Influence of mowing timing on the breeding success of an endangered ground-nesting migratory bird, Whinchat (Saxicola rubetra)

24 25 Ground-nesting birds in Europe are declining due to anthropogenic pressures that reduce 26 breeding productivity. Agricultural intensification under the Common Agricultural Policy in 27 Europe is a major driver of this for farmland species , including Whinchat (Saxicola rubetra), 28 in Ireland. We collected data over five breeding seasons in the Shannon Callows, a stronghold 29 for this species where seasonal flooding limits agricultural intensification. We recorded habitat 30 characteristics, breeding activity, and phenological events to determine population growth rates 31 and assess mowing impacts on breeding success. Mean fledging date was July 4th (± 9.28 days). 32 We calculate that for the population to remain stable, with 80.6% brood survival, mowing 33 should be delayed until July 14 th (+ 9.28 SD days to allow for natural variation). It is well 34 documented in the literature that early mowing significantly impacts Whinchat productivity. 35 However, when modelling mowing date against Whinchat breeding success weak significance 36 was found that disappeared when increasing sample size through simulations, suggesting that 37 mowing timing alone is not a strong determinant of reproductive success . Other factors 38 including habitat structure, prey availability, predation risk, and weather conditions likely 39 interact to influence breeding outcomes and warrant further study. We present a valuable 40 framework with five years of breeding data that can inform Whinchat conservation and 41 highlight the need for longer-term studies considering additional factors including how weather 42 conditions affect mowing and prey availability. 43 44 Key words : Whinchat, mowing, breeding success, conservation recommendations , agri -45 environmental measures 46 47 48 .CC-BY-NC-ND 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted July 19, 2025. ; https://doi.org/10.1101/2025.07.16.665079doi: bioRxiv preprint 3

49 European bird populations have suffered declines in recent decades, including substantial 50 losses in breeding numbers (Keller et al. 2020; Burns et al. 2021) . Agricultural species have 51 undergone particular ly steep declines, largely driven by the impacts of the Common 52 Agricultural Policy (CAP) and subsequent intensification of agricultural systems, which has 53 negatively affected specialised bird species (Donald et al. 2001; Newton 2004; Reif and 54 Vermouzek 2019; Rigal et al. 2023) . Agricultural land is the most widespread land use in 55 Europe occupying approximately 40% of the land surface area (Eurostat 2021) and ground-56 nesting birds in these landscapes are particularly vulnerable and at risk of decline (Bas et al. 57 2009; McMahon et al. 2020; McMahon et al. 2024) . Examples include Corncrake Crex crex 58 (Green and Stowe 1993), Skylark Alauda arvensis (Krebs et al. 1999; Donald et al. 2002), and 59 Whinchat Saxicola rubetra (Müller et al. 2005; Grüebler et al. 2012; Grüebler et al. 2015a; 60 Grüebler et al. 2015b) which have declined in population and in breeding range. The pressures 61 on these species are typically a result of agricultural intensification (Douglas et al. 2023) which 62 leads to habitat loss (Traba and Morales 2019), increased generalist predators (McMahon et al. 63 2020) and changes in mowing practices (Tome et al. 2020). 64 65 Whinchat Saxicola rubetra are an Afro-Palearctic migrant that have declined in population and 66 breeding range across Europe (Müller et al. 2005; Grüebler et al. 2012; Grüebler et al. 2015a; 67 Grüebler et al. 2015b). They are considered an indicator species of low intensity, high nature 68 value farmland and land -use changes (Britschgi et al. 2006; Orłowski et al. 2017) . 69 Overwintering habitat conditions have minimal impact on the species’ survival (Hulme and 70 Cresswell 2012; Blackburn and Cresswell 2015) . Mortality during migration, particularly for 71 first year birds may be impacting populations (Blackburn and Cresswell 2015) , however 72 migration barriers, present during spring migration at least ha ve not been shown to effect 73 behaviour of juveniles compared to adults (Blackburn et al. 2019). The issues with migration 74 are likely due to the quality and availability of stopover sites and further study is needed to 75 address this (Cresswell et al. 2025) . Nonetheless, survival at breeding grounds remains a 76 problem indicating that European conservation measures are urgently required. The European 77 breeding population is at risk with a ten-year trend of -33% (2014-2023) (PECBMS 2025). In 78 Ireland, a 76% decline in Whinchat breeding habitat has red -listed the species and designated 79 it as high conservation priority (Balmer et al. 2013; Gilbert et al. 2021) . The Irish population 80 is estimated at 51-100 pairs (Crowe et al. 2021). The species has two strongholds remaining in 81 Co. Wicklow in the east the Shannon Callows in the Midlands (Balmer et al. 2013). 82 83 Whinchat use botanical meadow grassland (Fischer et al. 2013) , which was once widespread 84 in many European countries, but is now disappearing with the continued transition to intensive 85 agriculture. In Ireland, meadow habitat is restricted to a few pockets of unimproved agricultural 86 land (Fossitt 2000). The majority of farmland in Ireland is improved grassland associated with 87 dairy farming which is less likely to be associated with the participation in agri-environmental 88 schemes (Sheridan et al. 2011; Sheridan et al. 2017). Whinchat require high insect production, 89 varied vegetation and sufficient perches (Murray et al. 2016). As they nest on the ground, they 90 can be impacted by mowing practices (Tome et al. 2020) and therefore they also require 91 sufficient cover until birds can fly. After leaving the nest, chicks will remain hidden in the grass 92 for ~7 days being fed by their parents, (Collar 2005; Tome and Denac 2012; Storchová and 93 Hořák 2018) and during this time are still vulnerable to mowing . Changing conditions , 94 associated with changing mowing regimes, on breeding grounds (Vickery et al. 2014) are 95 considered the biggest threat to the Whinchat. The Shannon Callows, a seasonally flooded 96 grassland area along the banks and islands of the River Shannon, is such an unimproved 97 agricultural grassland. Mowing in the Shannon Callows has historically been highly dependent 98 .CC-BY-NC-ND 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted July 19, 2025. ; https://doi.org/10.1101/2025.07.16.665079doi: bioRxiv preprint 4 on weather conditions (Heery 1993), and staggered throughout the area due to different land 99 ownership (Martin et al. 2023) . Mowing regimes have also been controlled to a degree since 100 1993 through grassland management measures of various agri-environmental schemes for 101 Corncrake conservation (Copland et al. 2012 ). The Shannon Callows are designated as a 102 Special Protection Area for the protection of breeding Corncrake Crex crex, and as such were 103 included in agri-environmental schemes to control mowing for this species (Copland et al. 104 2012), which would have benefitted Whinchat. Corncrake are now locally extinct in the area 105 meaning many hay meadows cannot be included in these schemes. 106 107 An important role of restructured agri-environmental policy is to halt the decline of European 108 bird populations. For this, species specific measures implemented over a European spatial scale 109 has been recommended (Broyer et al. 2014b) . This research will focus on strengthening the 110 scientific backing for the creation of a Whinchat agri-environmental measure (AEM), which 111 should be implemented at the continental level before declines worsen. There are AEMs in 112 Europe which include controlling and postponing mowing (Broyer et al. 2014a) and the 113 provision of fallows to create perches (Küblbeck et al. 2024) . The control of grazing and 114 stocking densities is another candidate AEM which could impact Whinchat breeding success 115 (Murray et al. 2016; Calladine and Jarrett 2021). Studies suggest hay mowing be delayed until, 116 at least, July 1st in Europe to protect meadow passerine populations against extinction (Fischer 117 et al. 2013; Broyer et al. 2017) . Studies from Ireland have suggested July 27 th as a suitable 118 mowing date although this was a pilot study based on a small sample size over one year (Kenny 119 et al. 2015). 120 121 The current study aims to update previous recommendations (Kenny et al. 2015) with an 122 additional four years of data for the Shannon Callows. This study aimed to characterise the 123 breeding activity of Whinchat in the Shannon Callows , to determine a safe mowing date to 124 support a sustainable population. As ground-nesting birds, Whinchat are particularly 125 vulnerable to mowing practices. By analysing five years of breeding data, we aim to provide 126 informed conservation recommendations to help the viability of this population. 127 128

129 Study site 130 The Shannon Callows is a protected area of lowland wet meadows and extensively grazed 131 pasture in central Ireland. The area is approximately 50km long with a high diversity of semi-132 natural habitats. Land here is managed for silage and hay and much is unsuitable for intensive 133 agriculture. It is designated as a Special Protection Area for the protection of wintering birds, 134 their habitat, and breeding Corncrake Crex crex under the Birds Directive (2009/147/EC) and 135 as a Special Area of Conservation under the Habitats Directive (92/43/EEC). The extent of the 136 Special Protection Area is shown in Fig 1. 137 Study species 138 Whinchat breed in the Shannon Callows, which is one of their strongholds in Ireland (Balmer 139 et al. 2013). Chicks typically leave the nest at 13.66 days (± 0.85 SD) (Storchová and Hořák 140 2018; BTO 2023), but in some places up to 15 days old (Tome and Denac 2012). They can fly 141 at days 17-19, but after this they still remain near the nest and dependent on their parents for 142 9-15 more days with independence around day 29 -30 (Collar 2005; Tome and Denac 2012; 143 Storchová and Hořák 2018) . Up until day 30, fledglings remain within approximately 100 144 meters of the nest site (Tome and Denac 2012) allowing them to be reliably assigned to specific 145 .CC-BY-NC-ND 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted July 19, 2025. ; https://doi.org/10.1101/2025.07.16.665079doi: bioRxiv preprint 5 nests. In this study we record fledging date once fledglings are observed. As they remain on 146 the ground or in the nest for several days after fledging our date is likely overestimated. Our 147 focus was on when chicks had sufficient mobility to avoid mowing and therefore fledging date 148 in this study can be defined as such rather than the strictly biological interpretation. 149 Field methods 150 We collected field data on breeding whinchat in the Shannon Callows in 2014, 2016, 2018, 151 2019 and 2020. In 2018 a comprehensive Whinchat census was carried out across all suitable 152 habitats. Subsequently, in 2019 and 2020, surveys were limited to previously confirmed sites. 153 For 2014 and 2016, only fledging date data were available. Surveying was conducted at a slow 154 pace, with transects spaced 50-100m apart, depending on accessibility. In areas where drains 155 or ditches restricted movement, potential perch points such as trees, fences and plant stalks 156 were scanned for bird activity. Nest disturbance was minimised throughout the survey period. 157 158 159 Fig. 1. Location of the Shannon Callows in central Ireland where the study was undertaken. 160 Data: NPWS, Special Protection Areas (SPA), 2024. CC BY 4.0. 161 162 Data collection focused on habitat characteristics, breeding activity and phenological events. 163 Key recorded dates included pair confirmation, first and last observations, breeding, fledging, 164 and mowing. The variables used in the analysis are shown in Table 1. Nests were considered 165 successful if they produced fledglings. To standardize temporal data, date was converted to 166 ordinal day, accounting for leap years. 167 168 Surveys were conducted from April to August, with site visits rotated to ensure comprehensive 169 coverage. Observations took place between 06:00–12:00 and 16:00–20:00 Irish Standard Time 170 (IST). During this period, sunrise ranged from approximately 07:00 in April to 05:00 in June, 171 and sunset from 20:00 in April to 22:00 in June. At the beginning of the breeding season, males 172 sing from perches, allowing both visual and auditory detection. Male perch locations were 173 recorded using GPS coordinates, with behavioural observations made from approximately 80 174 meters to minimize disturbance. A second observer confirmed territory boundaries each June 175 to ensure accuracy. Territories were classified following Bibby et al ., (2000), requiring a 176 .CC-BY-NC-ND 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted July 19, 2025. ; https://doi.org/10.1101/2025.07.16.665079doi: bioRxiv preprint 6 minimum of three observations over a 10-day period. Once pairs were established , spot 177 mapping was used to estimate territory boundaries (Bibby 2000). Territories were monitored 178 every 3-5 days for 10 -20 minutes to record key breeding behaviours, including pairing (birds 179 observed perching together), breeding activity (alarm calling, carrying food), and fledging 180 success (direct observation of fledglings). 181 182 Table 1. Definitions of terms used in the study to model and assess breeding Whinchat success 183 in the Shannon Callows, Ireland in relation to timing of mowing. 184 Term Description Breeding date Date adult observed carrying food OR both adults observed alarm calling (both indicate hatched young) Fledging date Date young were first observed in territory (recently fledged birds will hide in grass so likely overestimated). Used to indicate when birds could avoid mowing activity. Predicted hatched date Estimated from breeding date Final status Success – bird fledged; Failure – breeding pair were recorded but no fledglings Mowing day Date mowing was observed within that nest’s estimated territory Predicted Fledging Date Predicted hatching day plus 15 days (assumed maximum time chicks take to fledge) Mowing Day in Relation to Fledge Timing Difference between the predicted fledging date and the mowing date 185 Cumulative proportions 186 The population growth rate (λ) was calculated with 187 188 λ = PA + PJβ, 189 190 where PA is the probability of annual adult survival, P J is the probability of juvenile survival 191 from fledging to the following breeding season, and β is the production of female fledglings 192 per pair per breeding season (Pulliam 1988) . When λ equals 1, population size remains 193 constant, when it is greater than 1 the population is increasing and when it is less than one the 194 population is decreasing (Pulliam and Danielson 1991) (see Table 2 for justification of 195 estimates). A literature review was undertaken to identify these whinchat vital rates. 196 197 198 199 200 201 202 203 204 205 206 207 208 209 .CC-BY-NC-ND 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted July 19, 2025. ; https://doi.org/10.1101/2025.07.16.665079doi: bioRxiv preprint 7 Table 2. Vital rates used to calculate the population growth rate (λ) with justification and 210 sources for each. 211 Value Justification Literature Probability of annual adult survival (PA) 0.52 Can be considered a true estimate of survivability due to highly site faithful birds in Nigeria (Blackburn and Cresswell 2016) Probability of juvenile survival (PJ) 0.276 Mean of 3 estimates (Schmidt and Hantge 1954; Müller et al. 2005; Border et al. 2017) Breeding productivity (β) 2.61 Mean of 8 estimates (5.21/2) (Gray 1973) (5.6); (Fuller and Glue 1977) (4.96); (Pudil 2001)(5.48); (Gritschik and Baranovsky 2004)(4.64); (Müller et al. 2005) (4.9); (Frankiewicz 2008)(5.6); (Fischer et al. 2013) (4.73); (Shitikov et al. 2015)(5.79) Statistical modelling and resampling 212 First, we created a new variable called Predicted Fledge Date under the assumption that chick 213 fledging occurs 13.66 (± 0.85 SD) (Storchová and Hořák 2018; BTO 2023) days following the 214 Predicted Hatching Date . This is the stricter biological definition of fledging, referring 215 specifically to when chicks leave the nest. Given the absence of data regarding the Predicted 216 Hatching Date for the 53 available observations, the dataset was subsequently reduced to 43 217 observations for further analysis. Subsequently, a second new variable, Mowing Day in 218 Relation to Fledge Timing, was calculated. 219 220 This variable was calculated as follows: 221 222 Mowing Day in Relation to Fledge Timing = Mowing Date – Predicted Fledge Date 223 224 Thus, for this new variable ( Mowing Day in Relation to Fledge Timing ) a negative value 225 indicates that mowing occurred prior to the chicks fledging, while a positive value denotes that 226 mowing took place after the chicks had left the nest. 227 228 A generalized linear model (GLM) with a binomial error distribution was applied to assess the 229 relationship between Whinchat breeding success and timing of mowing relative to fledging. 230 This analysis was performed using the glm() function from the stats package in R. The 231 significance of the predictor variable was evaluated using Wald’s z-tests. 232 233 To further assess the robustness of the results, three complementary statistical approaches were 234 carried out. First, bootstrapping was carried out, where t he variability of model coefficients 235 .CC-BY-NC-ND 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted July 19, 2025. ; https://doi.org/10.1101/2025.07.16.665079doi: bioRxiv preprint 8 was estimated using a resampling procedure with 1000 replicates, implemented via the boot 236 package (Davison and Hinkley 1997) . Confidence intervals for the model estimates were 237 computed using the percentile method. After that, a five-K-fold cross-validation procedure was 238 conducted using the caret package (Kuhn 2020) to evaluate model predictive performance. The 239 root means square error (RMSE), means absolute error (MAE), and R2 were used to assess the 240 model’s explanatory power . Finally, we also performed M onte Carlo Simulations to explore 241 model sensitivity to sample size. Thus, 10000 simulated observations were generated based on 242 the distribution of existing data. The impact of increasing the sample size on coefficient 243 estimates and statistical significance was examined using the simglm package (LeBeau 2022). 244 245 Plots were generated using the ggplot2 package (Wickham 2016) in R. All statistical analyses 246 were conducted using RStudio (Team 2024) and were performed at a significance level of α = 247 0.05. 248

249 General description of data 250 Over five survey years in the Shannon Callows a total of 181 Whinchat territories were 251 monitored, consisting of 157 breeding pairs and 24 single males. Between 2018 and 2020, 68% 252 of nests successfully produced fledglings. In 2018, 19 pairs (68%) fledged young, followed by 253 35 (69%) in 2019 and 29 pairs (66%) in 2020. The earliest recorded fledging date was on the 254 9th of June 2016 while the latest was the 28th of July 2019. On average, fledging occurred on 255 July 2nd. Since fledging date was recorded based on the first observed presence of young birds, 256 these dates are likely over estimated. A summary of the total number of nests per year and the 257 ordinal day of fledging is provided in Table 3. 258 259 Mowing data was also collected from 2018 to 2020. Across this period, 54 sites were mowed 260 with 20 failed nests and 33 successfully fledging. In contrast, 69 sites remained unmowed, with 261 20 nests failing and 49 successful fledging. 262 263 Table 3. Summary statistics of Whinchat fledging dates in ordinal day (OD) for the study years, 264 including sample size (N), median, mean, standard deviation, variance, and standard error of 265 the mean. 266 Year N Median Mean Standard deviation Variance Standard error 2014 18 183.0 183.6 11.05 122.14 2.60 2016 16 183.5 181.7 9.78 95.56 2.44 2018 19 179.0 179.8 5.98 35.73 1.37 2019 35 184.0 185.4 9.66 93.31 1.63 2020 29 183.5 185.0 8.84 78.07 1.63 Overall 117 183 183.5 9.28 86.03 0.97 267 Cumulative proportions 268 The population growth rate (λ) was calculated as the sum of adult survival probability and the 269 product of juvenile survival probability and the number of female fledglings produced per pair 270 per breeding season (Pulliam 1988), equalling 1.240. This theoretically allows the population 271 to remain unchanged even with a loss of 19.4% of broods in a breeding season (1-(1/1.24036) 272 = 19.4%), meaning 80.6% of broods must survive. In this study, fledging was defined as the 273 point at which chicks are first observed outside the nest (see Table 1 ). This is likely an 274 .CC-BY-NC-ND 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted July 19, 2025. ; https://doi.org/10.1101/2025.07.16.665079doi: bioRxiv preprint 9 overestimation of actual fledging date as chicks will remain hidden in the grass for some time. 275 However, it is an indication of when chicks have sufficient mobility to avoid mowing which is 276 the focus here. We plotted data (Fig 2) from five breeding seasons (n = 117) and fitted a linear 277 regression model (y = mx + b) to determine the x -axis value in OD, corresponding to 80.6%. 278 This was 279 280 x = 80.6 - (-4.416) / 0.269 281 282 with x = 194. 4, or the 14 th of July. This has been rounded up to the 14 th of July (ordinal day 283 195) to identify the earliest full day the threshold had been reached. 284 285 286 Fig. 2 Cumulative proportion of mobile Whinchat broods by date as ordinal day (OD) for 5 287 years of Whinchat breeding data in the Shannon Callows, Ireland with a regression line fitted. 288 The intercept was calculated to identify the date where 80.6% of nests were mobile (indicated 289 by the dashed line). This is the 14th of July. 290 Mowing timing effect in Whinchat breeding success 291 The GLM results showed that mowing timing relative to fledging was a significant predictor 292 of breeding success (β = 0.099, SE = 0.051, z = 1.976, p-value = 0.048), indicating a positive 293 association between mowing occurring later in the season and fledging success (Fig 3). 294 295 The bootstrapping analysis supported the stability of the model estimates. The bootstrap bias 296 was small (0.016 for the mowing timing coefficient), and the 95% confidence interval (0.015, 297 0.270) did not include zero, indicating a statistically significant relationship. 298 299 The cross -validation results indicated moderate predictive performance, with an RMSE of 300 0.454, MAE of 0.386, and R 2 = 0.218. This suggests that mowing timing alone explains only 301 21.8% of the variability in breeding success, highlighting the need for additional explanatory 302 variables to provide greater clarity on the breeding success of Whinchat based on our dataset. 303 However, Monte Carlo simulations revealed that when sample size was artificially increased 304 to 10000 observations, the significance of the mowing timing variable disappeared (β = 0.004, 305 .CC-BY-NC-ND 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted July 19, 2025. ; https://doi.org/10.1101/2025.07.16.665079doi: bioRxiv preprint 10 S.E. = 0.003, z = 1.448, p-value = 0.148). This result suggested that with a larger dataset, the 306 observed effect may not be biologically meaningful and that other ecological factors are likely 307 to influence breeding success. 308 309 Fig. 3: Breeding success probability of Whinchat in relation to mowing date relative to fledging 310 timing in The Shannon Callows (Ireland, n= 43 nests). The x -axis represents the number of 311 days between mowing and fledging (negative values indicate mowing occurred before 312 fledging, positive values indicate mowing occurred after fledging). The y -axis shows the 313 predicted probability of breeding success derived from a GLM (solid black line), with a 95% 314 confidence interval (shaded area) estimated using a smoothing function. Raw data points are 315 shown, with green dots indicating mowing events before fledging and pink dots indicating 316 mowing events after fledging. 317 318

319 320 Data from other studies demonstrate that mowing impacts on Whinchat breeding success are 321 significant (Tome et al. 2020). However, the full magnitude of its effect in an Irish context has 322 not been fully investigated except with data from a single year (Kenny et al. 2015) and given 323 this population is at the western extent of the breeding range, representing an ecological niche, 324 we feel these new data are important . In this study we use five years of data to study the 325 characteristics of Whinchat breeding in the Shannon Callow in Ireland. Using vital rates from 326 the literature along with data we collected on fledging date, we calculated a mow date that 327 would allow the population to remain stable. We also directly test for the effect of mowing in 328 the Shannon Callows and find a statistically significant relationship between mowing timing 329 and breeding success. However, the significance was lost when sample size was increased 330 through simulations. Therefore, it appears other variables are impacting Whinchat breeding 331 success in the Shannon Callows and this study therefore provides important insights for 332 conservation applications. Additional research into these other variables for example weather, 333 predation, food availability and vegetation would be beneficial. Nonetheless, it is still likely 334 that if early mowing occurs (Tome et al. 2020) in a large enough area this would significantly 335 impact on Whinchat breeding success in the Shannon Callows. 336 337 .CC-BY-NC-ND 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted July 19, 2025. ; https://doi.org/10.1101/2025.07.16.665079doi: bioRxiv preprint 11 Using previously published Whinchat vital rates from across their breeding range for the 338 probability of juvenile survival (UK (Border et al. 2017), Switzerland (Müller et al. 2005) and 339 Germany (Schmidt and Hantge 1954) ), breeding productivity (UK (Gray 1973), (Fuller and 340 Glue 1977) , Czech Republic (Pudil 2001) , Belarus (Gritschik and Baranovsky 2004) , 341 Switzerland (Müller et al. 2005) , Poland (Frankiewicz 2008), Germany (Fischer et al. 2013) 342 and European Russia (Shitikov et al. 2015) ) and one wintering population in west Africa 343 (Blackburn and Cresswell 2016) , we estimated the percentage loss of Whinchat broods that 344 could, theoretical, maintain a stable population. Currently, such demographic data are 345 unavailable for the Irish Whinchat population. Therefore, our objective is to provide a 346 conservation framework rather than a definitive implementation strategy. Should estimates of 347 vital rates for the Irish population become available in the future , they can be readily 348 incorporated into this model . It is also important to acknowledge that apparent survival may 349 differ significantly from true survival in Whinchat populations (Shitikov et al. 2015) , and 350 potentially between sexes (Müller et al. 2005; Grüebler et al. 2008) , meaning that our 351 calculations should be considered informed estimates rather than precise predictions . We 352 calculated the population growth rate as 1.24, indicating that the population could theoretically 353 remain stable with a loss of 19.4 % of broods in a breeding season which is equivalent to a 354 survival rate of 80.6%. By fitting a linear regression to five years of fledging data, we estimated 355 that 80.6% of chicks had fledged by July 14th (OD = 204). This date represents when mowing 356 could theoretically begin without major impact on Whinchat breeding success. We defined 357 fledging as the first observation of chicks out of the nest, which may slightly overestimate true 358 fledging but reflects when they can likely avoid mowing. To account for natural variation, we 359 added one standard deviation (9.3 days) to the mean threshold date, yielding July 23rd. 360 Therefore, mowing should ideally be delayed until after July 23rd to reduce risk to late fledging 361 chicks. 362 363 There are three feasible ways for this to be interpreted. Firstly, choose 80.6% of nests and 364 protect with delayed mowing until all chicks have fledged. This does not account for stochastic 365 processes which may reduce survival such as predation, adverse weather or sample outliers. 366 Another option would be to protect 100% of nests, until 80.6% have fledged. This is the most 367 time consuming and expensive option and mowing will need to be delayed on the largest area. 368 It can be difficult to be certain of nest numbers and therefore this would make it difficult to 369 protect a certain number without high survey effort. Lastly, and what we recommend, is to 370 protect 89.8% of broods. The options suggested, require careful consideration and further 371 investigation by agro -economists, policy experts and conservationists. The benefits must 372 outweigh or at least equate the costs for conservation to be sustainable. The location of 373 protected nests is also a consideration, as nests in certain habitat types such as edge habitats 374 have been shown to have lower productivity in other ground nesting species (Sheridan et al. 375 2020). 376 377 Although our analysis indicates a statistically significant relationship between mowing timing 378 and Whinchat breeding success (p = 0.048), the strength of this association is relatively weak. 379 When the sample size was increased through Monte Carlo simulations, the significance of the 380 predictor was lost, suggesting that the effect may not be robust. While preliminary results 381 suggest that delayed mowing may improve fledging success, the overall explanatory power of 382 the model remains limited. This implies that mowing timing alone is unlikely to be a strong 383 determinant of reproductive success. 384 385 Nonetheless, mowing is a widely recognized threat to Whinchat populations and deserves 386 further investigation, particularly in our study area, the Shannon Callows. The plausible drivers 387 .CC-BY-NC-ND 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted July 19, 2025. ; https://doi.org/10.1101/2025.07.16.665079doi: bioRxiv preprint 12 of declines are breeding failures due to bird casualties (adult and/or juvenile) (Müller et al. 388 2005; Grüebler et al. 2008; Grüebler et al. 2012) and reduced food availability (Vickery et al. 389 2001; Britschgi et al. 2006). Both of which can occur from intensified grassland management 390 and early mowing (Feehan 2003). Previous research has demonstrated that early or intensive 391 mowing can negatively impact Whinchat survival (Tome et al. 2020), and more broadly, reduce 392 species richness in grassland ecosystems (Canonne et al. 2024). As agricultural intensification 393 continues to promote earlier mowing regimes, targeted conservation measures are increasingly 394 necessary. 395 396 Furthermore, recent studies highlight that factors such as habitat structure, predation risk, and 397 weather conditions also play significant roles in Whinchat breeding success. For instance, 398 vegetation heterogeneity and the presence of perching sites are crucial for nesting and foraging 399 efficiency (Murray et al. 2016) . Predation risk, particularly from mammals during the early 400 fledgling stage, can significantly affect juvenile survival (Naef‐Daenzer and Grüebler 2016) . 401 Additionally, adverse weather conditions, such as increased rainfall and temperature extremes, 402 have been linked to reduced fledgling survival and recruitment rates (Halliwell et al. 2023). 403 404 Edge avoidance also affects ground nesting birds in Ireland (Sheridan et al. 2020) and may 405 influence nest selection in Whinchat. Other significant pressures on the population is increasing 406 the likelihood of Whinchat replacement brood production at specific rare locations (Müller et 407 al. 2005), the dissolution of pair bonds with the onset of mowing (Grüebler et al. 2015b) , 408 predation pressure from winged and terrestrial predators (Tome and Denac 2012) and 409 detrimental weather conditions with rain, in particular, affecting nestling success (Öberg et al. 410 2015). Given these complexities, future studies should incorporate these additional variables 411 to better explain variation in breeding outcomes and support more effective management of 412 Whinchat populations. 413 . 414 415 Also mowing on the Shannon Callows has historically been dependent on weather conditions 416 (Heery 1993). Weather conditions can be particularly impactful on the Shannon Callows. Flood 417 extent can be a determinant of the timing of breeding, particularly if there is summer flooding 418 which can directly cause nest failure. In 2019 and 2020, there was a wet summer which reduced 419 early mowing, and mowing was recorded to have directly affect much less nests. In comparison 420 in 2018 there was a very wet spring followed by a dry summer which are normal conditions. 421 Had the summer of 2019 and 2020 been more typical, there may have been more mowing and 422 a more significant impact on Whinchat. 423 424 Controlling mowing operations is still required and may improve Whinchat fledging success 425 (Grüebler et al. 2012; Broyer et al. 2014b; Denac 2015; Horch and Spaar 2015; Siems-Wedhorn 426 2015). Meadow passerine populations in Europe have responded well with even short -lived 427 delayed mowing projects across Europe (Broyer et al. 2016). Ireland is in a unique position to 428 pursue the restoration of Whinchat populations on The Shannon Callows, a largely undisturbed 429 50km linear stretch of pasture and wet meadow landscape. Delayed machinery practice has 430 previously been successfully implemented to improve the numbers of corncrakes in Ireland 431 (McDevitt and Casey 2004) , demonstrating merit for future conservation measures on the 432 Shannon Callows. 433 434 While our results indicate that delayed mowing may enhance Whinchat fledging success, the 435 overall explanatory power of the model remains limited, and the statistical significance of 436 mowing timing diminishes under larger simulated sample sizes. This suggests that mowing 437 .CC-BY-NC-ND 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted July 19, 2025. ; https://doi.org/10.1101/2025.07.16.665079doi: bioRxiv preprint 13 timing alone is unlikely to be a primary driver of reproductive success in this species. A more 438 comprehensive understanding of breeding outcomes in our study area will likely require the 439 integration of additional ecological variables. Future research should consider incorporating 440 factors such as habitat structure, prey availability and hunting opportunity, predation risk, and 441 weather conditions, all of which are known to interact with land management practices and 442 influence reproductive performance in this species. 443 444 Acknowledgments 445 446 This research has been supported by the Irish Research Council, (Grant/Award Number: 447 ‘GOIPG/2017/1028’). The authors gratefully acknowledge the BirdWatch Ireland Banagher 448 team, Se án Kelly and Shane Cully, along with Kieran Kenny for their expert advice and 449 fieldwork assistance. 450 451 Statements and Declarations 452 453 The authors have no conflicts of interest to declare. 454 455 Author contributions: Aimée Gray contributed to the study conception and design , sourced 456 funding, carried out data collection and commented on previous versions of the manuscript. 457 458 Grace Walsh wrote the manuscript and inputted into the statistical analysis. 459 460 Eva de la Peña carried out the statistical analysis and commented on previous versions of the 461 manuscript. 462 463 Alex S. Copland contributed to the study conception and design, sourced funding, helped with 464 data collection and commented on previous versions of the manuscript. 465 466 Barry J. McMahon contributed to the study conception and design , sourced funding, inputted 467 into the statistical analysis and commented on previous versions of the manuscript. 468 469 All authors read and approved the final manuscript. 470 471 Data availability statements 472 Data is available on request 473 .CC-BY-NC-ND 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted July 19, 2025. ; https://doi.org/10.1101/2025.07.16.665079doi: bioRxiv preprint 14

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