Migratory Stopover Ecology

Bird migration is arguably one of the most fantastic events in the natural world. Birds have developed incredible adaptations that have enabled them to adopt this unique lifestyle.  Migration is also perhaps the most dangerous period during a migratory bird’s annual cycle. Each journey is perilous.

Places for birds to rest and refuel along their migration routes — known as stopover sites — are crucial to a successful migration. Most songbirds migrate at night. Each day during migration, they must find a safe place to stop, rest, and refuel in order to maintain their health and continue their journey.

Increasingly, these sites are located in urban areas. The Rouge River Bird Observatory has been a pioneer in the long-term study of urban migratory stopover sites. The vast majority of published papers on stopover ecology have focused on coastal or rural sites, and frequently cover a study period of only 2 or 3 years — the length of time a graduate student takes to complete a degree. RRBO’s work is unique with its focus on an urban, inland site, and especially its long-term nature.

RRBO research: Birds gain mass at our urban site during fall migration

Gray-cheeked Thrush

An analysis of weight and fat data for over 2,000 individual Catharus thrushes banded by RRBO on the UM-Dearborn campus during the fall seasons 1992 to 2006 was published in the peer-reviewed Wilson Journal of Ornithology in 2009.  The three focal species, Gray-cheeked Thrush (C. minimus), Swainson’s Thrus (C. ustalatus), and Hermit Thrush (C. guttatus), do not breed at our latitude and are considered passage migrants through our site.

Our study found that the majority of individuals of all three species gained significant fat and weight during their stopover on the UM-Dearborn campus.

Swainson's Thrush

This paper, A fifteen-year study of fall stopover patterns of Catharus thrushes at an inland, urban site was the first long-term study published of its kind on urban stopover ecology in North America.

The findings of mass gain sufficient for these birds to continue their migration is especially important because in an urban site such as ours, migrant birds are faced with unique challenges, including a high level of human disturbance which may decrease foraging opportunities, and a preponderance of non-native fruits.

What resources are important for migratory birds at urban stopover sites?

Catharus thrushes rely heavily on fruit during fall migration. Because most migrant birds, including those that typically eat insects, also incorporate fruit into their fall migratory diet, the thrushes make great surrogates for a whole suite of migrant species.

Hermit Thrush

Much of the native fruit, to which North American birds are evolutionarily adapted, has been depleted by other species by the time the bulk of the thrushes move through. This is especially true for Hermit Thrushes, which are late migrants. The remaining fruits are predominantly introduced, invasive species such as buckthorn and honeysuckle. Since thrushes gain mass on a diet high in non-native fruits, it demonstrates that some introduced plants do perform ecological functions.

RRBO is now focusing on answering questions such as which fruiting plants and shrubs commonly found in urban areas — both native and introduced — are most important to migratory birds? What is their relative availability? If highly invaded urban natural areas are to undergo restoration efforts, which introduced plants should be left to provide resources for migrant birds while native plants become established? Understanding these dynamics will influence bird conservation and habitat management decisions.

Because fruit passes through birds so quickly (and it is thought that many birds, especially migrants, choose fruit that can be processed quickly and efficiently), it is likely that seeds found in fecal samples of the birds that we capture represent fruit consumed on-site. RRBO began collecting samples from Catharus thrushes in 2007. In order to determine if resident birds are competing with migrants for the same fruits, or depleting certain fruiting species before migrants arrive, collection of samples from all species of birds, concentrating on American Robins and Gray Catbirds, began in 2009.

As urbanization transforms native ecosystems, understanding how birds use urban habitat as migratory stopover sites is becoming more critical to bird conservation. RRBO is dedicated to contributing meaningful data and expanding our knowledge to help migratory birds.

Student research opportunities are available, and we welcome collaboration with other institutions.

What you can do to help birds during migration

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