As indicated previously, the female Bell Vireo leaves the nest unoccupied several hours at a time in the transition period between completion of the nest and the start of egglaying. Such behavior early in the morning certainly would facilitate deposition of cowbird eggs. Early in the nesting period the mere presence of a cowbird egg in the nest prior to the laying of the host's first egg leads to abandonment of the nest. This seems to be correlated with the relative strength of the nesting tendency; anyhow cowbird eggs laid in later nests prior to the appearance of the host's own eggs did not cause the nesting birds to desert. The Bell Vireo does abandon the nest when all but one of its own eggs have been removed by the cowbird. Mumford (1952:232) records the removal of a cowbird egg by the host birds and I recorded a similar instance involving nest 2-b (1960). On May 14, 1960, I found one punctured cowbird egg on the ground about 10 feet west of this nest. Occasionally a cowbird egg is buried beneath the lining of a nest. Mumford (1952:23) observed this in mid-May in 1951 and I observed pair 8 (1960) actively covering with building material a cowbird egg on July 5, 1960. Covering a cowbird egg constitutes effective removal. Since the egg cannot be turned, an adhesion develops. TABLE 12. INCIDENCE OF COWBIRD PARASITISM OF THE BELL VIREO COMPARED WITH OTHER PASSERINES IN THE STUDY AREA IN 1959 AND 1960. ========================================================= | Bell | Other | Vireo |passerines --------------------------------------+-------+---------- Total nests examined containing | | at least one host egg | 35 | 43 Total nests parasitized | 24 | 14 Total number of cowbird eggs | 33 | 23 Per cent of nests parasitized | 68.6 | 32.6 Total number of cowbird eggs per nest | .94 | .54 --------------------------------------+-------+---------- The percentage of cowbird eggs hatched in relation to the number laid is relatively low. For instance, Mumford (1952:231) has only one record of a young cowbird successfully raised by a Bell Vireo. The data available in Bent (1950:260-261) also indicate that the percentage of cowbird eggs hatched is small. The Bell Vireo is less tolerant of cowbird parasitism than are many of the species so victimized, but is not so intolerant as the Robin, Catbird, and the Yellow-breasted Chat (Friedmann, 1929:193). SUMMARY 1. The behavior of a small population of Bell Vireos was studied in the spring and summer of 1959 and again in 1960 in Douglas County, Kansas, and results are compared with previous studies elsewhere.