Tests of Three Hypotheses of Hatching Asynchrony in the Common Tern

-We examined three hypotheses concerning hatching asynchrony in the Common Tern (Sterna hirundo). Survival of third-hatching "C-chicks" was significantly lower than that of "A-" and "B-chicks" in broods of three. In 2 yr when conditions did not appear favorable, survival was significantly higher in manipulated broods in which chicks hatched synchronously (73%) than in nonmanipulated broods (56%). Chicks in synchronous broods grew significantly faster than C-chicks and at a rate similar to Aand B-chicks. These results were inconsistent with the brood-reduction hypothesis, which predicts that hatching asynchrony will maximize brood success under conditions of food limitation. Chicks hatching from C-eggs grew significantly faster and survived at nonsignificantly higher rates when an older sibling was removed experimentally. These results were consistent with the hypothesis that C-chicks serve as insurance against loss of an older sibling. All three siblings, however, survived in 26% of nonmanipulated broods, which indicates that the sole function of the C-chick was not insurance. Predation on tern eggs was common and was correlated with numbers of migrating Ruddy Turnstones (Arenaria interpres). The percentage of time adults incubated was lower when only one egg had been laid than when two or three eggs were laid, and egg predation was most frequent during this initial stage. Although hatching asynchrony did not maximize chick survival, incubation before laying is completed may maximize overall nest success by protecting eggs from predators. Received 19 July 1989, accepted 28 April 1990. MANY birds begin incubation before completion of a clutch of eggs. Hatching asynchrony is the result. Survival frequently decreases with hatching order (for reviews, see O'Connor 1978, Clark and Wilson 1981, Hahn 1981). One of the most widely cited explanations of hatching asynchrony is Lack's (1954) "brood reduction" hypothesis (i.e. hatching asynchrony is an adaptation for adjusting brood size to an unpredictable food supply). Asynchrony creates a size hierarchy among siblings. Subsequent sibling competition or more parental attention to larger siblings leads to starvation of the youngest chick when conditions are poor. Resources are not wasted on the chick least likely to survive, and starvation of the entire brood is prevented. Alternatively, the last egg may serve as "insurance" against loss of an earlier-laid egg or an older sibling in species in which the entire brood rarely fledges or survives to breed (Graves et 3Present address: Department of Zoology, Eastern Illinois University, Charleston, Illinois 61920. al. 1984). Hatching asynchrony ensures that the last sibling, in which the least resources have been invested, will be the one to die if the older siblings survive. A decrease in egg size with laying order can accentuate the size hierarchy within the brood through which the brood-reduction and insurance strategies operate (Slagsvold et al. 1984). In Common Terns (Sterna hirundo; Nisbet 1973) and in larids in general (e.g. Parsons 1976), hatching is asynchronous, the last egg tends to be relatively small, and survival of the youngest sibling is reduced. Differential mortality by hatching order is usually viewed as adaptive in larids (but see Parsons 1976), either in terms of brood reduction (Langham 1972, Hahn 1981), insurance (Graves et al. 1984, Quinn and Morris 1986), or a strategy that combines both functions (Nisbet and Cohen 1975, Braun and Hunt 1983, Hebert and Barclay 1986). Few studies of larids, however, have offered strong experimental support for either hypothesis (but see Hahn 1981, Graves et al. 1984). 696 The Auk 107: 696-706. October 1990 This content downloaded from 207.46.13.77 on Tue, 17 May 2016 06:23:12 UTC All use subject to http://about.jstor.org/terms October 1990] Hatching Asynchrony in Terns 697 In contrast, several recent hypotheses view differential mortality within broods simply as a side effect of hatching asynchrony or of incubation before the clutch is completed (reviewed by Hussell 1972, Clark and Wilson 1981). In fact, in apparent contrast to the predictions of the brood-reduction hypothesis, the majority of experimental studies that compare normal, asynchronously hatching broods with manipulated, synchronously hatching broods have failed to detect higher survival rates in asynchronous broods (see reviews in Amundsen and Stokland 1988, Skagen 1988). Specifically, relatively high survival was recorded in synchronous broods of gulls (Hebert and Barclay 1986; but see Hahn 1981), cormorants (Shaw 1985, Amundsen and Stokland 1988), herons (Fujioka 1985, but see Mock and Ploger 1987), and passerines (Slagsvold 1982; Haydock and Ligon 1986; Gibbons 1987; Skagen 1987, 1988; but see Magrath 1989). In most of these studies chick growth was also similar in synchronous and asynchronous broods (see Werschkul 1979). Although only a few of these studies indicated that conditions were food-limited (Hebert and Barclay 1986, Skagen 1988, Magrath 1989), they call into question the general applicability of Lack's hypothesis. Stokland and Amundsen (1988) suggested that selection pressures to begin incubation before clutch completion deserve critical attention. We tested the brood-reduction and insurance hypotheses in the Common Tern under conditions of food limitation. We also attempted to determine whether onset of incubation before clutch completion could reflect predation pressure during the egg-laying period (the "egg protection hypothesis"; Parsons 1976). In this species, partial incubation begins with the first of 3 eggs (Nisbet and Cohen 1975), and the chicks hatch asynchronously, usually over 1.53 days (Courtney 1979). Mortality is higher for last-hatching "C-chicks" than for "A-" or "Bchicks" (Langham 1972, Nisbet 1973). Our objectives were to determine (1) whether manipulated broods with synchronous hatching were as successful as nonmanipulated broods; (2) the frequency with which all 3 chicks in a brood survived; (3) whether chicks hatching from C-eggs had higher survival rates after removal of an older sibling; and (4) whether constancy of incubation was related to frequency of egg