Organochlorine insecticides and mercury in predatory bird livers (PBMS)

The PBMS monitors contaminant concentrations principally in liver tissues and in addled or deserted eggs. The choice of matrix depends on the likely site of toxicological significance for any particular compound and/or the species of bird monitored. For example, SGAR residues are measured in the liver because it is the target organ for these compounds (where they interfere with the production of clotting factors) and contains high-affinity binding sites to which SGARs bind. Conversely some contaminants are embryotoxins and monitoring residues of these compounds in eggs is therefore more appropriate. Furthermore, the populations of some species, such as golden eagle (Aquila chrysaetos) and merlin (Falco columbarius), are at low densities in Britain and it is not possible to collect sufficient carcasses each year to obtain a statistically robust sample for monitoring liver concentrations. However, it is possible to collect reasonably large numbers of failed eggs for these species. Thus, monitoring contaminant levels in eggs allows us to study the impacts of chemicals on these rarer species. The sampling of carcasses for the PBMS relies on members of the public who, in response to advertisements, collect and send in any dead predatory birds that they find.

Taxidermists and wildlife rehabilitation centers also contribute significant numbers of specimens. However, only birds that have been freeliving prior to death, or at most have been in captivity for 7 days, are used by the PBMS. Volunteers submit the carcasses through the post using approved packaging (95kPa ADR [Road] compliant). Approximately 400 birds are submitted to the PBMS each year. Typically, the scheme receives most submissions in the autumn (September and October), and the early spring (February to April inclusive). Together, these five months account for approximately 60% of the birds submitted each year and predominantly consist of juveniles (birds that have hatched in the current or previous calendar year). This is reflected in the age ratio (juveniles to adults) of the whole sample which, over the last 10y, has been approximately 1.8 : 1, where adults are defined as individuals that have hatched before the previous calendar year. On receipt, the details of the bird are logged and the carcass is stored at -20 degrees Centigrade until a macroscopic postmortem examination is carried out.

The postmortem examination is conducted to determine age, using the European Union for Bird Ringing (EURING) classification (11), sex, body weight, and putative cause of death. Commonly, the birds have died from road traffic accidents, other types of collision, or starvation (17.5%, 24.9%, and 30.5% of sample respectively); other causes each account for the deaths of no more than 10% of all birds received. Additional observations include an assessment of the amount of fat reserves in the body, moult score (primaries, secondaries, tail and body feathers) (12), and the presence of hemorrhaging in the body organs. Where possible, whole organs are excised, weighed, and various tissues (liver, brain, kidneys, pectoral muscle, subcutaneous fat) are archived in glass jars at -20 degrees Celsius.

Only individuals that are licensed by the relevant regulatory statutory body can collect bird eggs in the UK. Licensed collectors visit nests as part of other studies, predominantly ringing exercises, and send to the PBMS eggs that are known to be addled or deserted. On receipt, the length, breadth and weight of each egg and the weight of its contents are recorded together with information on date of collection and provenance. Each egg is cracked using a scalpel, the contents removed, an approximate degree of development assessed, and the contents are homogenized prior to archiving at -20 degrees Centigrade in glass jars. The egg shell is rinsed, left to dry for 4 weeks at ambient temperature, then weighed. An egg shell thickness index (3) is calculated as shell weight (mg)/length (mm) _ breadth (mm).

At present, the core monitoring program of the PBMS involves analyzing contaminant concentrations in a subsample of the archived liver and egg samples each year. The livers from a stratified sample of sparrowhawks received each year, the livers of all herons (Ardea cinerea), and the contents of one egg from each clutch of merlin, golden eagle, and white-tailed sea eagle (Haliaeetus albicilla) eggs that have been received each year are analyzed for PCBs (35 individual congeners, total PCBs and sum PCB Toxic Equivalencies, PCB-TEQs), OC insecticides, and mercury. Approximately 10 gannet (Morus bassanus) eggs, collected from colonies on Bass Rock (North Sea) and Ailsa Craig (Irish Sea) are also analyzed for the same contaminants. Gannet eggs are usually collected biennially from each colony. The livers of a stratified sample of barn owls, and the livers of all kestrels (Falco tinnunculus) and red kites (Milvus milvus) received each year are analyzed for four SGARs, difenacoum, bromadiolone, brodifacoum, and flocoumafen.

Analyses for all the compounds are carried out by the Centre for Ecology & Hydrology, for which the methods of determination are described elsewhere (13, 14). The results of the monitoring are published in annual reports and scientific papers (13-16) and on a dedicated webpage (17). The volunteer collectors are also sent a post mortem report (carcasses) or biometric data (egg) for each sample they submit and a summary of the contaminant levels for those samples that are analyzed chemically. This level of feedback to collectors is essential in maintaining a strong relationship between the PBMS and the volunteer base that supports it.

The PBMS frozen archive contains samples dating back to 1968. It consists of approximately 17 500 tissue samples from 6000 individual birds and 9000 egg contents; the majority (82%) of tissue samples are from 1980 onwards. This archive constitutes a valuable resource for a range of environmental toxicology studies and other conservation-orientated research work. The archive facilitates assessment of the exposure to and effects of chemicals in predatory birds that is wider in scope than the core PBMS activities (18, 19), and can be used for piloting and refining new monitoring (20). It also provides a resource for projects that aim to assess the impacts of a range of nonchemical threats. In the past, these have included illegal trading and hunting (21), and quantifying the fine scale genetic structure of golden eagle populations (22). References available by downloading associated documentation.