Manure heaps attract farmland birds during winter

ABSTRACT

Capsule: Abundance and species diversity of both common farmland birds and species of conservation concern were significantly higher at manure heaps compared to open farmland, especially for the Tree Sparrow Passer montanus and Yellowhammer Emberiza citrinella, two declining granivorous farmland birds.

Over the last decades European farmland has been substantially homogenized. This process originally started earlier in Western Europe, but nowadays has been recorded in Eastern Europe as well (Tryjanowski et al. 2011). Structurally diverse and complex farmland with mosaics of different crop types and non-crop habitats have been replaced with intensively managed large-scale monocultures to meet increasing food requirements and reduce the costs of agricultural production. One of the negative side-effects of these changes is the substantial decline of farmland birds, as homogenous intensively used agricultural landscapes do not provide them with crucial resources for nesting and foraging (Flohre et al. 2011). Consequently, farmland birds that were previously common in many regions have rapidly declined, which also leads to substantially reduced distributional ranges and local extinctions of many species (Fuller et al. 1995).

In the modern agricultural landscape, all features and habitat elements which increase heterogeneity are of key importance for persistence of farmland biodiversity and thus should be promoted if we are to halt further biodiversity decline (Fahrig et al. 2011). Within farmland, manure stored before being spread as field fertilizer, is often the only element that increases spatial complexity of the otherwise flat and homogenous landscape. Manure can provide birds with food (invertebrates, grain residues) which can be especially important during winter when food availability is substantially reduced (Orłowski et al. 2014). Moreover, manure heaps can be used as lookouts by birds of prey or shelters for other species (Goławski & Kasprzykowski 2011, M. Šálek pers. obs.). One may, therefore, expect that manure heaps benefit local bird communities, but there are few empirical studies demonstrating such an effect (but see Goławski & Kasprzykowski 2011). As manure storage is changing following changes to European Union (EU) regulations, there is an urgent need for the empirical evaluation of the impact of manure heaps on farmland birds, including species of conservation concern.

In this study, we investigate abundance and species richness of farmland birds during winter in an agricultural landscape of the Czech Republic. More specifically, we compare bird communities and abundance of single species in the vicinity of manure heaps and in adjacent open farmland. We predict that both abundance and species richness of wintering farmland birds (including species of conservation concern) will be higher at manure heaps than in crop fields.

Our study was performed in intensive agricultural landscapes (approximately 450 km²) in the Czech Republic, Central Europe, particularly in southern and central Bohemia and central Moravia. The land-use was mainly dominated by arable habitats with a smaller proportion of forests, grasslands, non-crop patches and human settlements. The arable fields were mainly used for intensive cultivation of cereals, maize and oilseed rape, with small areas of catch crops and seed-rich cereal stubble. The majority of arable fields during winter were covered with low-vegetation or were ploughed. The grassland patches were predominantly hayfields and intensive pastures with short-sward vegetation during winter. Finally, the forests were mostly secondary coniferous, mixed stands or shrub vegetation embedded within the agricultural matrix.

The manure heaps consisted of manure (especially manure of cows and horses) and hay or straw used for bedding of livestock. The manure heaps usually did not exceed 20 m in length and 5 m in height (online Appendix S1a). For the selection of study plots we first used satellite maps of high resolution (Google Maps 2014, Seznam Maps 2015) or experience from previous studies, where we searched for manure heaps within the study area. Secondly, as many manure heaps were ephemeral and their distribution varied among years, we confirmed their presence in late autumn (November 2016, i.e. before the start of bird counts) and recorded the accurate position of individual manure heaps using a handheld global positioning system (GPS) device. Thirdly, the positions of manure heaps (n = 43 in total) were projected in GIS (QGIS 2012) and subsequently we selected paired control points (n = 43) in the farmland with similar environmental conditions (landscape composition, elevation, soil fertility, etc.) within a distance of 1000 m of the surveyed manure heaps. The control points in the farmland were always situated in the same habitat and with a similar proportion of surrounding land-use composition. All survey points were situated more than 200 m from the human settlements or farmsteads.

Birds were surveyed using a point count method (Gregory et al. 2004) where all heard or seen individuals within a 200-m buffer of the point centre were recorded during a 10-minute period. The birds were counted immediately after the recorder arrived at survey points. The surveys were conducted from 07:00 to 12:00 Central European Summer Time, under favourable weather conditions (no rain, snowfall, mist or strong wind). We also did not count birds during high snow cover (>10 cm), although such conditions were rare during the study period. Individuals or flocks flying over the study plots, which did not land during the count, were excluded, as they were assumed to not be associated with the study locality (Šálek et al. 2015a). All localities were visited once during the winter period (December 2016 to January 2017). The manure heaps and control points in the farmland were visited on the same day, approximately at the same time and in random order. Birds were surveyed by the first author (with three exceptions). Only farmland birds (i.e. species foraging mainly in farmland) were included in further analyses (online Appendix S2). We also distinguished species of conservation concern as those included in the Red List of birds of the Czech Republic (Šťastný et al. 2017).

We analysed paired data of birds observed at manure heaps and in nearby farmland with the help of Wilcoxon rank test for paired samples. In total, 43 such pairs were included. We performed the test for each species separately, and for the four diversity indices: abundance of all species, abundance of species of conservation concern, pooled species richness and species richness of species of conservation concern.

In total, we recorded 2049 individuals belonging to 25 farmland bird species (online Appendix S2). The Yellowhammer Emberiza citrinella was the most abundant species (n = 1583 individuals) followed by Tree Sparrow Passer montanus (n = 426) and Linnet Carduellis cannabina (n = 186). Eight species of conservation concern were recorded (online Appendix S2).

The pooled abundance of all farmland bird species was significantly higher at manure heaps (44.2 individuals on average) as compared to control farmland (3.5 individuals; Wilcoxon test for paired samples, V = 33, P < 0.001). While in the latter the pooled abundance rarely exceeded 10 individuals (once, on one site with 100 individuals), at 30 manure heaps sites the abundance exceeded 10 individuals (Figure 1). A similar pattern was observed in the case of pooled abundance of species of conservation concern (V = 4, P = 0.002). Species richness of all species was also higher at manure heaps (reached up to 8) than in control farmland (only once exceeded 3) and the difference was significant (V = 14, P < 0.001). Similarly, species richness of farmland birds of conservation concern was higher at manure heaps but in this case variation was small (between 0 and 2 species; V = 11, P < 0.001).

Figure 1. Abundance and species richness of all farmland bird species and farmland bird species of conservation concern. Paired sites are linked with lines, slight jittering along the x-axis is added to avoid symbol over-plotting. Note that the y-axes of the two left subplots are presented in a non-linear scale.

Abundance recorded at manure heaps was significantly higher than in control farmland in the case of three species: Tree Sparrow (Wilcoxon test for paired samples, V = 5, P = 0.002), Yellowhammer (V = 0, P < 0.001) and Common Chaffinch Fringilla coelebs (V = 0, P = 0.009, Figure 2) and on the verge of significance (P < 0.1) in the case of another four species (Common Buzzard Buteo buteo, Meadow Pipit Anthus pratensis, Magpie Pica pica and Hooded Crow Corvus cornix; not shown on the figure).

Figure 2. Abundance of three farmland bird species (Tree sparrow, Yellowhammer, Chaffinch) at manure heaps and in adjacent control farmland. Other explanations see Figure 1.

Our study suggests that manure heaps may play an important role for farmland birds during winter. In line with our predictions, we recorded a higher abundance and species richness of farmland birds at manure heaps as compared to adjacent open farmland. Species of conservation concern were also attracted by manure heaps.

The preference of farmland birds for manure heaps and their vicinity is probably mainly driven by a combination of suitable foraging and roosting conditions. In particular, manure heaps contain a variety of grass and other seeds consumed by cattle and horses, or from hay or straw used for bedding of livestock (Goławski & Kasprzykowski 2011, online Appendix S1b). Moreover, long-term stands of manure heaps are often enclosed by seed-rich ruderal and other wild plant vegetation. In contrast, the farmland habitats mostly consist of intensively and uniformly used crop fields or grassland with minor representation of food-rich ruderals or semi-natural patches. The wild plant and grass seeds are crucial food resources for many granivorous birds in winter, including Yellowhammers and Tree Sparrows (Summers-Smith 1988, Orłowski et al. 2014) that were the most abundant species at manure heaps (see also Goławski & Kasprzykowski 2011). Additionally, manure heaps also contain various invertebrates that are also important food for birds. During winter, birds may also use manure heaps as suitable roosting sites, especially during long nights with low ambient temperatures when fresh manure can offer warmer microclimate conditions (Goławski & Kasprzykowski 2011). Furthermore, birds of prey or corvids can use manure heaps as lookouts when hunting, as these are often the only elevated structures in otherwise open and flat areas. Perching sites are important as they can reduce energetic costs for raptors that are hunting (Riegert et al. 2007).

Manure heaps are one of several food-rich structures associated with livestock farming, like farmsteads with animal production that, in comparison with open farmland, harbour higher species richness and abundance of farmland birds during winter (Šálek et al. 2018). These structures seem to provide birds with slightly different resources, for example, farmsteads offer nesting sites (Rosin et al. 2016), while manure heaps mainly provide access to food (Goławski & Kasprzykowski 2011, Šálek et al. 2018). All these structures, however, are disappearing in the modern European agricultural landscape. Although there are no statistics about the number of manure heaps in farmland, most probably their number has markedly declined during recent decades, as the number of cattle decreased by almost 60% during 1989–2016 which resulted in a 29% decrease of farmsteads with cattle breeding during 2000–10 (Šálek et al. 2015b, 2018). Moreover, the manure used in agriculture is an important source of water pollution and the recent EU Nitrates Directive aims at reducing this source of pollution by introducing manure storage regulations (Eurostat 2013). Our results, however, demonstrate that manure heaps are important for both common and rare farmland birds during winter and may help them to survive this critical period. We therefore conclude that new regulations promoting manure storage facilities (e.g. closed containers) may negatively affect many farmland bird species, in contradiction of other EU regulations which aim to halt biodiversity loss.

Acknowledgements

We would like to thank Vojtěch Brlík, Miroslav Bažant, Jaroslav Závora and Marina Kipson for their help during the fieldwork.

Additional information

Funding

This work was supported by The Czech Academy of Sciences (RVO 68081766).