Tree Swallow

The tree swallow is a migratory songbird only found along the Tittabawassee River from around mid-April through early July; however, given their abilities to perform highly acrobatic flight patterns, they can easily be spotted foraging in the floodplain fields and along the river. Our research aims to verify that the tree swallow population that resides within the Tittabawassee River floodplain is not experiencing adverse effects from site-specific contaminants, specifically dioxins and furans.

Tree swallows possess many ideal attributes of a study species for assessing bioaccumulative compounds in the environment, including being obligate cavity nesters, having an almost completely aquatic-based diet, having a widespread distribution, being limited in foraging range during nesting, being resistant to disturbances while nesting, and having been commonly used as a study species for past site-specific risk assessments.

To determine their dietary exposure to contaminants, site-specific diets of the tree swallow are determined through the collection of bolus samples. Bolus samples are collected from nestling birds between the ages of 3 and 12 days old by putting a restrictive device around their throat to prevent them from swallowing prey items brought to the nest by adults while still allowing them to breathe normally. The device is left in place for 30 to 45 minutes, after which the samples are collected and devices removed. The site-specific dietary composition is then determined by identifying the individual insects represented. These prey items are then collected from the study area by the research team and analyzed for contaminant concentrations. For the tree swallow, this includes a variety of different small flies. These collections allow us to estimate the concentration of contaminants tree swallows along the Tittabawassee River are exposed to through their diet.

Contaminant concentrations in the tissues of tree swallows are also analyzed to verify that their exposure has been characterized correctly. Specifically for the tree swallow, tissues collected include nestling tissues and eggs.

The health of the tree swallow population is assessed through the evaluation of several productivity measurements, including nest occupancy, nest success, clutch size, hatching success, and fledgling success.

Tree swallow data have been collected from reference areas in Sanford, Michigan, and the Pine and Chippewa rivers in and around the Chippewa Nature Center to downstream of Midland, Michigan, ranging to the Shiawassee National Wildlife Refuge and beyond to near the confluence of the Saginaw River with Saginaw Bay.

Integrating the findings from all lines of evidence will provide us with the information necessary to estimate the risk of adverse effects to tree swallows residing within the Tittabawassee River floodplain.