Developing monitoring methods for cryptic species : a case study of the Australasian bittern, Botaurus poiciloptilus : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Manawatū, New Zealand
Difficult-to-detect species (here-after, ‘cryptic’) are problematic to monitor. This is because
sampling is often restricted by logistic complications, and species-detectability tends to be
low and/or highly variable. Such challenges create data that are complex to interpret, and
contain biases that cannot be estimated, making results less meaningful. Yet there is a need
to monitor such species as they are also often rare.
In this thesis I review 30 publications, covering 28 different species, to demonstrate that
challenges experienced across cryptic species fall into four categories: visually-cryptic,
behaviourally-cryptic, spatially-cryptic and temporally-cryptic. The Australasian bittern
(Botaurus poiciloptilus) is an appropriate case-study for examining the process of developing
a monitoring method for cryptic threatened species because they have all four cryptic
characteristics. Yet bitterns are also endangered, and what is left of their habitat is underthreat.
Currently the most feasible monitoring method available for bitterns is counts of
male calls (booms) during the breeding season. However, calling-rate is known to be variable
and difficulties in accessing some sites restricts sampling possibilities.
I fitted a range of generalised linear mixed models to 461 15-min call-counts, conducted in
a range of conditions, during two breeding seasons at Whangamarino wetland, to identify
factors affecting calling-rate-per-individual-bittern (CRPI). Results showed that CRPI was
predictable in terms of time-of-day, time-of-year, cloud-cover, rainfall and certain moon
parameters, but some spatial and temporal variation remained unexplained. Additionally, I
showed that recorders are a cost-effective practical solution to logistical constraints
restricting sampling possibilities at some sites. Furthermore, I show that abundance can be
estimated from calling-rate by correcting for effect sizes of factors affecting CRPI. Results
obtained using 269 15-min sound-files at two sites (Whangamarino wetland and Lake
Whatuma) show that these abundance derivations are accurate but imprecise. To
understand more about how call-based methods can be used to monitor bitterns, I radiotracked
six males throughout the optimum monitoring-period to confirm that these birds
have high site-fidelity, therefore, validating territory-mapping method assumptions. The
approach used in this thesis is applicable to any cryptic species, as illustrated with the Guam
rail (Gallirallus owstoni) in my final discussion.