| dc.description.abstracteng | The Common Agricultural Policy (CAP) of the European Union has often been identified as a driver of recent agricultural intensification and thus of biodiversity loss. All major taxa, including vascular plants, arthropods, mammals, and birds, have undergone dramatic population declines on farmland during the second half of the 20th century. In that context, I started my PhD project on the seasonal habitat use and breeding performance of the Eurasian Skylark (Alauda arvensis), one of the current CAP’s greatest “loser” species. During the spring and summer of 2017 to 2019, I collected comprehensive data on the breeding biology of the Skylark population in the heterogeneous farmland south of Göttingen, Lower Saxony, in Germany. The new insights gained as a result of the project are presented in this dissertation.
Chapter 1 gives an overview of the crisis facing European farmland birds, the interference of agricultural intensification with the breeding biology of Skylarks, and my thesis objectives. Intensified land use is associated with many processes, such as an increased application of pesticides and fertilizers or landscape homogenization. However, the widespread cultivation of dense-growing crops likely poses the most significant challenge to breeding Skylarks, as they strongly depend on accessible vegetation for both nest building and foraging. Crop growth during the breeding season is thought to induce an increasing paucity of suitable nesting and foraging habitats. Therefore, this PhD project aimed to investigate the seasonal changes in habitat use and breeding performance due to the growing crop vegetation. All findings are interpreted to provide recommendations for improved Skylark conservation.
Chapter 2 deals with the decreasing suitability of winter cereals as nesting habitat, which is expected to be the primary driver behind the decline of Skylarks by curtailing the potential time for breeding of this multi-brooded species. My co-authors and I investigated: (i) the extent to which Skylarks in our study area prematurely cease nesting activity, switch nesting habitats, or breed on high-risk linear structures, like tramlines, when winter cereals have become impenetrable. Moreover, we explored: (ii) if nest success decreases throughout the breeding season, and (iii) how often Skylarks make a successful breeding attempt per year. To answer our research questions, we radio-tagged 28 adult Skylarks in April of 2018 and 2019 and documented the habitat use and breeding success for half of them during a full breeding season. Additionally, we searched for Skylark nests of untagged pairs, resulting in 96 total monitored nests. All, except one radio-tagged Skylark, showed breeding activity beyond July 1st. A mixed-effect logistic regression model (GLMM) revealed that the probability of starting a breeding attempt did not decrease seasonally. Therefore, our study could not observe a premature termination of breeding. As indicated by an analysis of centroid shifts of Skylark home ranges, three-quarters of tagged Skylarks kept their home range throughout a breeding season. All home ranges were composed of winter cereals and at least one other habitat type, with only two exceptions. Skylarks switched nesting habitats away from winter cereals to crops like sugar beet or set-aside from late May onwards. According to a Mayfield logistic regression, corn and linear structures were high-risk nesting sites but only played a minor role in breeding. A further Mayfield logistic regression revealed that nest success did not decrease over time and was exceptionally high, with a nest survival of ca. 54%. Taken together, we could not detect a curtailment of the breeding season, and Skylarks had time to make 1.5 to 1.8 breeding attempts, of which 0.8 were successful. We conclude that crop diversity in our study area guaranteed individuals a chance for successful nests throughout the breeding season.
Chapter 3 investigates the assumption that the growing crop vegetation gradually hampers Skylarks’ access to food and thus decreases food availability in modern farmland. We analyzed (i) habitat selection concerning prey biomass/diversity, vegetation cover, and distance to foraging sites, (ii) the general and seasonal habitat use, and (iii) seasonal development of foraging parameters (e.g., the feeding frequency) as indicators of food availability. During the breeding seasons of 2018 and 2019, we documented the foraging flights of chick-raising pairs at 51 nests. Furthermore, we collected data on vegetation cover, arthropod abundance, and distance to the foraging sites within the surroundings of 42 of these nests. As revealed by a generalized additive mixed model and an analysis of relative variable importance, Skylarks selected foraging habitats mainly based on the distance to the nest, with a clear preference for habitats closer than 112 m. The most frequent habitat types within Skylark home ranges (winter wheat, sugar beet, corn, annual flower strips, and field paths) could not be ranked according to their overall relevance as foraging habitat in a compositional analysis. GLMM models showed a seasonally decreasing use of sugar beet and annual flower strips and increasing use of winter wheat relative to their availability in home ranges. According to linear mixed-effect models and a linear regression model, none of the analyzed foraging parameters worsened over time. Instead, the feeding frequency even increased with the ongoing breeding season. We suggest that the heterogeneous farmland in our study area allowed Skylarks to establish diverse home ranges, which enabled spatial synergetic effects with flexible adaptations of habitat use to changing conditions and prevented any deterioration of foraging parameters.
Chapter 4 emphasizes seasonal changes of nest locations within winter cereals. In this study, my co-authors and I tested if Skylarks actively avoid high-risk linear structures, like tramlines and field edges, as nesting sites in the early breeding season. Between 2017 and 2019, we located 32 nests in winter cereals before the end of May and showed that Skylarks nested 2 m further away from linear structures than expected if nest location was random. Furthermore, we found a higher nest predation risk on linear structures and a shifting of nesting sites towards them later in the breeding season. Because we interpret the initial avoidance as the ability of Skylarks to assess predation risk at fine spatial scales, we present the idea of additional tramline fragments as a modification of Skylark plots for conservation management.
Chapter 5 identifies the heterogeneous composition of our study area as the probable reason we did not find a seasonal curtailment of the breeding season or a seasonal decrease in food availability. I relate our results to other research that pointed to the benefits of farmland heterogeneity. I also interpret it as the most significant advantage of our studies providing arguments for crop diversification on the individual level and not based on breeding pair densities. Additionally, I discuss potential shortcomings of my PhD project, like the impossibility of studying the influence of bad weather due to the extraordinarily hot and dry conditions during the fieldwork period. The chapter finishes with suggestions for future research on Skylarks. | de |