The study done shows the mercury levels in different organism’s blood, by tracking these levels the mercury can be tracked as it moves through the food web. This looked at species around the Shenandoah River in Virgina where there was mercury contamination. The experiment tested 13 terrestrial-feeding bird species that bred within 50 m of the river, 5 aquatic-feeding species that had direct contact and species at an uncontaminated site. Both aquatic and terrestrial-feeding birds had similar levels which were significantly higher than the uncontaminated birds. This suggests that the birds do not need direct contact to be affected; if something they ate was contaminated it would be passed on to them. Predators typically only receive 10% of the energy from their prey, so they must eat more to receive sufficient energy for survival; this means if spiders are contaminated and the birds must eat many to get sufficient energy, then they will have much more mercury in their system then one individual spider.
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