Female biased mortality caused by anthropogenic nest loss contributes to population decline and adult sex ratio of a meadow bird
Introduction
Adult survival is one of the most important demographic parameters affecting population size, and it is a key proximate mechanism that underlies the decline of many bird species (Peach et al., 1999, Sæther and Bakke, 2000, Newton, 2004). Hence, human-induced changes in the environmental and ecological conditions affecting survival patterns are expected to have strong impacts on population trends. Understanding these man-made factors is crucial for developing models of population viability and management strategies aimed at preventing declines or extinction of endangered species. Harvesting natural resources such as hunting or fishing is a common anthropogenic source of adult mortality. However, also accidental killing of adults has been shown to affect population parameters, for example by long-line fishing in sea birds (Tuck et al., 2001, Awkerman et al., 2006), by crap trapping or road use in turtles (Aresco, 2005, Browne and Hecnar, 2007, Dorcas et al., 2007) or by pesticide use in farmland birds (Mineau, 2002).
Grassland management has been intensified over the last decades, especially in the lowlands of central and north-western Europe (Smith and Jones, 1991, Green, 1996, Donald et al., 2001, Müller et al., 2005) and north-eastern North America (Troy et al., 2005). Changes in the temporal patterns of harvesting, in particular mowing of grassland earlier in the season, and intensification of farming practice allowed more frequent grass cuts. Advances in mowing technology increased the speed and efficiency of mowing machines and new storage techniques such as silage favoured even earlier mowing dates. The changes in mowing phenology are considered as one of the main causes of population decline in ground-nesting grassland birds, strikingly reducing their nest survival, since early mowing occurs at a vulnerable time in their breeding cycle (Jenny, 1990, Green et al., 1997, Schifferli, 2000, Perlut et al., 2006). Eggs, nestlings and fledglings are threatened of being killed or injured.
Effects of the timing of harvest on population size and source-sink dynamics are much more pronounced, if grassland management not only affects fecundity but also survival of adult birds. Laying and incubating females often leave very shortly before farming machines destroy their nest and might fail to escape in time from lethal casualties. Thus, if mowing takes place when females still incubate (i.e. in the pre-hatching stage of the brood) female mortality might be increased with marked consequences on future numbers of breeding pairs (Ferrer and Hiraldo, 1992). Although, male-skewed adult sex ratios are common in wild bird populations (Donald, 2007), accidental killing of females would result in a further distortion of the species-specific sex ratio. We then expect an increase in breeding territories occupied by unmated males. Increased female mortality would also reduce annual reproduction of the population by reducing the number of replacement clutches. However, whether changes in grassland management immediately affect survival of ground-nesting females remains unclear because quantifying adult mortality caused by agricultural activities has so far been inhibited by the difficulties in finding carcasses (Balcomb, 1986, Rivera-Milan et al., 2004) and in separating mortality from dispersal or emigration after nest losses.
A species that suffers from agricultural changes in grassland management is the whinchat (Saxicola rubetra), a long-distance migrant and ground-nesting passerine of open grasslands. In the last decades, it disappeared from the lowlands of central Europe (Hagemeijer and Blair, 1997), and recently marked declines in populations have been reported also from Alpine Switzerland (Schmid et al., 1998), where intensification on grassland started later than in the lowlands. There is now an altitudinal gradient with intensive meadow management in areas of the valley bottoms and more traditional farming on the higher slopes of the valleys (Schifferli et al., 1999). Intensively managed grassland fields provide unfavourable food conditions for whinchats (Britschgi et al., 2006) and in these low quality breeding habitats nest success is too low to maintain the local population (Müller et al., 2005). Immigration (probably from the slopes, where breeding success is higher) seems to balance the number of breeding pairs in the valley bottom, but overall population size is declining continuously. This situation allows studying the mechanisms of a current whinchat population decline on the small scale of an alpine valley and provides insight into the whinchat extinction process in the 20th century across the lowlands of central Europe.
Here, using radio-telemetry and colour ringing of adults, we quantify the sex-specific survival patterns throughout the breeding season in an Alpine whinchat population. We analyse territory mapping and nest survival data from 1988 to 2007 to study changes in the pre-hatching nest destruction rate and in the proportion of territories occupied by unmated males. We present strong evidence that mowing early in the season directly affects female survival and that this female-biased mortality affects the extent of population decline of whinchats.
Section snippets
Study area and study population
Adult breeding season survival was studied throughout the breeding seasons of 2002 and 2005 in an Inner-Alpine valley of Switzerland (lower part of the Engadine valley, 46°50′N, 10°23′E). We selected three grassland plots near the bottom of the valley (Pradellas near the village Ramosch, Quadra near the village Sent and Cutüra near the village Ardez), known to be occupied by whinchats, covering c. 7% of the suitable whinchat habitats in the valley. In the years of the survival study, meadows
Results
In total, we caught 37 male (7 unmated) and 34 female whinchats. In 2002, birds were colour-ringed (16 males, 14 females), whereas in 2005 they were also radio-tagged (21 males, 20 females). The nests of 22 paired males and 24 females were destroyed by mowing. After the first mowing event, 15 paired males and 14 females were not located any more in their former home range. Three encounter models received substantial support (ΔQAICc < 2; Table 2). However, due to the principle of parsimony we
Discussion
This study suggests that modern meadow management results in an enlarged sex-biased mortality of breeding whinchats. In particular, we found that (1) in the last 20 years, whinchat nest destruction by mowing shifted from the nestling stage to the pre-hatching stage, (2) grass harvesting before hatching of clutches resulted in accidental killing of females by mowing machines, (3) female survival during the breeding season was lower than male survival, and this difference arose in the periods of
Acknowledgements
We thank A. Bassin, A. Koller, M. Liechti, F. Märker, E. Rey, A. Ryser, H. Schmaljohann and T. Wicki for helping to carry out the fieldwork. World Wildlife Found Switzerland, Pro Natura Switzerland, BirdLife Switzerland, Karl-Mayer foundation, Däniker foundation and the Animal Protection Society of the canton Zurich supported the field study in the Engadine in 1988.
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