In a recent study, Justin Shew and colleagues found nest survival improved with policy-based management and establishing native grasses but conclude finer-scale details often have superior predictive ability from a multi-scale perspective. Read more about their findings here.

A video summary of their work is also available.

Grassland and farmland bird populations have been declining around the world and these declines are primarily attributed to grassland habitat conversion to agricultural practices. In addition to this, many of the historical drivers (i.e., fire and large grazing ungulates) that once maintained, floristically diversified, and ensured a mosaic of successional stages, are no longer prevalent in our contemporary and highly fragmented grassland landscapes.

Set-aside programmes and policy-based management

Various governments have recognized the value of setting aside agricultural private lands into conservation practices that not only benefit wildlife but also reduce soil loss and improve water/air quality. In the United States, the most popular set-aside private lands programme, which offers economic incentives to landowners, is the Conservation Reserve Program (CRP) administered by the U.S. Department of Agriculture’s Farm Service Agency (FSA). Technical assistance for CRP is provided by the Natural Resources Conservation Service (NRCS). Enrolled landowners are required to manage their grassland conservation cover mid-way through their 10-15 year CRP contract to promote early-successional habitat for wildlife.

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An example digitized landscape within a field centred 1600 m radius circle used to assess patch and landscape metrics in relation to grassland bird nest survival. The field is outlined in red and the black outline is a 100m field buffer used in characterizing various patch-scale metrics. Nest-site characteristics, microhabitat, and management represent non-digitized analysis steps.

Opportunities and questions

Because of this unique opportunity to conduct research directly linked to policy-based management and landowner decision making, we set out to understand how specific and broad management options to CRP landowners could influence nest survival of grassland birds. The team looked at effects of tractor-based management, specifically light-strip discing, herbicidal spraying, and spraying combined with forb interseeding, on nest survival in north-western Illinois.  Over the course of three years assigned fields were managed in their entirety, starting fall 2011 and ending by spring of 2014, according to NRCS guidelines. Additionally, we aimed to understand how management in relation to multi-scale habitat, ranging from nest-site characteristics to landscape context, affected nest survival of: (1) red-winged blackbirds (Agelaius phoeniceus), an extremely abundant and facultative species with lower conservation value; (2) dickcissels (Spiza americana), a grassland obligate that overwinters outside of the U.S. and has a much higher conservation value; (3) the ground-nesting community; and (4) the above-ground nesting community. Nesting data was collected during 2011-2014 breeding seasons, with 2011 representing a year of pre-management data collection.

Finer-scale and temporal results

From a multi-scale perspective, finer-scale habitat characteristics and temporal variables often predicted grassland bird nest survival better than management or landscape context. Blackbird nests survived more when nests were in denser and taller vegetation and above-ground nesters had greater nest survival in fields with higher forb diversity. Conversely, more forb cover at above-ground nesting sites decreased nest survival. Also, blackbird and dickcissel nests survived better earlier in the breeding season compared to later. This result alone should guide managers and policy makers to avoid conducting management that could destroy nests earlier in the breeding season where nests have a higher chance of surviving and are more aggressively defended (Shew et al. 2016) – likely contributing more offspring to the overall population.

Management-focused results and explanations

However, our management focused analysis (step 5 in the publication) revealed that above-ground nesters, which included mostly field sparrows (Spizella pusilla) and common yellowthroats (Geothlypis trichas), responded positively to light-strip discing conducted on fields the fall before the breeding season when compared to control and pre-managed fields. Light-strip discing involved pulling agricultural disc harrows with a tractor and removing 50% of the residual vegetation in strips across the field. The on average 3.7% increase in daily nest survival from discing management equated to an on average 30% increase in probability of surviving the nesting period (PSNP) for above-ground nesters. Herbicidal spraying and spraying with interseeding treatments also generally improved nest survival of above-ground nesters compared to reference conditions.

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A male dickcissel (Spiza americana) perching on a dead forb stalk (Henry County, Illinois, USA). Policy-based management often promoted such forbs, which were sometimes undesirable to landowners but used by certain birds for perching and nesting substrates. Greater forb diversity likely reduced nest predator search efficiency and attracted more arthropod food resources for grassland birds. Photo by J. Shew.

Although distinct management treatment effects were not as apparent in other analyses, the more a field was managed across the study and when conducted the fall before the breeding season, regardless of type, management generally improved nest survival for both dickcissels (~4.4% PSNP increase) and above-ground nesters (~15.4% PSNP increase) compared to unmanaged situations.  Even the establishment of native warm-season grasses generally improved nest survival for blackbirds, dickcissels, and ground nesters (average 9.1% PSNP increase across groups) compared to fields dominated by non-native and cool-season smooth brome grass (Bromus inermis).

Policy-based management can indeed improve nest survival of grassland birds, which is likely explained by management creating vegetation structural heterogeneity and floristic diversity that reduces predator search efficiency, while also improving arthropod food resources for birds. Although landscape composition was generally not important in these analyses, we do not recommend ignoring landscape contexts. Instead we advocate focusing management in beneficial landscapes that support positive meta-population dynamics, such as increased field colonization and persistence, of focal species.

Read the full article Finer‐scale habitat predicts nest survival in grassland birds more than management and landscape: A multi‐scale perspective in Journal of Applied Ecology.