to land after dark, and burrows may be occupied by more than one pair, or they
may be unoccupied. It is possible to assess whether burrows are occupied using
playback methods (although you need to know or measure the response rate),
endoscopes, smell, or by planting toothpicks around the entrance to the burrow
and seeing whether these get knocked over (but beware pre-breeding birds that
are prospecting for nest sites). Multiple occupancy of burrows is difficult to
detect and remains a problem. Steinkamp et al. (2003) provide a practical and
detailed review of survey methods for seabirds and colonial waterbirds.
2.3.9 Counting roosts and flocks
Counting large aggregations of birds away from breeding colonies poses many
of the same problems as counting birds in colonies, but with some additional
considerations:
- If disturbed by the counter, birds are unlikely to return to the same place;
observers need to maintain a distance. - Birds may be closer together than when they are in nesting colonies where
they tend to space themselves out, so great care is needed to count those
present. - Flocks often contain several species; it is necessary to count each separately.
- Some aggregations, such as roosting flocks, form for only short periods,
often when light conditions are poor. Counts of nocturnal roosts often
require the use of photography or of counts of groups of birds joining the
roost.
Stationary flocks of up to 500 birds can be counted directly with relative
ease if conditions are good. For larger flocks, and for rapidly moving flocks,
photography or estimation methods are needed. A common method when
estimating very large flocks is to count, say, 10, 20, 50, 100, or 500 birds and then
estimate what proportion of the flock this represents. An important consideration
when using this method is that birds in flocks do not tend to be evenly distributed,
with higher densities in the center of the flock and lower densities at the periphery.
Alternatively, for wading birds feeding on open mudflats and waterbirds on lakes,
the flock can be broken down into smaller counting units using natural features of
the habitat or distant landmarks. When birds are in dense groups, accurate counts
are only possible by counting from above, or by counting them as they enter or
leave an area. Care is needed so that counting does not disturb the birds; count
from concealed or raised positions. The exception to this rule is the flushmethod
in which birds are deliberately flushed into the air in order to get a better count of
numbers (see Steinkamp et al. 2003). Coastal birds might be more easily counted
Field methods| 45