1. Introduction

Researchers of wild animals are increasingly conducting
projects that include measures of circulating hormone levels
or the distributions and abundances of white blood cells as
a means to evaluate health or stress in animal populations.
While the specific goals of the projects may vary, they all have
in common certain methodological approaches; animals are
captured in the wild and a suite of samples (tissue, blood)
and/or measurements are collected from them that allow the
investigator to gauge the health state of the population or
to assess health- or immune-related parameters (e.g., [1–3]).
One of the most commonly assessed pieces of information in
such projects is data on the numbers of circulating leukocytes
(white blood cells) in the animals, which can be gathered
from examination of stained blood smears on microscope
slides. As the primary line of defense against foreign pathogens,
leukocytes make up an important component of the
innate or nonspecific immune system [4–6], thus their concentrations
are often of particular interest. Indeed, interest
in obtaining data on leukocyte numbers is rapidly growing
among animal ecologists (e.g., [7–11]). However, one aspect
that investigators must consider in any research project involving
capture of wild animals is the method of capture
[12, 13]. This may be particularly relevant when multiple
capture techniques are used across studies.
Two recent reports have described the effects of trapping
on basal levels of stress hormones in wild animals; Fletcher
and Boonstra [14] examined populations of meadow voles
(Microtus pennsylvanicus), while Lynn and Porter [15] studied
house sparrows (Passer domesticus). In both cases,
levels of plasma stress hormones were compared in animals
captured with baited cage traps (where animals freely enter
and remain for some period of time before sampling) to