Bird populations--United States, Avian biology
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 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 in 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 asynchrony of summer broods. Thus, brood reduction in summer was probably an incidental and nonadaptive outcome of hatching asynchrony.
Murphy, Michael T., "Breeding Patterns of Eastern Phoebes in Kansas: Adaptive Strategies or Physiological Constraint?" (1994). Biology Faculty Publications and Presentations. 77.