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Dit jaarverslag presenteerd de achtergrond van het onderzoek en een overzicht van de voorlopige resultaten van de monitoring van de gruttopopulatie, predatoren en alternatieve prooien, insecten, bodemleven en landgebruik in 2021 en/of voorgaande jaren. Ook beschrijft het de inzichten en ideeën die daaruit worden gehaald voor verdere analyse van de data en toekomstig onderzoek.

Studies of interactions among earthworms as prey for visually foraging predators required a field method that measures earthworm availability (i.e., the density of surfacing earthworms). We present such a method by counting surfacing earthworms at night by an observer lying prone on a cart propelled by an observer across measured distances at constant low speed. The method was applied in dairy farmland grasslands in The Netherlands during October and November 2011. We quantified the numbers of surfacing earthworms as well as those measured during standard hand‐sorting sampling (i.e., total abundance based on soil counts), distinguishing clay or peat soils and grasslands with either monocultures or species‐rich vegetation. Managed grasslands with different soil types showed opposing correlations between surface availability and total abundance of earthworms. This emphasizes the importance of direct measurements of earthworm availability if the goal of the study is to explain the behavior of either visual earthworm predators or earthworms themselves.

Earthworms are an important prey for the endangered meadow birds of northwest Europe. Although intensive grassland management with high manure inputs generally promotes earthworm abundance, it may reduce the effective food availability for meadow birds through desiccation of the topsoil, which causes earthworms to remain deeper in the soil. We studied the response of Red Worm Lumbricus rubellus, a detritivore, and Grey Worm Aporrectodea caliginosa, a geophage, to soil moisture profiles in the field and under experimental conditions. Surfacing earthworms were counted weekly in eight intensively managed grasslands (treated with high inputs of slurry by slit injection) with variable groundwater tables in the Netherlands. At each count, soil penetration resistance, soil moisture tension and groundwater level were measured, while air temperature and humidity were obtained from a nearby weather station. The response to variation in the vertical distribution of soil moisture was also experimentally studied in the two earthworm species. In the field, earthworms’ surfacing activity at night was negatively associated with soil moisture tension and positively by relative air humidity. Surprisingly, there was no effect of groundwater level; an important management variable in meadow bird conservation. Under experimental conditions, both L. rubellus and A. caliginosa moved to deeper soil layers (>20 cm) in drier soil moisture treatments, avoiding the upper layer when moisture levels dropped below 30%. Synthesis and applications. We propose that in intensively managed grasslands with slurry application, topsoil desiccation reduces earthworm availability for meadow birds. This can be counteracted by keeping soil moisture tensions of the top soil above −15 kPa. We suggest that the late raising of groundwater tables in spring and the disturbance of the soil by slit injection of slurry increase topsoil desiccation. This decreases earthworm availability when it matters most for breedi

Most Dutch dairy farmland today is fertilized with slurry, a mixture of cattle dung and urine. As a food source for earthworms, this type of organic fertilizer is of lower quality than the traditionally used farmyard manure consisting of dung with some urine and mixed with bedding material such as straw. Earthworms living in dairy farmland belong to two distinct ecotypes, the detritivores and the geophages. Detritivores rely on farmyard manure as a food source more than geophages and therefore the type of organic fertilizer is expected to determine their relative abundances. In turn, this would affect higher trophic levels, as detritivores, when they come to the surface to collect food, are particularly important as prey for birds and mammals. Here we tested the hypothesis that dairy farmland fertilized with slurry will contain fewer detritivorous earthworms (thereby becoming less attractive for earthworm predators) by quantifying the abundance of the two earthworm ecotypes in 45 grasslands fertilized with either slurry, farmyard manure, or both. To determine the availability of detritivores for earthworm predators, we quantified earthworm surface availability by counting surfacing earthworms in the field and compared these numbers with densities belowground. To study the direct effects of different organic fertilizer types on earthworms, we measured their growth rates under controlled constant conditions using either slurry or farmyard manure, with hay as a control. We found that detritivores occurred in the highest densities in grasslands only fertilized with farmyard manure and that they also grew better on farmyard manure than on slurry. These differences were not found in geophages. Detritivores made up 25% of the total earthworm abundance in the soil, but contributed 83% to the surfacing earthworms at night; detritivore earthworms will thus be the main prey for visually hunting earthworm predators. The few dairy farmers using farmyard manure to fertilize their

In een volle zaal op biologische boerderij Polderzicht kwam een zeer divers gezelschap van natuurbeheerders, ondernemers, beleidsmakers, adviseurs, onderzoekers en andere deskundigen bij elkaar om informatie uit te wisselen over het gebruik van ruige stalmest. Het programma bestond uit drie presentaties, een rondleiding en een heerlijke biologische lunch.

Earthworms (Lumbricidae) are important prey for many birds. Based on theirown feeding ecology, earthworms can be separated into two ecotypes: the detritivoresthat feed on organic material and the geophages that feed on soil particlesand organic matter. Detritivores collect their food on the surface during thenight when they are exposed to nocturnal predators. Hungry animals tend toshow more risk-prone behaviour and may therefore be more vulnerable to birdpredation, so we expect well-fed detritivorous earthworms to visit the surfaceless frequently. In this study, we tested this hypothesis in dairy farmland in theprovince of Fryslân, The Netherlands. Two uniform grasslands were split, witheach half receiving either an early (1 February 2014) or a late (14 March 2014)farmyard manure application. Every two weeks, nocturnal surface activity ofearthworms was measured by counting surfacing earthworms from a slowlypushed cart. Furthermore, soil samples were taken for total abundances and tomeasure individual body conditions of earthworms. As predicted, the density ofsurfacing earthworms was on average 2.5 times higher in the fields before farmyardmanure was applied. Immature detritivores had significantly lower bodymasses in fields not yet manured, suggesting that these growing earthwormswere hungry. Differences in surfacing behaviour and body mass disappearedafter all fields had been given farmyard manure. We conclude that hunger forcesdetritivorous earthworms to the surface. After manure application, they appearsatisfied and avoid the risk of depredation by birds by staying away from the soilsurface. To promote earthworm availability for meadow birds, spreading farmyardmanure on the surface should occur as late in spring as possible. In thisway, hungry earthworms are forced to the surface and are available as meadowbird prey for longer periods.

Ruff Philomachus pugnax staging in the Netherlands forage in agricultural grasslands,where they mainly eat earthworms (Lumbricidae). Food intake and the surface availabilityof earthworms were studied in dairy farmland of southwest Friesland in March–April2011. Daily changes in earthworm availability were quantified by counting visible earthworms.No earthworms were seen on the surface during daytime, but their numberssharply increased after sunset and remained high during the night. Nevertheless, intakerates of individual Ruff in different grasslands measured during daytime showed the typicalHolling type II functional response relationship with the surfacing earthworm densitiesmeasured at night. Radiotagging of Ruff in spring 2007 revealed that most, if not all,feeding occurs during the day, with the Ruff assembling at shoreline roosts at night. Thisraises the question of why Ruff do not feed at night, if prey can be caught more easilythan during daytime. In March–May 2013 we experimentally examined the visual andauditory sensory modalities used by Ruff to find and capture earthworms. Five maleswere kept in an indoor aviary and we recorded them individually foraging on trays with10 earthworms mixed with soil under various standardized light and white noise conditions.The number of earthworms discovered and eaten by Ruff increased with lightlevel, but only when white noise was played, suggesting that although they can detectearthworms by sight, Ruff also use auditory cues. We suggest that although surfacingnumbers of earthworms are highest during the night, diurnal intake rates are probablysufficient to avoid nocturnal foraging on a resource that is more available but perhapsless detectable at that time.

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Capsule Context-specific equations are needed to reconstruct diet composition and intake rate of Red Knots by the use of shell fragments retrieved from droppings.Aims To explore whether the method to reconstruct Red Knot diet described by Dekinga & Piersma [Dekinga, A. & Piersma, T. 1993. Reconstructing diet composition on the basis of faeces in a mollusc-eating wader, the Knot Calidris canutus. Bird Study 40: 144–156] accurately predicts the diet of Red Knots Calidris canutus canutus outside Northwest Europe, at Banc d'Arguin, Mauritania.Methods Feeding experiments with captive Red Knots on the bivalves Dosinia isocardia or Loripes lucinalis were carried out at Banc d'Arguin, the main wintering area of Red Knot subspecies C. c. canutus. Ingested diets were compared with the reconstructed diets derived from the general method developed by Dekinga & Piersma (1993). Droppings collected over multiple years were also analysed to evaluate the calibration method from this study.Results Of the total ingested shell mass (DMshell) in both bivalves, approximately 65% of the shell mass was retrieved in the droppings (DMdrop). Therefore, dry mass of droppings in the field (DMdrop) has to be multiplied by 1.547 to calculate the ingested dry mass (DMshell). For size estimations of ingested shells from droppings, hinges should be used for D. isocardia and hinges including tops for L. lucinalis.Conclusion The correction factor of 1.547 found here is 50% larger than the factor 0.993 for heterodont bivalves from Europe established by Dekinga & Piersma (1993). Application of the published factor would lead to serious underestimation of energy intake rates based on dropping frequencies and dropping content (by as much as 35%), although it would have small effects on the relative species composition of the diet. Having shown that such correction factors can differ among sites and prey we recommend their determination in new ecological contexts.