Poorer Health in Migrating Catbirds That Eat Fruits from Invasives?

Need another reason to get rid of non-native, invasive shrubs and to plant or otherwise encourage native shrubs in your yard or when conducting a habitat management project?

A recent study (see attachment below) by scientists compared the health of migrating gray catbirds -- primarily berry and fruit feeders -- that spend time in habitats where mostly exotic berries are available, such as those from non-native bush honeysuckle and autumn olive, versus habitats where they can fuel up on native berries like spicebush, winterberry, and gray dogwood.

gray catbird

Migratory gray catbirds that fuel up on fruit from native shrubs appear to have better immune system functioning and more circulating antioxidants than catbirds that eat fruits of honeysuckle, autumn olive, and other invasive shrubs./T. Berriman

Catbirds that the researchers captured and studied in areas with abundant fruit-producing native shrubs had better immune system function and more circulating antioxidant compounds than birds in habitats choked with invasives. The results for another type of bird that also depends on eating fruit during migration, Swainson's thrush, weren't as clear.

Yushi Oguchi, Robert Smith, and Jennifer Owen conducted their field research near East Lansing, Michigan. In a paper published in The Condor, they write that “Migration is a physiologically demanding activity. Recent studies suggest that migrating birds can improve their immune and antioxidant status during stopover, implying that variation in stopover habitat can affect migrants’ health.”

The scientists also noted that catbirds using native-dominated shrublands retained their body mass, while those in exotic-dominated shrublands lost mass.

Ogushi, Smith, and Owen used mist nets to capture birds during the autumn migration, then withdrew small amounts of blood from their subjects before freeing the birds to continue south. Altogether they caught and studied more than 400 gray catbirds and a similar number of Swainson’s thrushes.

They also studied the chemical components of fruits produced by both native and exotic shrubs.

The researchers suggest that “measuring immune and antioxidant condition in addition to refueling performance will better reveal habitat quality for migrating birds.”

They believe more studies are needed to explore “links among the energy and nutritional contents of [migratory birds’] diets, refueling performance, immunity, and antioxidant capacity,” and to “explicitly test whether health recovers during stopover” feeding periods.

Nevertheless, their results are another piece of information that conservation-minded citizens and landowners should consider when deciding what kinds of shrubs to use for landscaping, or in projects aimed at creating good-quality young forest and shrubland habitat for wildlife such as American woodcock, New England cottontails, and songbirds.