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 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.  Unfortunately, stopover sites are in decline.

For those of you who think there's nothing left to discover, let me assure you that in the field of migration stopover ecology, many mysteries remain.   The pressing question is: What determines whether an area is important as a stopover site?

The logical answer is resources.  An important stopover site will have high quality habitat that is rich in resources, enabling migrants to rapidly refuel and continue on their way.  However, for any given site, many other factors influence whether or not it is valuable as a stopover.  Some are:

-- Is the site near an ecological barrier, such as a large body of water?  These sites usually concentrate migrants, sometimes regardless of habitat quality.
-- Is there a high risk of predation?  Suburban areas with their host of predators, or coastal areas which are favored migration routes of some hawks, can be most risky.
-- Does the habitat structure offer efficient foraging opportunities?

In order to assess whether a site is being used as a stopover, and to determine the quality of the habitat as a stopover, we can look at several things:

-- The variety and number of migrants using the site
-- How long they stay before resuming migration
-- How much weight they gain during their stay

Bird banding is perhaps the only way for us to answer these questions.  We can analyze birds that are recaptured one or more days after they were initially banded.  One might assume that large numbers of birds being recaptured and gaining weight means an area is an important stopover.  But some researchers maintain that abundance is not necessarily an indicator of a good stopover site.  Far from being straightforward, thoughtful consideration unveils a whole slew of additional variables:

-- How far does this species have left to travel on migration?  The further their goal, the less time they tend to spend in a stopover area, particularly in spring, even if an area is resource rich.
-- What is this species' migration window?  September 10 might be early in migration for a Yellow-rumped Warbler, but late for a Yellow Warbler.  As migration progresses, typically so does the pace, so the species might spend less time in a stopover area.
-- How old is the bird?  In the fall, young birds forage less efficiently and tend to be subordinate to older birds and thus are forced to use sub-optimal habitats. Therefore, they tend to stay longer and gain weight more slowly.  Some sites have more young birds or old birds, depending on where the site location (adults tend to travel in the middle of the species' migration route, young birds along the edges).
-- How fat was the bird when it arrived?  The leaner the bird, the more likely it is to stay at a site.
-- Is it spring or fall?  Migrants utilize different resources (more insects in spring, fruit in fall), so each season must be considered separately.

As you can see, these types of variables force researchers to consider each site, and often each species or age group, a bit differently.  This makes research projects time-consuming and complex, and answers elusive to ferret out!

The role of  the Rouge River Bird Observatory

RRBO conducts its research on the campus of UM-Dearborn, one of the last remaining large natural areas in highly urbanized southeast Michigan (here's an aerial view).  As an inland location not near an ecological barrier, it may offer a less biased view of the quality of stopover habitat.  For example, a fat depleted bird must take care of immediate energy requirements after a flight over the Gulf of Mexico upon first arrival on-shore, even if the habitat isn't ideal.  When there is no ecological barrier, a migrant may "shop around" for the best available habitat.

RRBO has gathered detailed information on migrants utilizing the campus, far more than can be explained on this page.  But for an overview of this complicated issue, let's look at some summary results.

We looked at the stopover patterns of 48 species of birds, those which are true migrants (not known to breed regularly) in our area. 

True Migrants recaptured at RRBO

	Percent recaptured	Mean stay in days	Percent of original body weight change
Spring	4.8% (N=4017)	3.5	+2.1%
Fall	8.0% (N=6920)	5.5	+4.0%

What does this mean?  These numbers seem very small, but in studies of this type, they are always conservative. Here's why:

-- Birds may not be captured on the first day they arrive, or recaptured on the last day they stay.
-- Banding tends to only capture 10% of the birds present in an area.
-- At RRBO, birds are released about a half-mile from the banding area, and have a large area to disperse to other than back to the banding area.

RRBO's recapture rates are actually quite good, and comparable even to areas with ecological barriers reported in other studies (but comparison is very difficult, see below).

Weight gains shown here also appear low.  However, due to the "fuel efficiency" of birds, a huge weight gain is not needed for them to continue with their migration, particularly in this region.  For example, energetic studies have shown that a Swainson's Thrush requires about 1.6 grams of fat to enable it to fly 6 to 8 hours overnight for a distance of about 170 miles.  This translates into around 5% of its body weight. Swainson's Thrushes at RRBO gain 4.5% of their original body weight in spring, and 9% in fall during their stay here (remember, the figures above are aggregate).

The future

Relatively few studies have been directed toward the needs of birds on migration.  Worse, many different methods were used to calculate or present weight gains.  Various species were examined, sometimes no effort was made to separate residents from migrants.  Some studies used baited nets or traps, an added resource which obviously influences both length of stay and weight gain.  Most studies took place in coastal areas near an ecological barrier.  Therefore, it's very difficult to offer a comparison of our data, or to even accurately compare two similar studies.

Clearly, unraveling the complexities of the needs of en route migrants requires a long-term commitment to research.  Intelligent, informed decisions on the conservation of stopover areas can only be made as researchers settle on standardized ways of gathering and analyzing data, and more sites across a wider range of habitats are investigated.  Coastal sites are increasingly well examined, while very few studies take place inland, and fewer still in urban natural areas -- making RRBO's research even more important.
 

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