Breeding patterns of eastern phoebes in Kansas: adaptive strategies or physiological constraint?

Data were collected on annual (1980-1983) and seasonal (spring vs. summer) variation in reproduction by the double-brooded Eastern Phoebe (Sayornis phoebe) to test the proposal that phoebes modify reproductive patterns on a seasonal basis and switch from being brood survivalists in spring to brood reductionists in summer. Clutch size did not differ between spring and summer broods nor among years, but spring nests fledged one more nestling than summer nests. In 1981 breeding began earlier, eggs were larger, and nestlings grew faster than in all other years. Clutch size and egg mass within spring clutches increased seasonally. Egg mass was also larger in summer clutches. These observations suggest that food is usually limited during the initiation of spring clutches. However, because spring broods were more productive than summer broods, I predicted that phoebes should act as brood survivalists in spring, but become brood reductionists in summer. All predictions were supported. During the spring: (a) clutches hatched synchronously; (b) egg mass increased significantly with laying order; (c) hatch order had little impact on nestling growth and; (d) last-hatched young fledged as frequently as their siblings. However, during the summer: (a) clutches hatched asynchronously; (b) egg mass did not consistently vary with laying sequence; (c) hatch order had a significant negative impact on growth; and (d) last-hatched young fledged only about 50% of the time. Thus, phoebes seemed to adaptively shift reproductive patterns seasonally, switching from a brood-survivalist strategy in spring to a brood-reductionist strategy in summer. However, I suggest that proximate responses to food availability provide a more parsimonious explanation for the observed patterns. The increase in egg mass with laying sequence was most likely the result of progressive increases in food availability in spring. Higher food availability and reduced energy demands during summer probably also allowed females to lay uniformly large eggs and start incubation sooner. The latter resulted in greater hatching asynchrony in summer clutches. The poorer growth and higher mortality of last-hatched young in summer resulted from a severe size disadvantage that was the result of the greater asynchrony of summer broods. Thus, brood reduction in summer was probably an incidental and nonadaptive outcome of hatching asynchrony. Received 21 January 1993, accepted 31 May 1993. FOOD HAS A DIRECT, proximate impact on avian reproductive processes (Davies and Lundberg 1985, Hussell and Quinney 1987, Martin 1987, Nilsson 1991) and variation in the availability of food may have greatly influenced the evolution of reproductive patterns of birds (Lack 1947, Howe 1976, 1978, O'Connor 1978, Martin 1987). For most birds, especially those that feed their young, future unpredictability of food is a major constraint, and females may often enter the breeding season without being able to predict optimal clutch size (i.e. size that produces maximum number of fledglings). Lack (1947) and Ricklefs (1965) argued that the combination of hatching asynchrony and brood reduction 1 Present address: Department of Biology, Hartwick College, Oneonta, New York 13820, USA. (i.e. starvation of smallest nestling during periods of food shortage) was an evolved, adaptive phenomenon that allowed parents to contend with unpredictability in food supplies. Asynchronously hatched clutches made it possible for parents to (1) fledge all of their young during occasional periods of high food availability, yet also (2) reduce brood size to a level commensurate with their feeding capacity when food was in short supply. Experimental evidence exists to support Lack's model (e.g. Magrath 1989, Hebert 1993). However, several other variables have been suggested as important factors in the evolution of hatching asynchrony (for review, see Magrath 1990, Nilsson 1993), the most widely acknowledged being nest predation (Clark and Wilson 1981, Hussell 1985; see below). Although generally presented as alternatives, it is possible for two or more fac-