TWEL Keith Norris Thesis

Body mass dynamics, stopover durations, and habitat conditions for migrant shorebirds in the southwestern Lake Erie marsh region

Keith Alan Norris, MS

Advisor:  Robert J. Gates

Thesis

Migration is energetically expensive for shorebirds.  Wetland and coastal areas along migration routes provide critical stopover habitat and food resources for shorebirds to rest and replenish fat reserves needed to complete migration.  Migration strategies and use of stopover habitats may differ between inland and coastal migrating shorebirds.  Further, loss of wetlands throughout inland North America has reduced the amount of stopover habitat available for migrating shorebirds.  Limited habitat can create shortages of food resources at major stopover locations which can cause energetic shortfalls that prevent shorebirds from completing migration in good body condition, potentially impacting survival and reproduction, thereby contributing to population declines.

I conducted research at an inland stopover site to examine within and cross-seasonal changes in shorebird body mass and to investigate relationships of body mass dynamics to stopover duration, food availability, and habitat conditions.  My investigation sought to determine if migrating shorebirds are resource limited, and if so, which season (autumn vs. spring) is most limiting.  Changes in shorebird body mass (e.g. loss or failure to gain adequate body mass) could indicate resource limitation.  Assessments of stopover durations provide additional insight into potential resource limitations.  Direct assessments of invertebrate food resources and habitat conditions provide additional insight into potential limitations encountered by migrating shorebirds. Knowledge of food and habitat resources and potential energetic limitations along shorebird migration routes is important for strategic habitat conservation planning because habitat objectives are set based on bioenergetics models that assume shorebirds are limited by habitat.

I analyzed data from 9,018 shorebirds banded during autumn (2006-2013) and 946 shorebirds banded during spring (2012-2013) migration at Winous Point Marsh Conservancy (WPMC) and Ottawa National Wildlife Refuge (ONWR) wetland complexes in the southwestern Lake Erie marsh region (LEMR).  This area is one of three shorebird stopover sites in the Midwest region identified as a Site of Regional Importance by the Western Hemisphere Shorebird Reserve Network.

Analysis of within-season recaptures (n = 783) indicated that shorebirds gained body mass during autumn and generally maintained body mass during spring in the LEMR.  Rates of body mass gain were 0.28 g/day for least sandpiper, 0.55 g/day for semipalmated sandpiper, 1.10 g/day for pectoral sandpiper, and 1.49 g/day for short-billed dowitcher over the full estimated stopover duration in autumn.  Autumn minimum stopover duration estimates were doubled to calculate full stopover durations in the LEMR of 12 days for semipalmated sandpiper (n = 301), 13 days for short-billed dowitcher (n = 69), 14 days for pectoral sandpiper (n = 35), and 16 days for least sandpiper (n = 281).  Rates of mass gain slowed and began to decline for semipalmated and least sandpipers after the full stopover duration period, but remained linear for pectoral sandpipers and short-billed dowitchers.

I assessed habitat conditions and invertebrate food resources from saturated mud and shallow water ≤5 cm in areas on or near where shorebirds were banded.  Habitat conditions varied between and within seasons with weather patterns and wetland management activities.  Primary shorebird foraging zones of saturated mud and shallow water ≤5 cm rarely dominated an area.  Food resources were highly variable across all seasons and sampling periods.  Means of median biomass levels ranged from a low of 3.67 kg/ha (95% CI ±4.61) in spring to a high of 12.14 kg/ha (95% CI ±5.68) in autumn. 

My research results can inform bioenergetics models used to set habitat objectives for the Upper Mississippi River & Great Lakes Region Joint Venture’s Shorebird Habitat Conservation Strategy.  My estimated rates of body mass gain, full stopover duration, and food resource levels applied to the Joint Venture’s bioenergetics model increased estimated foraging habitat requirements for four species by 166% in order to maintain population levels estimated by the Joint Venture.  The bioenergetics model was heavily influenced by the stopover duration value used in the model.  Using full stopover duration estimates rather than minimum stopover duration estimates doubled the amount of habitat needed.  Results indicate that shorebirds may presently be limited by habitat and food resources in the Joint Venture’s planning region.  Spring season is likely more limiting for shorebirds based on reduced invertebrate forage biomass and lack of gains in body mass gains.