What Do Grassland Birds Need for Migratory Habitat?

A new study sheds (a little) light on a slice of grassland bird life we know very little about.

We know a lot about grassland birds.  Sure, there’s a lot we don’t know too – but we certainly know a lot more about grassland birds than we do about leafhoppers, nematodes, or most other groups of grassland species.  So how is it possible that we know almost nothing about what kinds of habitat grassland birds need during migration?

Do lark sparrows look for the same habitat during migration as they use for nesting? We really have no idea.

I’ve wondered about this for a long time – ever since I did my graduate work on grassland birds back in the 1990’s.  At that time, I was one of a growing number of researchers investigating the importance of the size and shape of prairie habitats to grassland birds.  We now know that prairie size and shape are very important, and more recent studies are even showing that the amount of grassland in the neighborhood around a particular prairie influences whether or not grassland birds will nest there or not.  As we continue to refine our understanding of breeding habitat needs, however, we’ve made no significant progress at all on what grassland birds need for habitat during migration – even though we know that migration is an incredibly stressful part of a bird’s life cycle.  It’s reasonable to wonder, in fact, if population declines in many grassland bird species could be more strongly tied to migratory and winter habitat than to breeding habitat.

For more than a decade, the only available published information on habitat use by grassland birds outside of their breeding areas has been a very nice study of winter habitat use in south Texas from 1999.  That research found grassland birds using a much broader array of habitat types than expected, including brushy and even woodland habitats.  The authors, however, were quick to point out that just because birds were using particular habitats doesn’t mean that those habitats were preferred, and there is no data on how well birds were doing in those habitats.  Still, it’s intriguing to think that bird species that are so picky about finding big open grasslands for nesting might not hang around in the same habitat types at other times of the year.

Just last week, I talked to one of the authors (Patrick Doran, The Nature Conservancy – Michigan) of a new paper that gives us our first real peek into the migratory needs of grassland birds.  Bruce Robertson, Doran, and others conducted a study of how grassland birds utilize grassland and biomass crops as migratory habitat as part of a larger project to assess biomass crops and their potential impact on birds in Michigan.  Their data on bird use of switchgrass fields and prairies during migration showed that obligate grassland birds (species that rely solely on grasslands for breeding habitat) were positively tied to large grassland patches and tended to stay away from wooded areas.  In addition to those two variables, obligate grassland birds also tended to use grasslands with patchy vegetation structure (a mixture of tall and short structure) more than uniformly tall/dense grass.

Large prairies with heterogeneous vegetation structure may be favored by migratory grassland birds. In August and September, as many grassland birds are moving south, both seeds and insects are abundant in many prairies. Is one food source more important than the other? Is either important? Or are birds simply looking for a safe place to rest before continuing south?

This project was a great first step, but like all good research projects, it left us with more questions than answers.  What are those grassland birds using the habitat for?  Are they feeding significantly while there or just resting?  If they’re feeding, are they eating insects or seeds – or both?  Do grassland birds look for the same habitat structure (e.g., tall/dense or short/sparse, etc.) when choosing migratory stopover habitat as they do when choosing breeding habitat?  Knowing the answers to those questions would sure be helpful as we think about how to manage prairies.  For example, assuming habitat structure is important to grassland migrants, decisions whether to burn a prairie in the fall, early spring, or late spring could have very big implications for migratory birds.

One of the most interesting findings in the study by Robertson and his colleagues is the suggestion that grassland patch size might be important for migratory grassland birds.  Woodland migrants have been more extensively studied than grassland birds, and while the picture is not yet clear, it doesn’t appear that birds that require large woodlands for breeding habitat necessarily require the same for migratory stopover habitats.  As an example, a study in eastern South Dakota found that neotropical migrants seemed to use small farmstead woodlots and large Missouri River woodlands interchangeably.  Again, there are still lots of questions still to answer (e.g., are there differences in stress levels or weight loss between habitat types?) but that apparent ability for woodland birds to feed and roost in small woodland habitat during migration has important implications for conservation.  If larger woodlands were required for migratory stopover sites, our challenge to provide appropriate habitat would be much more difficult – especially in landscapes like those in North America’s Great Plains.  It looks like we might not get off so lightly in the case of grassland birds.  If this recent study is representative, the future of grassland bird populations may hinge on our capacity to increase the number of large (how large is large enough??) grassland habitat patches in our most fragmented landscapes.

I’m hoping that the study by Robertson and his colleagues will spawn others like it.  However, studying migratory grassland birds is difficult.  How many of you are comfortable identifying little brown grassland birds when they don’t sing – and when your only look at them comes as they are darting away from you and diving back into the grass?  Regardless of the difficulty, though, we really need the information, and that information doesn’t have to come soley from academic research.  Any birdwatcher or prairie manager could add critical information to what we know by simply keeping track of the grassland bird species they see during spring and fall migration, and noting any habitat characteristics they can (e.g., prairie size, distance from trees, habitat structure – tall, short, dense, sparse).  At worst, sharing field notes with each other might give us at least something to go on as we think about management, and at best could provide pilot data to help design more effective research projects than could be designed by starting from wild guesses.

It seems reasonable to expect that late summer haying reduces the quality of prairies for migrating grassland birds. On the other hand, maybe not?

Our Platte River Prairies are located just to the south of an incredible grassland landscape – the 12 million acre Nebraska Sandhills.  I sometimes wonder about the birds that nest there.  They aren’t lacking for breeding habitat quantity or quality, as far as we know, but they’re only in those big sandhill prairies for a small proportion of their year.  In order to survive to the next breeding season, those birds have to make their way south through some very fragmented landscapes, survive the winter in what can be perilous southern habitats, and then work back north through those fragmented landscapes again.  Our Platte River Prairies are probably among their first stops heading south and their last stops heading north.  Are there management practices that we’re doing that are contributing positively or negatively to the success of those migratory birds?  We really have no idea.  That makes me uncomfortable.

I imagine the birds are a little nervous too.

Report from the 2011 Grassland Restoration Network – Part 2: Grassland Birds

The Grassland Restoration Network’s 2011 annual meeting was at The Nature Conservancy’s Kankakee Sands prairie/wetland restoration site in Indiana.  This year, we focused more than usual on creating habitat for various animal taxa, and I previously reported on the herpetology portion of the meeting.  Here is a summary of the discussions we had regarding grassland birds.

While the diversity of insects in a prairie is strongly tied to plant diversity, grassland birds have no such relationship.  The species richness, abundance, and breeding success of grassland birds are mainly related to variables such as vegetation structure, habitat patch size, and the amount of grassland in the neighborhood around their nesting sites.  At Kankakee Sands, the 6,000 acres of restored prairie/wetland habitat are obviously increasing both the patch size and total amount of grassland in the neighborhood, so our discussions focused on vegetation structure.  The ideal situation, of course, is to have a diversity of vegetation structure scattered across the site so that all grassland bird species can find the habitat they require for nesting.

This kind of tall vegetation is typical of most of the restored prairie at Kankakee Sands (and most other tallgrass prairie restoration sites). While valuable for many species, it's not much good for grasshopper sparrows and other animals that require short or patchy habitat structure.

We talked mainly about two different approaches to creating habitat structure in order to accomodate a rich variety of bird species.  The first approach is to design seed mixes for each desired habitat structure type, and the second is to design seed mixes that promote overall biological diversity, and then manage for structure with fire, grazing, and other tools.  Kankakee Sands is employing both strategies, so we were able to look up close at each.

We first looked at a restored prairie that had been seeded with about 60 species of short and medium-height plant species, and that included bunchgrasses like dropseeds and little bluestem instead of tall and strongly rhizomatous species like big bluestem.  The resulting structure was very favorable for grasshopper sparrows and other species that prefer that short to mid-height structure.  The seeding was nearly a decade old and seemed to be maintaining its structure and species composition with only a couple of prescribed fires as management.  The short patchy vegetation is definitely a contrast to the majority of other seedings at Kankakee Sands (and most other tallgrass prairie restoration sites) which are mainly tall and rank.  Those taller sites provide excellent habitat for Henslow’s sparrows and species with similar habitat preferences, but don’t do much for grasshopper sparrows.

Chip O'Leary (left) describes grassland bird habitat and research results with participants of the 2011 Grassland Restoration Network workshop. This restored prairie was seeded with short and medium-height plant species (including bunchgrasses instead of tall and strongly rhizomatous species).

The second example we visited was one of the first seedings done at Kankakee Sands (in the late 1990’s).  From the beginning, the several hundred acre prairie has been dominated by grasses and has been low on forb species diversity.  For the last several years, the Conservancy has been experimenting with patch-burn grazing as a way to create more heterogenous vegetation structure and to increase forb diversity in this prairie.  To date, forb diversity has neither increased nor decreased, but habitat structure has certainly changed.  The recently burned (and thus currently grazed) portions of the prairie provided excellent grasshopper sparrow habitat, while other portions were more tall and rank.  Though no change has been detected in forb abundance or diversity, the staff has noticed that a few forb species seem to bloom less abundantly than in the past – including compass plant, prairie dock, and Canada milkvetch.  Because of that, we discussed the value of fencing out a significant portion of the prairie each year to ensure that those species were given a complete break from grazing pressure periodically.

The two methods of creating bird habitat (seeding design vs. active management) both seem to be working well so far.  Both the prairie seeded with short/bunchy vegetation and the grazed prairie had significantly different vegetation structure than did the majority of the tall rank prairies around them – and birds are responding to that structure.  However, there are still plenty of questions about the long-term future of both approaches to creating bird habitat. 

In terms of the seeding design approach, one potential downfall is that the site was seeded with considerably fewer plant species than most other tallgrass prairie seedings at Kankakee Sands.  The potential effect of this lower diversity on insects and other species is unknown.  In addition, planting short and medium height plant species in soil/climate conditions that typically favor tall species could result in a relatively unstable prairie community.  In the long term, the fact that those shorter plant species aren’t using all of the available light/soil/moisture resources could lead to encroachment by either tall grasses (defeating the purpose of the design) or invasive species (which create obvious problems).  The planting we looked at was located on dry sandy soils with very low organic matter, so it probably is less at risk for that kind of instability than if it had been located in wetter or heavier soils. 

A final potential disadvantage of the seeding design approach is that the location of the short/medium habitat structure is static.  Grazing and other management tools for manipulating structure can be moved around a site from year to year, creating a shifting mosaic of habitat conditions.  That kind of mobility could help keep predator or pathogen populations from building up under consistently favorable conditions at any one site (this is speculation).  In addition, the staff will have to be careful to avoid repetitive management treatments aimed at maintaining the same structure year after year – that management could consistently favor some plant species over others, further reducing the plant (and insect?) diversity of the prairie.

There are plenty of concerns about the patch-burn grazing strategy as well.  To date, the plant diversity in the grazed prairie we looked at has not gone down, but neither has it increased – though increasing plant diversity in a grass-dominated prairie is very difficult with any strategy.  Because the prairie started with few forbs, it’s hard to know what the impact of patch-burn grazing would be on a more diverse plant community, but that needs investigating.  We discussed the possibility that a higher stocking rate and the addition of a large exclosure that changes location each year could help with both habitat and plant diversity over time. 

The burned patch of the patch-burn grazed prairie at Kankakee Sands. While the compass plant in this photo is blooming under grazing, other individuals (of compass plant, prairie dock, Canada milkvetch, and others) appear to be blooming less frequently than they did prior to the introduction of cattle. While this doesn't kill the plants over the short-term, it is a concern down the road, and the Kankakee Sands staff is considering strategies to mitigate those potential impacts.

A higher stocking rate would lead to more intense grazing of the dominant grasses such as big bluestem that are likely preventing existing forbs from becoming more abundant.  Currently many of the grass plants inside the most-recently burned patch are only being moderately grazed, and that can actually induce those plants to divert extra resources into rhizome production – leading them to expand their footprint (not the objective here).  More intense grazing on those grasses could create better opportunities for seed germination and seedling establishment around those plants, and would create even shorter vegetation structure, which might help attract upland sandpipers and as well as grasshopper sparrows. 

Regardless of stocking rate, the use of a grazing exclosure would help ensure a periodic break from grazing for plant species that otherwise be vulnerable to annual grazing – even in the unburned (and lightly grazed) portions of the prairie.  With a higher stocking rate, the exclosure would become even more important.  In addition to protecting plants from grazing, it would also protect fuel for the next year’s burn.  An exclosure roughly 1/4 or 1/3 of the size of the prairie would probably sufficient for protecting both plants and fuel, and would still leave cattle access to both burned and unburned portions of the prairie – something is important when promoting selective grazing. 

Even with those potential alterations to the current patch-burn grazing system, there are still plenty of unknowns about the long-term impacts of cattle grazing at Kankakee Sands.  It seems clear that grazing can create a shifting mosaic of habitat structure, but whether or not it can maintain the kind of plant diversity (and other diversity) desired by the Conservancy at this site is still an open question.

Discussing those kinds of questions while standing on the ground, however, is the best part of our Grassland Restoration Network workshops.  We don’t all agree on the best strategies, because we are all still experimenting with our own ideas on our own sites – and none of us feel like we have all the answers.  Being able to see for ourselves what various restoration and management treatment results look like helps us better compare those results to what we see on our own sites.  While we don’t have all the answers yet, we’re certainly moving much closer to them as a group than we would be individually.