Trends in herring gull egg quality over four decades reflect ecosystem state

Egg quality (size, energy density) is important in determining early survival of birds. Here, we examine temporal (1981-2019) trends in herring gull (Larus argentatus) egg volume and energy density at breeding colonies on all five Laurentian Great Lakes. Temporal declines in egg volume were observed at 4/6 colonies on the upper Great Lakes (Lakes Superior, Michigan, Huron). On the lower Great Lakes (Lakes Erie, Ontario, and connecting channels) egg volume declined at 3/8 colonies and increased at one site. Egg energy density (kJ/g of egg contents) declined at 4/6 upper Great Lakes colonies and at 2/8 lower Great Lakes colonies. All of the upper Great Lakes colonies showed declines in either egg volume or energy density, or both, and these declines were related to dietary markers in eggs (fatty acids, stable nitrogen and carbon isotopes). On the lower Great Lakes and connecting channels, declines in egg volume or energy density were related to dietary endpoints in 3/5 instances. An information-theoretic approach indicated that trends in egg volume were best explained at the colony level while egg energy density trends were best explained by lake of origin. Diet-related declines in herring gull egg quality are likely a reflection of broad-scale ecosystem changes limiting aquatic food availability for gulls, particularly on the upper Great Lakes. These changes may be contributing to population declines in herring gulls and other surface-feeding aquatic birds. This study highlights the value of long-term monitoring of wildlife for identifying ecosystem change. Crown Copyright (C) 2020 Published by Elsevier B.V. on behalf of International Association for Great Lakes Research.