480 Dr. A. L. Thomson : Results of a Study of [Ibis, 
records for a given species come from Ireland and others 
from Portugal the former country must lie iu the route of 
the birds which travel to the latter. 
The most important unknown factor, from a statistical 
point of view, is the actual mortality rate and its geo¬ 
graphical and seasonal incidence. The student of the 
method, unfortunately, deals only with what may be called 
the “ recorded mortality,” and he can only attempt to guess 
what fraction of the whole it represents. Furthermore, the 
proportion between true and recorded mortality must vary 
in different circumstances according to the chances that 
exist of a dead bird being reported. It may be safely 
assumed, for instance, that a smaller proportion of actual 
deaths is reported from foreign countries than from the 
British Isles, while the fact that birds tend to meet death 
in different forms at different times of year, especially in 
the case of species shot for sport, may materially influence 
the matter. Even were it possible to estimate the true 
mortality, this would not afford a perfect index of the com¬ 
parative total numbers of birds present in particular areas 
at particular times. 
The foregoing considerations apply mainly to records 
which show actual movements, for it is true that in these 
cases a record usually originates through the death of the 
bird. But a similar and even more uncertain factor exists 
in the cases of more or less sedentary birds which are 
re-caught, often many times, at the places where they were 
marked. Records of this latter kind depend largely on the 
activity of the marker himself, who continues to trap birds 
for further marking and in the process constantly recovers 
birds he had previously released. Not only may the 
marker’s efforts be erratic for personal reasons, but he will 
be largely governed by the greater facility with which birds 
are caught at some seasons as compared with others. An 
uncertain factor is therefore introduced into the marker’s 
ovn records of birds recovered, while a much greater one 
must be allowed for if these records are compared with those 
