Research Paper

INTRODUCTION

Grasslands are one of the most drastically transformed ecosystems in the world (White et al. 2000), and they face increasing pressure from the expansion of agriculture, urbanization, and anthropogenic climate change (Newbold et al. 2017, Nanni et al. 2020, Williams and Newbold 2020, Bardgett et al. 2021). Grasslands provide a wide variety of resources and services, working as reservoirs of biodiversity and playing important roles in carbon storage, water regulation, and climate regulation (Bardgett et al. 2021). Despite their importance, there is an alarming lack of legal protection of grassland ecosystems across the world, with only an estimated 8% of the remaining grassland surface inside protected areas (Dudley et al. 2020). Hence, it is fundamental that private landowners engage in conservation efforts and sustainable practices to preserve grassland biodiversity and ecosystem services (Seigel and Lockwood 2010, Azpiroz et al. 2012).

Birds are a valuable group for grassland conservation initiatives because of their symbolic value, ease of recognition and quantification, and the potential of some threatened species to serve as bio-indicators (Browder et al. 2002, Fraixedas et al. 2020). The breeding success of birds is a key component of their population dynamics (Xiao et al. 2017) because it has a direct impact on the recruitment of new individuals. Usually, predation is the main cause of nest failure in birds (Martin 1993a), and it is particularly high in grassland ground-nesting birds (Martin 1993b, Colombo and Segura 2023). Predation may be influenced by landscape features, such as the amount and proximity of habitat edges providing refuge to more predator types, which should increase predation rates (Lahti 2001, Hovick et al. 2012, DeGregorio et al. 2014a). However, the evidence is inconsistent for grassland birds, with many studies finding no effect of these edges (Jones and White 2012, Perkins et al. 2013) or even a reverse effect for some species (Renfrew et al. 2005). Nonetheless, the effect of edges on nest success is still a topic of concern due to the ongoing fragmentation and reduction of large continuous areas of natural grasslands (Baldi et al. 2006, Pretelli et al. 2015). Predation can also increase as a consequence of fragmentation because of the local reduction in the abundance of apex predators that control the abundance of mammalian mesopredators (Browne et al. 2021), which opportunistically feed on nests of ground-nesting birds (Jackson et al. 2018, Browne et al. 2023). Other landscape features that could affect nest predation in grasslands are the type of management in the surrounding area (i.e., grazing regime, crop type), and the presence of alternative prey for generalist predators (Simonsen and Fontaine 2016). Understanding the drivers of nest predation in grasslands is of crucial importance because some habitat characteristics and even predator abundance can be managed to achieve conservation goals.

On a smaller scale, predation can be affected by characteristics of individual nests and their immediate surroundings. For example, it is expected that birds select nesting sites that provide better concealment from predators, which should increase their chances of survival (Fogarty et al. 2017, Colombo et al. 2024). However, studies that evaluated the effects of vegetation characteristics on nest survival have found mixed results, in part due to large variability in measuring techniques and the presence of confounding factors such as changes in plant phenology (Borgmann and Conway 2015, Smith et al. 2018). This variability makes it difficult to find broadly generalizable patterns that are useful for land management. In addition, temporal factors such as nest age can greatly affect daily nest survival rates (Grant and Shaffer 2012, Colombo and Segura 2023). For example, nests have more parental activity during the nestling stage (i.e., constant arrivals and departures for feeding), which provides more cues for predators to find them (Skutch 1949, Martin et al. 2000, Şahin Arslan and Martin 2024). However, this hypothesis may only be valid when most predators are visually oriented (Roper and Goldstein 1997, Schaaf et al. 2018), and in some scenarios olfactory predators can be more important in determining nest success (Fogarty et al. 2017). Knowing the temporal patterns of nest predation can provide insights into the predator community and be helpful to make conservation efforts more cost and time effective.

Most studies about factors affecting nest survival are done on a species-by-species basis. Although useful for understanding each species’ breeding success and vulnerability to different sources of failure, single-species analyses have a limited range of habitat variables of interest due to how evolutionary pressures shaped nesting phenology and nest-site selection for each species (Latif et al. 2012). Including data from multiple species could uncover spatial and temporal patterns of predation that would otherwise be masked because not all situations are represented by nests of a single species. In this contribution, we used data on nest predation rates of an assemblage of ground-nesting passerines of the Pampas Grasslands of Argentina with the objective of finding general patterns of predation that respond to habitat or temporal variables useful for management. Our initial predictions were that (1) nests with lower concealment and in sparse grass would have higher rates of predation because they should be more detectable; (2) nests located near grassland edges would have higher rates of predation because edges provide habitat for habitat generalist predators; and (3) nests would have higher predation rates during the nestling stage because increased activity at the nest may attract more predators.

MATERIALS AND METHODS

Study area

The study site was a private farm in Punta Piedras, Buenos Aires province, Argentina (35° 20′ 02.04″ S; 57° 12′ 18.12″ W). The farm is located within the Flooding Pampa, a subregion of the Pampas Grasslands characterized by mostly natural grasslands growing on a soil with poor drainage, which produces frequent floods during the rainy season (usually winter through spring; Matteucci 2012). The mean annual precipitation in the area varies from 850 to 1400 mm, and temperatures range from 14 ºC to 16 ºC (SMN 2023). Given that these conditions do not favor crop production, most of the grassland surface is still used for low intensity grazing by domestic cattle, with a density of approximately 1 animal/ha in all survey plots (M. A. Colombo, personal observation). The property is divided by low wire fences (made of four evenly spaced horizontal wires, with the highest at 1 m from the ground) into large plots with natural grasslands dominated by native grasses (Nassella spp., Setaria spp., Leersia hexandra) and shrubs (Baccharis spp.), although many other native and exotic plant species are present to a lesser extent (Hummel et al. 2009, Roitman and Preliasco 2012). All plots contain the same grassland communities, which are generally short (< 1 m tall) and of intermediate density, allowing for easy walkthrough without leaving persistent trails (M. A. Colombo, personal observation). Scattered throughout the grasslands, there are patches of forests composed of native and exotic trees, locally known as Talares. Larger extensions of forests are found close to the shore of the De la Plata River, which form continuous rows 50–100 m wide and several kilometers long. The farm is intersected by wide dirt roads with generally low traffic.

Field procedure

Nest searching and monitoring

We used data from nests found during three field seasons (September to February 2017–2018, 2018–2019, and 2019–2020). Our nest searching and monitoring protocol was adapted from Winter et al. (2003): we searched for nests by systematically walking portions of the field with sweeping sticks or by dragging a 20 m-long rope between two people plus one observer behind, with the objective of flushing incubating birds that gave away their nest position. Additionally, we used behavioral cues (birds carrying nest material or food to their nestlings) to find nests during nest building and nestling stage.

Once we located a nest, we recorded its coordinates with a GPS device and placed a small flag four meters away from the nest to allow later visits during monitoring. We monitored nests every two to four days until the date of success or failure. We considered a nest successful if at least one fledgling left the nest. We classified failed nests as abandoned (eggs were cold after incubation had already started, or all the nestlings were dead in the nest without signs of predation), predated (the eggs or nestlings disappeared between visits before the expected fledging date), or trampled (nest was destroyed by cattle stepping or laying on them).

Habitat variables

After each nest was successful or failed, we measured vegetation variables in the nest surroundings. We obtained the height of the vegetation clump used as support or cover, in cm (hereafter clump height), the horizontal visual obstruction index (hereafter hVOI) as a measure of grass density, and the upper visual obstruction index (hereafter uVOI), as a measure of overhead concealment of the nest. We obtained the hVOI by placing a modified Robel pole (Robel et al. 1970) divided into 10-cm sections numbered from 1 (closest to the ground) to 10 (farthest from the ground). We placed the pole at the nest and registered the lowest visible section while observing from four meters away at one meter height, once from each cardinal direction (NSEW). The final hVOI score for each nest was the average of the four readings. We obtained the uVOI by placing an eight-centimeter diameter disc divided into eight black-and-white sections on the nest and recording the number of visible sections from directly overhead. If all sections were visible the score was zero, and if none were visible the score was eight.

Using the GPS location of each nest, we obtained the distance to the nearest grassland edges, i.e., boundaries with another type of habitat that is not a grassland, which were either roads or forests, by analyzing a SPOT 6 satellite image (1.5-m spatial resolution) in software QGIS (QGIS Development Team 2023). The base image was provided by the CONAE (Comisión Nacional de Actividades Espaciales).

Temporal variables

We classified each nest according to the season it was found, so that the variable was coded as a three-level factor, i.e., season (1)) 2017–2018, season (2)) 2018–2019, and season (3)) 2019–2020. We defined the clutch initiation date for each nest as the day the first egg was laid. Within each season, we relativized clutch initiation dates to the earliest calendar day observed over the entire study period, which we assigned a value of zero. Because most nests were found after the clutch had been initiated, we calculated the initiation date based on hatching date or nestling age, considering the length of the incubation period for each species (see details in Colombo and Segura 2023).

Data analysis

We selected species to include in our study based on the following criteria to ensure they were comparable and accessible to a similar diversity of predators:

• they should build their nests among the grasses and be found using the same methodology (cannot use bushes or trees as nest support),
• they should be small passerines (< 50 g) with an altricial nesting cycle (including stages of construction, egg laying, incubation, and nestling rearing),
• their nests should be active throughout the entire field season (September to February),
• and at least five nests should have a complete dataset available (including a regular visit schedule and measurements of all variables of interest).

This study includes data on some species that were previously analyzed in other studies with different objectives: Grassland Sparrow Ammodramus humeralis (Colombo et al. 2021), Hellmayr’s Pipit Anthus hellmayri (Colombo and Segura 2023), and Grassland Yellow-Finch Sicalis luteola (Colombo et al. 2024). New species included in this study are Correndera Pipit Anthus correndera, Short-billed Pipit Anthus furcatus, Yellowish Pipit Anthus chii, and White-browed Meadowlark Leistes superciliaris. All species are currently listed as Least Concern (LC) by the IUCN (2025).

We estimated daily nest survival rate (DSR) following the method proposed by Shaffer (2004), building generalized linear models (GLM) with a logistic-exposure link function, in which the probabilities of surviving each visit interval depend on the duration (in days) of the interval, and the response variable (survival) is coded as 1 = survived the interval and 0 = failed during the interval. To focus only on predation (DSRp), we removed all nest visits in which nests failed due to other causes from the analysis. For example, if a nest was found flooded on the third visit, we included data from the first and second visits and discarded the final one. Nests that were abandoned or trampled between discovery and the first visit were discarded. We kept all visits for successful and predated nests.

We evaluated the support of a set of candidate models of DSRp with different combinations of explanatory variables using Akaike’s information criterion corrected for small samples (AIC_c), which ranks models according to their support while penalizing for the number of parameters (Burnham and Anderson 2002). To reduce the number of uninformative parameters and avoid creating overly complex models (Arnold 2010), we followed a secondary candidate set approach (see details in Morin et al. 2020), first evaluating the support of models containing combinations of variables from each scale separately: nest-site scale included hVOI, uVOI, and clump height; study-site scale included distance to roads, distance to forest edges, and distance to any edge type; and temporal scale included clutch initiation (within season) and nest stage (a two-level factor indicating if the nest had eggs or nestlings during the interval). We used nest stage instead of linear nest age because species have different laying, incubation, and nestling periods, thus at the same nest age they could be in different stages.

In this initial step, only the variables that increased support over the null model (i.e., resulted in lower AIC_c value than a model with no covariates) were retained. We built the final model set using all possible combinations of these variables, including an interaction term with nest stage, because different predation pressures could exist for eggs and for nestlings. We evaluated the support of these models based on AIC_c values and evaluated the significance of each variable based on the 95% confidence intervals of their parameters. In all models, we included species and season as random effects for the intercept to account for differences in baseline nest survival among species and seasons. All statistical analyses were performed using R (R Core Team 2024), packages lme4 (Bates et al. 2024) and MASS (Venables and Ripley 2002).

RESULTS

We found 277 nests that met the required criteria, which comprised 123 nests of Grassland Yellow-Finch, 80 nests of Hellmayr’s Pipit, five nests of Correndera Pipit, nine nests of Short-billed Pipit, nine nests of Yellowish Pipit, 33 nests of Grassland Sparrow, and 18 nests of White-browed Meadowlark. For all species combined, 171 nests were predated (61.7%), 86 were successful (31%), 10 were abandoned due to unknown causes (3.6%), 8 were abandoned after intense rain (2.9%), and 2 were trampled by cattle (0.7%). We found 71 nests in the 2017–2018 season, 93 in 2018–2019, and 113 in 2019–2020. The earliest clutch was initiated on 21 September and the latest on 6 February (Fig. 1). Details on the nest characteristics of each species are provided in Appendix 1. The total sample size used for our analyses of DSRp was 1038 visit intervals that accounted for 2751 exposure days. Based on the constant model, an average nest had a DSRp of 0.934 (± 0.005 SE).

Among nest-site variables, only uVOI improved model performance relative to the null model and, among temporal variables, only nest stage improved model performance (Table 1). None of the study-site variables improved model performance, thus they were not retained to use in the final model set.

When evaluating the final model set, the best model included both uVOI and nest stage, plus an interaction term between them, receiving substantially better support than all other models (Table 1). This model indicated that overall DSRp was lower during the nestling stage, and that uVOI had a significant positive effect only during the nestling stage (Table 2); that is, during the nestling stage, nests with better overhead concealment had higher survival to predation (Fig. 2). Although the random effects showed that there was some variability in survival among species and seasons, all of the 95% confidence intervals included zero (Fig. 3).

DISCUSSION

We studied an assemblage of grassland ground-nesting passerines and found that, overall, nests had lower survival to predation during the nestling stage and that, during this stage, overhead visual concealment had a positive effect on survival. The results of this multi-species analysis allow us to draw interesting inferences about nest predation in the Flooding Pampas. First, we found support for our prediction that predation would be higher during the nestling stage, presumably because of the increased parental activity (Skutch 1949, Şahin Arslan and Martin 2024). Keeping in mind that this explanation is mostly true when the predators are diurnal and visually oriented (Şahin Arslan and Martin 2024) because they can see the adult birds approaching and leaving the nest, our results suggest that visual predators are the most important factor affecting nest survival in our study site. In other South American grasslands, de Aguiar et al. (2022) also suggested that most predators were visually oriented, although that study used artificial eggs, which do not accurately reflect predation during the nestling stage.

Nests with better overhead concealment had higher DSRp but only during the nestling stage. This suggests that predators are less likely to successfully follow parental activity to nests when these are more concealed, reinforcing the idea that predators are mostly visually oriented. Similarly, Remeš (2005) found that nest concealment had a positive association with nest survival during the nestling stage of the Blackcap Sylvia atricapilla in temperate forests, although poor concealment was compensated for by parental nest defense behaviors. Additionally, nests with dense vegetation cover could limit access to avian predators, even after they have found the location from the air (Bravo et al. 2022). This could explain why, during the nestling stage, better concealed nests are less likely to be predated even if a parent gives away their position. Although this is hard to assert based on our data, future behavioral research on the search strategy of avian predators will be valuable to understand the role of concealment during the nestling stage.

Only overhead (and not horizontal) concealment had a significant effect on DSRp, suggesting that aerial predators in our study site play an important role in shaping ground-nesting birds’ breeding success. Similar effects of overhead concealment have been found in forest species when most known predators were birds (Hannon et al. 2009, Israel et al. 2023). In open habitats, birds have been shown to be important predators of artificial nests (Ponce et al. 2018, Bravo et al. 2020, Vazquez and Amico 2023), although the relationship between avian predation and overhead concealment has not been tested in natural nests of grassland passerines. Anecdotal observations during our study period included nest predation by Long-Winged Harrier Circus buffoni, Crested Caracara Caracara plancus, Chimango Caracara Daptrius chimango, and Great Kiskadee Pitangus sulphuratus (M. Colombo, unpublished data), although more systematic records are required to assess the frequency and importance of nest predation by birds.

Although we rarely confirm predation events by snakes, they are probably another important component of the nest predator community (Weatherhead and Blouin-Demers 2004), particularly for ground nesting birds (Klug et al. 2010, DeGregorio et al. 2014b). Information about snakes’ feeding ecology and behavior is scarce, but most of the species found predating nests are presumably diurnal and locate nests by sight (see Weatherhead and Blouin-Demers 2004), which could contribute to nest predation rates being higher during the nestling stage when activity at the nest is higher (Stake et al. 2005, Londoño et al. 2023). We observed one predation event by a Patagonian Racer Philodryas patagoniensis, a known diurnal nest predator (López and Giraudo 2008, Giambelluca 2015, Browne et al. 2023) commonly seen at our study site (M. A. Colombo, personal observation), which occurred during the nestling stage. Snakes are unlikely to be affected by overhead concealment because they are ground dwelling; therefore, they may not explain the interaction with this variable. We believe that a study focused on the habitat use and population status of snakes in the Flooding Pampas would be very valuable to understand this predator-prey dynamic.

Mammals are another important group of nest predators to consider. We found a high predation probability (estimated ~85% for an average nesting cycle) compared to some studies in other regions, where predation probability ranged from 42% to 81% (MacDonald and Bolton 2008, Johnson et al. 2012). High rates of predation could be a consequence of mammalian mesopredators being abundant due to the lack of natural top predators that control their populations (mesopredator release hypothesis; Crooks and Soulé 1999). Browne et al. (2023) found similar predation rates (81%) for the Strange-tailed Tyrant Alectrurus risora in northern Argentina where natural top predators were absent, but found lower rates (70%) in protected areas with confirmed presence of Pumas Puma concolor. Some mesopredators with good vision like the Pampas fox Lycalopex gymnocercus (commonly seen at our study site) could follow parental activity to a nest, although predation by these medium-sized carnivores is likely incidental because they usually focus on larger prey (Yanes and Suárez 1996, Flemming et al. 2019). In addition, most mammals rely on olfactory cues to locate nests (Coppedge et al. 2007, Conover et al. 2010, but see Pretelli et al. 2023) and are probably not greatly affected by overhead visual concealment. Therefore, they might not be the most important predators at our study site, although we must consider the potential role of additional unmeasured factors, such as nest temperature or the scent produced at each nest stage.

Contrary to our prediction, we did not find any effect of proximity to edges on predation rates of the ground-nesting passerine assemblage. Grassland-forest edges can provide more habitat for snakes due to the availability of both shaded and open areas that provide thermal options (Weatherhead and Blouin-Demers 2004), whereas road edges can prompt some mammalian predators to walk alongside and incidentally find nests more frequently (Depalma and Mermoz 2019). We believe that edge effects on nest predation could be easier to detect in more fragmented systems, in which the edge/area relationship is higher as is typical in North American pastures (see for example Renfrew et al. 2005, Perkins et al. 2013). At our study site, all plots were large (> 100 ha), providing more open grassland where nests could be safer from edge predators but also more exposed to grassland-specific predators (Grant et al. 2006, Ellison et al. 2013). A larger-scale study involving field sites of smaller sizes could help elucidate the lack of expected edge-effects and the importance of preserving large grassland areas that are still present in the Flooding Pampa. In addition, other variables describing edges in more detail (e.g., height and density of adjacent forests, amount of edge, or type of grassland in adjacent fields), not measured in this study, could have a greater influence on nest predation. More research at the landscape level would also be beneficial to understand and predict the effects of fragmentation on nest predation (Chalfoun et al. 2002).

We analyzed the effects of habitat and temporal features on nest survival to predation, i.e., the main cause of nest failure, for an assemblage of species that share nesting characteristics. Although species-specific models are necessary for endangered or declining species (see for example Pucheta et al. 2018, Browne et al. 2021), this multi-species approach can be useful as a baseline for general management actions or recommendations in the region, especially given that the population statuses of many species are still unknown (MAyDS and Aves Argentinas 2017, IUCN 2025). Nonetheless, although statistically nonsignificant, there was some variation in survival across species and breeding season, so the nesting characteristics of each species and possible temporal variations in predator abundance should be considered when analyzing other grassland systems.

Our results suggest that small-scale features (like nest concealment) are the most important factors to manage to promote nest survival of ground-nesting passerines. This goal could be achieved on private lands, for example, by implementing low intensity grazing regimes with stocking rates below one livestock unit/ha, that preserve dense grass where birds can hide their nests (Aldabe et al. 2024). Policies promoting sustainable maximum stocking rates are key given how much of the world’s remaining natural grasslands are used for livestock grazing (Henwood 2010). In addition, because nests were more vulnerable to predation during the nestling stage, useful interventions like simple predator exclosures (Pucheta et al. 2018) should be more efficient if focused on this stage, while also avoiding the risk of inducing nest abandonment associated with earlier placement. Finally, our interpretations were somewhat limited by the lack of reliable predator identification; hence we emphasize the need for future studies using cameras to identify nest predators of Neotropical grassland birds.

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