Ruth Howison, Jos Hooijmeijer and Theunis Piersma report:
The Netherlands is home to over 85% of the northwest European breeding population of the continental subspecies of Black-tailed Godwits Limosa limosa limosa (Fig. 1). Black-tailed Godwits are an iconic meadow bird and the national bird species of The Netherlands. The Dutch population originated in the wet grassland meadows created by many generations of Dutch dairy farmers (Beintema 1986). However, due to intensification of agricultural practices (Kentie et al. 2015, Howison et al. 2018), the population has declined by over 75% since the first population size estimates in the 1960s (Kentie et al. 2016)
Figure 1. A Black-tailed Godwit, individually marked with colour-rings, issuing alarm calls to nearby chicks from an elevated position. (Photo: R.A. Howison).
For the past 15 years, the RUG team ‘Skries’ (most years consisting of ca. 10 paid field technicians and ca. 10 students and international volunteers) has carefully monitored godwit population events (establishment, nest locations, egg and chick production and postbreeding movements) within what is now a 11,400 ha study area in southwest Friesland (municipality Súdwest Fryslân). In addition, the locations of a small number of black-tailed godwits population have been tracked with solar PTT satellite transmitters (Microwave Telemetry, Columbia, MD).
In 2018, Europe experienced one of the hottest summers on record. Extreme temperature anomalys bring with them extreme weather as heat waves, droughts, and floods, which negatively impact agricultural as well as natural systems. For southwest Friesland, air temperatures in early March dipped below average, freezing most of the countryside (Fig. 2). However, soon May and June followed with maximum daily temperatures exceeding the upper limits of the long term average calculated over the past 28 years.
Figure 2. Daily temperatures (oC) from 1 January until 24 August 2018 plotted against the long-term average ±SD (from 1990 to 2018), obtained from Stavoren, The Netherlands.
Using vegetation indices measured at 16-day intervals by satellite imagery (see Howison et al. 2018 for methods) at the level of fields, we tracked the response of the grassland habitat in the province of Friesland. We compared the situation on 10 May 2018 (Fig. 3a), the moment when grass biomass is high, i.e. just before the widescale mowing, with the situation on 14 August 2018 (Fig. 3b), i.e. when the negative impacts of the drought on the vegetation were most evident. Based on this comparison, we mapped (Fig. 3c) where the impact of drought was light or even absent, i.e. where plant growth increased (Green), where drought had a negative impact, i.e. the vegetation index had decreased (Red), and where vegetation showed little change (Yellow).
Figure 3. Tracking habitat change with vegetation indices (MODIS EVI) between (a) 10 May 2018, (b) 14 August 2018, (c) Log change ratio between the two dates, where Red indicates a decrease in greenness, Yellow indicates little or no change, and Green indicates an increase, (d) The spatial distribution of Black-tailed Godwits, overlaying the log change ratio, during the postbreeding period 2018 (after 15 June 2018).
We then compared two measures of godwit distribution with this quantitative assessment of the impact of drought on the grassy vegetation. (1) By weekly counts of all godwits in all 3014 fields in our study area, we measured the godwit distribution during territory establishment, egg-laying and early incubation (26 March to 22 April 2018). (2) On the basis of the locations of individually colour-marked birds and godwits tagged with satellite transmitters, we quantified the distribution of postbreeding godwits fuelling up for southward migration from 15 June to 15 August (Fig. 3d).
During territory establishment and early nesting, when their movements are necessarily constrained, the godwits in southwest Friesland occurred on meadows which suffered slightly, yet significantly, less from drought than unused grasslands (Fig. 4). However, during the postbreeding period, the godwits concentrated at meadows where little change in plant growth had occurred during the drought (Fig. 3d & Fig. 4). We note also that southwest Friesland suffered more from the drought than Friesland as a whole (comparison of the two right bar in Fig. 4).
Figure 4. The effect of the summer drought measured as a loss in greenness (y-axis) was less in fields in southwest (ZW) Friesland used by godwits in the establishment and early nesting period (first column) than in southwest Friesland as a whole (third column). This effect was quite a bit stronger in the distribution of postbreeding godwits (second column). Note also that the effects of drought were more severe in southwest Friesland (third column) than in Friesland as a whole (fourth column). If the capital letters above the bars are different, the categories are statistically significantly different from each other.
The preference of Black-tailed Godwits for fields in southwest Friesland that, later in summer, suffered the least from the drought of 2018, suggests that the godwits are indicating fields with healthy ‘working’ soils, where capillary processes allow the groundwater to reach the plants even when water tables are low. Godwits indicate drought-resilient grasslands.
References
Beintema, A. J. (1986) Man-made Polders in The Netherlands: A traditional habitat for shorebirds. Colonial Waterbirds 9, 196-202.
Howison, R. A., Piersma, T., Kentie, R., Hooijmeijer, J. C. E. W. & Olff, H. (2018) Quantifying landscape-level land-use intensity patterns through radar-based remote sensing. Journal of Applied Ecology 55, 1276-1287.
Kentie, R., Both, C., Hooijmeijer, J. C. E. W. & Piersma, T. (2015) Management of modern agricultural landscapes increases nest predation rates in Black‐tailed Godwits Limosa limosa. Ibis 157, 614-625.
Kentie, R., Senner, N. R., Hooijmeijer, J. C. E. W., Márquez-Ferrando, R., Masero, J. A., Verhoeven, M. A. & Piersma, T. (2016) Estimating the size of the Dutch breeding population of Continental Black-tailed Godwits from 2007 – 2015 using resighting data from spring staging sites. Ardea 104, 213-225.
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