| Measurements | May (N 10) | June (N 8) |
|---|---|---|
| External depth | 61.6 mm. | 59.3 mm. |
| Depth of cup | 45.5 mm. | 46.3 mm. |
| Outside diameter | 57.3/55.5 mm. | 54.3/53.5 mm. |
| Inside diameter | 43.4/42.2 mm. | 45.5/43.9 mm. |
| Thickness of forward wall 1 inch below rim | 13.8 mm. | 7.6 mm. |
| Thickness of bottom | 11.3 mm. | 4.6 mm. |
EGGLAYING AND INCUBATION
Egglaying
Egglaying begins the first or second day after completion of the nest. The female sits in the nest occasionally for periods of five to twenty-five minutes on the day the nest is completed. This is interrupted by periods of nest-adornment and foraging; such activities sometimes keep the female off the nest for several hours. Prior to the laying of the first egg, only the female is seen on the nest, although the male is often seen sitting quietly within the nest tree a few feet from the female. The infrequency of the "congested" song and the alarm (eh-eH-EH) after the inception of "broodiness" indicates the waning of courtship behavior. As later in incubation only the "normal" song and the scold are heard.
Eggs are laid early in the morning prior to 5:30 a. m. according to Nolan (1960:232). The nest is usually left unoccupied for considerable periods after the first egg is laid, but, on the first day of laying, both sexes have been observed sitting for brief periods averaging ten minutes in length. Eggs are laid at one-day intervals until completion of the clutch. I found incubation to begin with the second egg.
Clutch-size
The average clutch-size of the Bell Vireo in Kansas, based on thirty-three records, is 3.39 eggs (Table 8). Seasonally, the largest average clutches are produced in the middle of the breeding season, that is, in June. Lack (1947:308-309) indicates that in European passerines the highest seasonal average clutch-sizes likewise occur in June. The largest average clutch-size in the Bell Vireo is presumably related to some aspect of the availability of food.
Table 8. Average Numbers of Eggs per Nest (Number of Records in Parentheses)[F].
| Year | May | June | July | Mean annual clutch-size |
|---|---|---|---|---|
| 1959 1960 | 3.0 (7) 3.3 (6) | 3.2 (12) 3.83 (5) | 3.0 (1) 4.0 (2) | 3.06 3.72 |
| 1959-1960 | 3.17 | 3.52 | 3.5 | 3.39 |