Mercury exposure of a wetland songbird, Agelaius phoeniceus, in the New York metropolitan area and its effect on nestling growth rate.

The potential for mercury accumulation in free-living passerine birds is now recognized to be much greater than previously assumed. However, lowest observable effect levels have yet to be well established for this taxonomic group and it is usually unknown whether levels observed in the wild are causing adverse effects. We measured total blood mercury (THg) levels and took repeated morphological measurements from nestling red-winged blackbirds (Agelaius phoeniceus; N = 39) in the New York metropolitan area to investigate whether mercury affected their growth rate. We also compared THg levels of nestlings (and parents; N = 14) between our two study sites, which included riparian habitats along a city river and surrounding ponds in a nearby suburb, to examine differences between birds within and beyond the urban core. THg levels ranged 0.009-0.284 ppm in nestlings and 0.036-0.746 ppm in adults. Adults and nestlings had significantly higher THg outside of the city than within, possibly due to the ability of rivers to flush contaminants and the higher methylation potential of ponds. Among our candidate sets, models containing THg had minimal support for explaining variation in nestling growth rate. Summed Akaike weights further showed that THg had little relative importance. Mercury pollution in our sites may be low, or feather growth may have been sufficient to protect nestlings from accumulating harmful mercury levels in living tissues.