The transfer of nutrients between marine and terrestrial systems has important ecological consequences, and animal movement is an important driver of nutrient transfer. Coastal birds forage in marine environments and breed in terrestrial habitats, and thus serve as vectors moving nutrients from the sea to the land. However, urbanization can influence the extent to which coastal birds forage in marine or terrestrial environments due to the availability of human subsidies. Establishing a reliable and straightforward indicator of marine foraging would be useful for assessing changes in the use of terrestrial vs. marine habitats in the face of urbanization and broader environmental change, and for understanding the flow of nutrients and energy between terrestrial and marine environments. Mercury (Hg) is a highly toxic heavy metal which bioaccumulates in marine food webs, and is a potential indicator of marine foraging. Methylmercury (MeHg), is present only in aquatic ecosystems and reaches elevated concentrations in the prey species of marine birds. Thus, high concentrations of MeHg would be expected for birds foraging in marine environments in comparison to those foraging on terrestrial sources. We hypothesized that the degree of marine foraging influences Hg uptake in coastal birds. To test this hypothesis, we combined GPS tracking data with measurements of Hg concentrations in the blood of herring gulls (Larus argentatus) along an urban gradient in the northeast United States. We examined Hg concentrations for 51 individual herring gulls tracked with GPS tags at study sites representing high, medium and low degrees of urbanization. Our results showed a strong and significant positive relationship between Hg concentrations and the proportion of herring gull foraging locations occurring in offshore waters. Hg concentrations differed significantly between herring gulls whose primary foraging habitat occurred in marine vs. terrestrial environments. Gulls in more urban colonies spent less time foraging in marine environments, and had significantly lower Hg concentrations than those at the more remote study. Our results suggest that Hg concentrations in blood can be used to reflect the extent of marine foraging for animals using both marine and terrestrial habitats. Hg concentrations could be valuable monitoring tool to assess how the use of marine foraging habitats changes through time (dietary shifts) or relative to environmental change such as urbanization.

海洋和陆地系统之间养分的转移具有重要的生态后果，而动物的运动是养分转移的重要驱动力。沿海鸟类在海洋环境中觅食并在陆地栖息地繁殖，因此成为将营养物从海洋转移到陆地的媒介。但是，由于可获得人类补贴，城市化会影响沿海鸟类在海洋或陆地环境中觅食的程度。建立可靠而直接的海洋觅食指标，对于评估面对城市化和更广泛的环境变化的陆地和海洋栖息地的使用变化，以及理解陆地和海洋环境之间的养分和能量流，将很有用。汞（Hg）是一种剧毒的重金属，会在海洋食物网中生物富集，并且是海洋觅食的潜在指标。甲基汞（MeHg）仅存在于水生生态系统中，并且在海洋鸟类的猎物中浓度升高。因此，与在陆地上觅食的鸟类相比，在海洋环境中觅食的鸟类有望获得高浓度的甲基汞。我们假设海洋觅食的程度会影响沿海鸟类对汞的吸收。为了验证这一假设，我们将GPS跟踪数据与鲱鸥血液中Hg浓度的测量值相结合（与在陆地上觅食的鸟类相比，在海洋环境中觅食的鸟类有望获得高浓度的甲基汞。我们假设海洋觅食的程度会影响沿海鸟类对汞的吸收。为了验证这一假设，我们将GPS跟踪数据与鲱鸥血液中Hg浓度的测量值相结合（与在陆地上觅食的鸟类相比，在海洋环境中觅食的鸟类有望获得高浓度的甲基汞。我们假设海洋觅食的程度会影响沿海鸟类对汞的吸收。为了验证这一假设，我们将GPS跟踪数据与鲱鸥血液中Hg浓度的测量值相结合（鸥）沿美国东北部的城市梯度。在代表高，中，低城市化程度的研究地点，我们检查了用GPS标签追踪的51只鲱鱼的汞含量。我们的结果表明，汞浓度与在近海中发生的鲱鸥觅食位置比例之间有很强的正相关关系。在主要觅食栖息地发生在海洋与陆地环境中的鲱鸥之间，汞浓度存在显着差异。在更多的城市殖民地中，海鸥在海洋环境中觅食的时间更少，并且汞含量明显低于更偏远的研究。我们的结果表明，血液中的汞浓度可用于反映使用海洋和陆地生境的动物在海洋中觅食的程度。