BREEDING PATTERNS OF EASTERN PHOEBES IN KANSAS - ADAPTIVE STRATEGIES OR PHYSIOLOGICAL CONSTRAINT

Data were collected on annual (1980-1983) and seasonal (spring vs. sum mer) variation in reproduction by the double-brooded Eastern Phoebe (S ayornis 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 fl edged one more nestling than summer nests. In 1981 breeding began earl ier, eggs were larger, and nestlings grew faster than in all other yea rs. Clutch size and egg mass within spring clutches increased seasonal ly. Egg mass was also larger in summer clutches. These observations su ggest that food is usually limited during the initiation of spring clu tches. 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 h ad little impact on nestling growth and; (d) last-hatched young fledge d as frequently as their siblings. However, during the summer: (a) clu tches hatched asynchronously; (b) egg mass did not consistently vary w ith laying sequence; (c) hatch order had a significant negative impact on growth; and (d) last-hatched young fledged only about 50% of the t ime. Thus, phoebes seemed to adaptively shift reproductive patterns se asonally, switching from a brood-survivalist strategy in spring to a b rood-reductionist strategy in summer. However, I suggest that proximat e responses to food availability provide a more parsimonious explanati on for the observed patterns. The increase in egg mass with laying seq uence was most likely the result of progressive increases in food avai lability in spring. Higher food availability and reduced energy demand s during summer probably also allowed females to lay uniformly large e ggs and start incubation sooner. The latter resulted in greater hatchi ng asynchrony in summer clutches. The poorer growth and higher mortali ty of last-hatched young in summer resulted from a severe size disadva ntage that was the result of the greater asynchrony of summer broods. Thus, brood reduction in summer was probably an incidental and nonadap tive outcome of hatching asynchrony.