Tree Swallow (Tachycineta bicolor)
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 aimed to verify that the tree swallow populations that reside within the Tittabawassee River floodplain were not experiencing adverse effects from site-specific contaminants, in particular, 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 define their dietary exposure to contaminants, site-specific diets of the tree swallow were determined through the collection of bolus samples. Bolus samples were 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 was left in place for 30 to 45 minutes, after which the samples were collected and the device removed. The site-specific dietary composition was then determined by identifying the individual insects represented. These prey items were then collected from the study area by the research team and analyzed for contaminant concentrations. For the tree swallow, these prey items included a variety of different small flies. These collections allowed us to estimate the concentration of contaminants tree swallows along the Tittabawassee River were exposed to through their diet.
Contaminant concentrations in the tissues of tree swallows were also analyzed to verify that their exposure had been characterized correctly. Specifically for the tree swallow, collected tissues included eggs and nestling tissues.
The health of the tree swallow population was assessed through the evaluation of several productivity measurements, which included 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, and study areas 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 provided us with the information necessary to estimate the risk of adverse effects to tree swallows that reside within the Tittabawassee River floodplain.