The Eastern Bluebird (Sialia sialis) is a sec-structures built in the abandoned cavities unless an ondary (non-excavating) cavity-nesting spe-obvious disturbance (predation, inclement weather, cies that readily accepts nesting boxes, and conflict with other species) appeared to have many studies (Laskey 1943, Thomas 1946, prompted the move to a new site. I banded 909 bluebirds with a standard aluminum Peakall 1970, White and Woolfenden 1973) band (size 1B) and one to three plastic, colored leg have reported on breeding phenology, clutch bands. Nestlings were banded when 8 to 12 days size, and nesting success in various parts of old. Adult birds were captured with mist nets and its range. In the northern United States some a variety of manual and automatic traps attached to bluebirds are double-brooded; first nests are nest boxes. Although a few birds were caught and banded throughout the breeding period, most unusually begun in early spring and second nests banded adults were captured as soon as possible after are initiated during summer (Peakall 1970). their arrival in March and April. Banding early in the Many bluebird pairs, however, rear only a season was necessary in order to obtain complete single brood in either the spring or the sum-breeding histories because bluebirds may change termer, and do not attempt second nests (pers. ritories and nest sites in early spring (e.g., four different pairs successively occupied a single nest box observ.), perhaps because of a limited num-between 3 Auril and 13 Mav 1976). ber of suitable nest sites (Lack 1968:176, Most bluibirds left the' study ' area in winter; ar-Stewart 1973). rival dates for these birds (migrants) are the dates In this paper I examine bluebird nesting of initial sightings in the study area. An arrival date is not necessarily the date of territory establishment adaptations, particularly those relating to the (males) or date of pair bond formation (males and temporal organization of the breeding season, females). A few bluebirds (non-migrants) wintered and compare nesting performance in year-in and around the study area; I considered their arling and adult birds. To test the hypothesis rival dates as the earliest dates after which I reguthat breeding costs energy and reduces the larly saw them in the breeding areas. Clutch size refers to completed clutches only; I physiological condition of the adults (Rickfollowed Zwickel (1975) in considering clutches comlefs 1974:261), I also compare nesting success plete "if incubation was underway, or in a very few of bluebirds breeding in summer-those who cases, almost certainly underway" provided there was reared a spring brood vs. those who did not. no subsequent increase in the number of eggs present Such a comparison has not been made in in the nest. Date of onset of general laying (GLD) is defined as the earliest date of a season on which studies of other multi-brooded, cavity nesting two or more females laid their first eggs, and the species (Kessel 1957, Summers-Smith 1963, mean GLD date is the average of...
