for similar sampling situations. Inferences about movements can also be based on
counts of birds. For example, Johnson and Grier (1988) drew inferences about
duck movements based on year-to-year changes in abundance estimates in differ-
ent regions. Although reasonable inference is sometimes possible, confidence in
the findings is limited because of the influence of other demographic variables. It
is difficult to attribute changes in abundance to the action of a single demographic
parameter, when all four parameters act in concert to determine abundance. In
this chapter, we therefore consider only methods based on marked individuals,
because such methods are well developed and permit separate estimation of both
survival and movement rates.
5.2 Tag type and subsequent encounters
Robust estimation of rates of survival and movement usually entails capturing
and marking birds with individual marks so that they can be recognized at
subsequent encounters (see Chapter 4 for methods of capture and marking). The
kind of mark applied determines the appropriate method for re-encountering
marked birds. For example, if birds are tagged with satellite transmitters then
re-encounter data are downloaded from satellites. If birds are tagged with standard
radio-transmitters (Chapter 6), then re-encounters are obtained via receivers
that can be handheld or mounted on vehicles or fixed structures. If bird tags are
visible from a distance (e.g. color rings or legbands, patagial tags, neck collars; see
Chapter 4) then re-encounters occur as repeat observations by investigators and,
in some cases, members of the public. If bird tags are not visible from a distance
but can only be read from a bird in hand (e.g. standard metal rings or legbands,
passive integrated transponder [PIT] tags; see Chapter 4), then re-encounters
occur via recaptures by investigators or by recoveries of dead birds by members of
the public. Some investigators use so-called “batch marks” (e.g. dyes or marks of
a single color) to identify birds caught at a particular time or location, or hatched
in a particular year. Such batch marks are much less useful than individual marks
for the purpose of estimating rates of survival or movement. We do not discuss
their use here.
5.3 Survival rates
5.3.1Radio-telemetry
Field sampling for survival studies using radiotelemetry usually involves a single
study area that is small enough to be traversed within a few days at most. Thus,
investigators with radio receivers try to cover the area at specified sample periods
120 |Estimating survival and movement