Trace, macro and rare earth elements in a sedentary avian predator: Relationships with age and dietary tracers

The tawny owl (Strix aluco) has been proposed as a key species for pan-European monitoring of pollutants because of its long lifespan and sedentary behavior. We investigated the occurrence of a wide range of essential and non-essential elements in tawny owls from central Norway and compared elemental concentration between adults and nestlings and to dietary tracers (δ13C and δ15N). Concentrations of 61 elements were analyzed using whole blood and body feathers collected from adult females (n = 52) and their nestlings (n = 61). Most detected elements were present in higher concentrations in adult feathers or blood compared to nestling feathers or blood. Mean mercury (Hg) blood concentration in adults (0.069 μg/g) exceeded the mean threshold detrimental to health (0.03 μg/g), but no molar Hg:Se ratio exceeded 1:1, so selenium (Se) is in excess in all matrices to protect against adverse effects of Hg. Cadmium (Cd) exceeded sublethal concentrations associated with oxidative stress (0.0005 μg/g) in both adult (0.0006 μg/g) and nestling (0.0010 μg/g) blood, but levels did not exceed concentrations associated with adverse health effects. Cerium (Ce), neodymium (Nd), and yttrium (Y) were detected at the highest mean concentrations of rare earth elements (REEs) in all matrices. Values of δ15N indicated that adults (7.6 ± 0.9‰) fed at lower trophic levels and with a narrower dietary niche during feather growth than did nestlings (9.0 ± 1.8‰), a finding perhaps related to nestlings consuming a higher proportion of migratory passerines. We found no relationship between relative trophic position (δ15N) or dietary carbon source (δ13C) and elemental concentrations in adults, but data on δ15N indicated biomagnification of several of the elements in nestlings, many of which were REEs. This emphasizes the need for further investigation of the biomagnification potential of REEs and their potential source, such as migrating birds.