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Nests in the grass

bobolink canada research

Bobolink populations have undergone a dramatic decline in recent decades. Bird Ecology and Conservation Ontario (Canada) have been undertaking research to address this; Zoé Lebrun-Southcott tells more about the project and its findings to date…

On June 3, 2016, I was crouched in a hay field, watching a patch of grass for the return of a female bobolink. Sure enough, several minutes later, she was back. She disappeared and I watched closely as several of the grasses moved, ever so slightly, as she ran along the ground to the left of where I’d lost sight of her. Luckily, it was a still day, allowing me to see the movement in the grass and where it stopped. I approached cautiously and when I was a few feet from the patch I had been eyeing, the female flushed. I gently moved the grasses to look at the ground and found her nest at the base of a clover plant. She had four eggs.

2016 was the first year of Bird Ecology and Conservation Ontario’s (BECO) new research program on birds at risk in agricultural landscapes. Most of our efforts were focused on bobolink, a threatened songbird species whose population has declined more than 75% in Ontario over the last few decades. Bobolink nest in hay fields, pastures, and other grasslands. The male’s charismatic bubbly song—sometimes described as the R2-D2 song—and helicopter-like flight displays make this species unmistakable.

A bobolink nest (Photo credit: Gerald Morris)

Bobolink face threats throughout their life cycle, but on the breeding grounds in Ontario probable causes of population decline include a decrease in the amount of hay and pasture and changes in agricultural practices, including earlier and more frequent hay harvesting. Other species that nest in agricultural grasslands (e.g., eastern meadowlark, Savannah sparrow, grasshopper sparrow) are facing similar population trajectories. Overall, grassland bird populations are declining at an alarming rate in Ontario. The goal of our birds at risk in agricultural landscapes program is to work with the agricultural community to gather new ecological data that can help address challenging conservation issues for these farmland birds at risk.

In early June of 2016, while I was trying to find nests in hay fields in Simcoe County, southern Ontario, a small team of BECO field biologists was searching for bobolink nests in cattle pastures in Renfrew County, eastern Ontario. Finding bobolink nests is challenging. Females can be stealthy around nests and males may mate with more than one female, resulting in multiple nests in a territory. The goal of our project in hay fields in Simcoe County was to capture and attach miniature radio transmitters to a handful of birds, to evaluate how well we could track their movements, habitat use, and possible re-nesting after nests were destroyed by the hay harvest. Our research in pastures in Renfrew County was a collaboration with the Ontario Soil and Crop Improvement Association, to evaluate conservation strategies to benefit nesting bobolink on rotationally-grazed beef cattle farms.

Monica Fromberger and Gerald Morris flag the location of a late-season nest. Markers are placed a few metres away, to avoid potentially attracting predators to nests. (Photo credit: Jennifer Horvat)

Early in the morning on June 6, I received an email from the farmer whose hay field I had been nest searching in a few days earlier. They had to cut the hay in part of the field, that day. The weeds were going to seed, they didn’t want to use herbicides, and the weather looked good for the next few days, so they couldn’t delay. It was earlier than they had been planning to cut, but the weather and the vegetation growth had changed their plans. When my partner and I returned on June 7, half of the field had been cut. The first nest I had discovered on June 3 was in the section that had been harvested, about a metre from the edge of the cut. It now had five eggs, two of which were broken, and the parents were nowhere to be found. Bobolink typically raise only one brood per season. A nest failure usually results in no young produced that year, but some bobolink will re-nest if a failure occurs early enough in the breeding season. This pair might have had enough time to attempt a re-nest; unfortunately, we hadn’t had a chance to tag them to track their next move. Luckily, the half of the field that hadn’t been cut was where most of the nesting bobolink were located and we were able to capture some of the adults tending to these nests, as well as a few in another hay field, before the hay was harvested.

Meanwhile, in the Renfrew County pastures, cattle were being rotated through paddocks and most female bobolink were sitting on eggs. Nests in pastures run the risk of being trampled when pastures are grazed during the breeding season. Shorter vegetation following grazing can also mean less cover and more vulnerability to predation. In 2016, we designed a two-year crossover experiment to test if strategically-placed bobolink refuges—areas that aren’t grazed by cattle during the nesting season—can have a meaningful impact on conservation efforts for this species. Refuge paddocks of about three hectares were located within six rotationally-grazed pastures, ranging in size from 15 to 59 hectares. Each of the six pastures had 5 to 17 paddocks for rotational grazing.

During our first field season working in hay fields and pastures we learned a lot about monitoring bobolink … and about the complexities of grass-based farming from the farmers we worked with.

A female bobolink, with a beak full of arthropods, watches alertly for danger before delivering the food to her young. (Photo credit: Jennifer Horvat)

Our one-year pilot radio telemetry project enabled us to test methods of capturing, tagging, and tracking bobolink, which were overall successful, except for the challenge of relocating birds that dispersed widely after the hay harvest. In the future, we hope to conduct a larger study, tagging birds in fields cut at different times to better understand the frequency of re-nesting and habitat use after displacement. But, this project is on the shelf for now, until wildlife tracking technology advances, providing more effective and affordable tracking methods that will allow us to quickly relocate birds that disperse to areas far from their breeding fields.

In 2017, our second field season working with bobolink, we returned to Renfrew County to continue studying bobolink in pastures. For the bobolink refuge experiment, we switched the grazing treatment in paddocks so that refuge paddocks not grazed in 2016 were grazed in 2017 and vice versa. We also added another experiment to our project: testing if bobolink could tolerate some light grazing by cattle early in the breeding season and still fledge young. We worked with farmers to identify paddocks that their cattle could graze during late May and early June, at a lower grazing pressure than a typical year. Bobolink arrived in these four paddocks after migration and established territories. The cattle entered shortly afterwards and began grazing, but were left in these paddocks for up to eight days only. In some paddocks, the bobolink remained; in others, some birds moved to an adjacent field. After the cattle left, some of the birds that had vacated the paddocks returned to build a new nest. A couple of nests were trampled, but these birds stayed and also re-nested. These paddocks weren’t grazed again until July, when most bobolink nests have typically fledged. More than half of the bobolink nests in these lightly grazed paddocks subsequently fledged young, which is similar to the frequency of nest success in fields undisturbed by agricultural activity. Even though the vegetation was shorter than in un-grazed fields, we did not observe an increase in the frequency of nest predation.

Zoé Lebrun-Southcott attaches a small backpack radio transmitter to a male bobolink. (Photo credit: Rebecca Howe)

In the eight paddocks included in the refuge experiment, we monitored 28 bobolink territories when paddocks were un-grazed, compared to 25 territories when paddocks were grazed. Fifty-four percent of bobolink territories had evidence of fledging when paddocks were un-grazed during one year of the experiment, compared to 16% when paddocks were grazed in the other year—more than three times as many territories fledged young when refuge paddocks were un-grazed.

Additionally, we examined the impact of grazing on fledging success for the 118 bobolink territories we monitored in all grazed paddocks across the six farms over the two years of our study. Perhaps not surprisingly, our models showed that the best variable for predicting the probability of young fledging from a territory was the amount of grazing pressure during the middle of the nesting season (late May through late June). Grazing pressure early and late in the nesting season did not have as much of an impact, nor did the date cattle first entered paddocks, overall grazing pressure across the season, or the year. Our model predictions indicated that the mean probability of young fledging from a territory decreased across the increasing range of mid-season stocking rates (i.e., number of cattle x days grazed / area grazed [ha]), from a 0.53 probability of fledging when stocking rate was 0, to a 0.04 probability of fledging when stocking rate was 174. Our data also clearly indicated that territories rarely fledged young when mid-season grazing pressure was high. Only one of the 17 bobolink territories we monitored in paddocks with a mid-season stocking rate greater than 100 fledged young.

Cattle on one of the rotationally-grazed pastures where we tested bobolink stewardship strategies. (Photo credit: Andrew Campomizzi)

Though our results are not unexpected and perhaps intuitive, it’s important to quantify the relationships between agricultural practices and bobolink fledging success, to be able to develop science-based conservation practices. Currently in Ontario, the generally-accepted stewardship practice to support bobolink in hay fields and rotationally-grazed pastures is to delay agricultural activity until after mid-July, when the majority of bobolink nests have fledged. However, delaying the cutting and grazing of fields is not an option for many farmers. For stewardship practices to be more widely applicable, we need to identify and test additional conservation strategies that balance the needs of the birds and of the farmers. Results from our two-year study in rotationally-grazed pastures indicate that there may be some opportunities for grazing during the nesting season that can be tolerated by bobolink, but more research is necessary to fully understand the impact of grazing at different intensities and at different times.

We plan to return to these questions in the future, but this year we’re returning to work in hay fields. With funding incentives in place to support farmers in delaying the hay harvest, and little monitoring of the ecological impact of these programs, we’re interested in collecting data on the effects of delayed haying on the birds. How many bobolink nests are benefitting from delaying the hay harvest in these fields compared to when the fields would normally be cut?

Similar to our research in rotationally-grazed pastures, we’re also interested in testing additional strategies in hay fields beyond delaying agricultural activity until after mid-July. It’s common in Ontario for farmers to initially cut and bale the exterior of fields, to avoid driving over and damaging cut hay when they harvest the interior of the field. Because bobolink nests are more often located towards the centre of fields rather than near the edges, we’re interested in the idea of delaying the harvest in the interior of the fields only, allowing farmers to harvest some high-quality hay early in the season, while potentially protecting the majority of nests in a field until they fledge. This year, we’ll begin investigating if this practice provides an overall benefit to bobolink and we’ll start discussing the feasibility of this strategy with participating farmers.

When hay is harvested, bobolink nests have little chance of survival. Nests that are not directly destroyed are left exposed and are typically predated shortly after the harvest. (Photo credit: Andrew Campomizzi)

Spring is late to arrive in Ontario this year. It’s mid-April and as I write this article, freezing rain is coating the trees outside and much of the province is under ice and snow storm warnings. The migratory songbirds that have already arrived have mostly stopped singing for now. Bobolink won’t arrive until early May, but they’re on their way, making the long trip from South America. These remarkable birds have one of the longest annual migrations of all North American songbirds, with a round-trip flight covering approximately 20,000 kilometres each year.

Bobolink, like many other migratory songbirds, face threats on their non-breeding grounds and migration routes as well as their nesting grounds. Bobolink overwinter in large flocks, feeding mostly on seeds in the grasslands, agricultural fields, and marshes of Bolivia, Paraguay, northeastern Argentina, and southwestern Brazil. Habitat loss and exposure to pesticides are two factors considered to be significant threats to the species on its non-breeding grounds.

Facing such steep population declines, the future of the bobolink and other bird species that nest in agricultural grasslands is uncertain. Conservation of these species will require significant changes. But one thing is for certain, to make any real progress on the conservation of these farmland birds will require working with, and not against, the farmers who create the habitat these species depend on.

Header Image: A male bobolink Credit: Gerald Morris.

About the Author: Zoé Lebrun-Southcott is Executive Director and Wildlife Biologist at Bird Ecology and Conservation Ontario (BECO), a non-profit organisation she co-founded with her partner, Andrew Campomizzi, in 2013. For more information, visit

BECO’s bobolink research on rotationally-grazed beef cattle pastures was a collaboration with the Ontario Soil and Crop Improvement Association, with funding from the Species at Risk Partnerships on Agricultural Lands (SARPAL) program, an Environment and Climate Change Canada initiative. Additional funding for BECO’s birds at risk in agricultural landscapes program was provided by the Government of Ontario, Echo Foundation, the Ontario Trillium Foundation, and the CICan Clean Tech Internships program.

The views expressed herein are solely those of the author.