981) proposed that asynchronous hatching evolved as a means of reducing the