described by Ingram (1978). A more recent refinement is that tissue adhesives
such as Vetbond, can be used to close the wound. A noninvasive technique—
magnetic resonance imaging—has been used to visualize internal organs of birds
(Romagnano et al. 1996) and to assess testicular maturity (Czisch et al. 2001).
However, the current availability of suitable MRI machines and their costs mean
that this technique is unlikely to become widely used in the near future.
9.3 Ecotoxicology
Toxicology is concerned with the harmful effects of chemicals in man and other
species. Two species of birds are used in standard OECD testing protocols—
Japanese quail and bobwhite quail. Historically, the toxicity of chemicals has been
measured as the median lethal dose (LD 50 ) following acute exposure to the chemical.
More recently, acute exposure has been used to assess the no observed effect
dose (NOED)—the highest dose that produces no lethal effects. Alternatives to
acute toxicity tests, which merely classify chemicals as harmful toxic or very toxic, are
now preferred because these require the use of far fewer animals (Timbrell 1995).
Ecotoxicology is concerned with how the harmful effects of chemicals on
individuals impact on populations and ultimately upon ecosystems (Walker et al.
1996). These chemicals may be anthropogenic (pollutants, pesticides) or natural
chemicals occurring at toxic concentrations as a result of human activity (conta-
minants). In its widest sense, ecotoxicology ranges from molecular effects to
effects on ecosystems. Consequently it encompasses a wide range of technologies.
Most studies in ecotoxicology involve measurements of pollutant residues in
tissues or assessments of the physiological changes caused by pollutants. Pollutant
residues can be measured in dead birds, but only if the pollutant is fairly stable and
not rapidly metabolized. For example, residue levels of organochlorine pesticides,
polychlorinated biphenyls, and mercury have been monitored in birds of prey in
the United Kingdom since the early 1960s (Newton et al. 1993). Marked declines
in the populations of birds of prey coincided with the introduction of organochlo-
rine pesticides, an effect later attributed eggshell thinning (Ratcliffe 1970; Peakall
1993) caused by DDE, a metabolite of DDT, and to direct lethal effects of the
more toxic organochlorines. Tissue residues of labile or rapidly metabolized
pollutants cannot easily be measured. However, it may be possible to measure the
physiological effects of such pollutants to assess exposure. Such responses are
called biomarkers (Peakall 1992; Walker et al. 1996).
One of the most commonly used biomarkers in avian ecotoxicology is the inhi-
bition of cholinesterase (Thompson 1991). Cholinesterase metabolizes the neuro-
transmitter acetylcholine in nerve synapses. Organophosphorus compounds and
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