Bird Ecology and Conservation A Handbook of Techniques

processes that give rise to the data. Following the original work of Cormack
(1964), Jolly (1965), and Seber (1965), the Cormack–Jolly–Seber (CJS) model
requires two sets of parameters. Capture (or resighting) probability, pi, is the
probability that a bird present in the study area during sampling occasion iis
recaptured (or resighted) during that period. Survival probability, i, is the pro-
bability that a bird alive and in the study area at sample occasion iis still alive
and in the local population at sampling occasion i1.
Consider a 2-period study in which a bird is captured and released in period 1
and is either recaptured in period 2 (capture history 1 1) or not (capture history 1 0).
Figure 5.1 presents a tree diagram of the possible events and their associated para-
meters. The probability for a particular capture history is obtained by working
backwards from the history and by multiplying the parameters associated with
each branch of the tree that led to the history. Thus, given release in period 1, the
probability associated with capture history 1 1 is simply  1 p 2. There are 2 different
paths leading to capture history 1 0, and when we sum the two products of
parameters along these paths we obtain:  1 (1p 2 )(1 1 )
1  1 p 2 as
the probability associated with a bird released in period 1 and not recaptured in
period 2. The two summed components reflect the two possible sets of events
producing this capture history; the bird could have survived until period 2 and
not been caught then, or it could have died or permanently emigrated between
periods 1 and 2.
Now consider a longer capture history. Given that a bird is first caught and
released in period 2, the probability associated with the capture history 0 1 0 1 0
can be written in terms of these parameters as:

P(0 1 0 1 0 | release in 2)
 2 (1p 3 ) 3 p 4 (1 4 p 5 ).

Survival rates| 125

Period 1

Animal caught,

marked, and

released

Animal

alive

Animal

p (^2) caught^11

1
1–

1
1–p 2
10
10
Animal
not caught
Animal
dead or
emigrated
Period 2 Capture history
Fig. 5.1Tree diagram of events and their probabilities for a bird released in period 1
of a 2-period capture–recapture study under the CJS model.