(4) That the discrepancy between the field data and the life-table data 
is the result of a comparison between successful pairs and the com 
bination of successful plus unsuccessful pairs. This seems to me 
to be an hypothesis that is tenable at this time. It implies, 
however, that only about 30 per cent of the females succeed in 
raising l young to the banding stage. 
Population Dynamics 
The North American data on the number of young Barn Owls in 
the nest (4.2 by Gallup in California, 4.0 for broods noted in the U. | 
S. banding files) agree rather well with the .6 calculated by Schifferli 
for Switzerland. Against these the British data published by Blaker 
(1933)-~3.8 eggs, 3.0 hatching, 2.8 young leaving the nest--offer a 
remarkable contrast. At least three hypotheses may be offered to 
explain this discrepancys (1) that the British data were collected 
in a relatively short period and are representative of a low point 
in the barn owl's "cycle" of productivity; (2) that British barn 
owls differ from Swiss barn owls in their population dynamics in 
somewhat the same way as Lack and Schifferli (1948) found the starlings 
of these two countries do; and (3) that some as-yet-unimom bias exists 
in the manner that Blaker's inquiry operated. These interesting ques- 
tions can best be settled by a study of brood sizes reported in the 
British banding files over a period of years and perhaps also by cal- 
culations of mortality rates in the British barn owl population. 
Of equal interest is the striking discrepancy in mortality 
rates now available for Swiss and American populations of this species: 
for birds in their first year of life (from the time of banding), 76 
per cent calculated by Schifferli (1949) and 51-58 per cent in ny 
study; 57 per cent per year for adult Swiss birds, and about 28-37 per 
cent for American ones. 
No convincing explanation is offered in this study for the 
apparent discrepancy between productivity in the &merican barn owl 
and subadult and adult mortality rates. Although the mortality 
rates calculated here are derived from small samples, it seems cer- 
tain that they are low among the adult birds and probably of the 
order of 30 per cent per year. The high productivity of the success- 
ful females must be counterbalanced by some major phenomenon in the 
life history and ecology of this species. This possibility can be 
reeassessed when a larger sample of banded birds is available for 
study. At the moment, I have no solution to the problem to offer. 
oummary 
Nestling barn owls have been banded in North America in 
every month except February and December, 86 per cent of the banded 
nestlings being ringed from April to July inclusive. Recoveries of 
the young birds are reported throughout the 12 months following band- 
ing. Small-sample analyses yielded first-year mortality rates 
(from the date of banding) of 51 per cent per year (2 samples) to 
li2 
