Juliana Dänhardt

Optimal departure decisions of songbirds from an experimental stopover site and the significance of weather

Recent models have worked with the assumption that birds try to minimize either time, energy or predation risk during migration, or some combination of these. The few empirical studies available have suggested that time minimization may be the most common strategy. One way of distinguishing between strategies is to study the departure decisions of migrating birds. We supplied migrating European robins, Erithacus rubecula, with food ad libitum in the field and monitored their changes in body mass prior to departure. The overall mass gain rate (k(tot), the ratio of daily mass increase to lean mass) of 10 birds using the feeding station was on average 0.05 (range 0.03-0.09). Departure fuel loads (f(dep), the ratio of fuel mass to lean mass) were on average 0.53 (range 0.35-0.66). There was no significant correlation between f(dep) and k(tot), which indicates a strategy of minimizing the energy cost of transport, rather than minimizing time but other aspects of the fuel deposition pattern suggest that time minimization may also be important. Whereas the behaviour of the robins was difficult to interpret in the light of optimal migration strategies, the importance of weather was striking. The robins selected the best weather conditions (tail wind, high air pressure and no precipitation) within the likely period of departure

Consequences of organic farming and landscape heterogeneity for species richness and abundance of farmland birds.

It has been suggested that organic farming may benefit farmland biodiversity more in landscapes that have lost a significant part of its former landscape heterogeneity. We tested this hypothesis by comparing bird species richness and abundance during the breeding season in organic and conventional farms, matched to eliminate all differences not directly linked to the farming practice, situated in either homogeneous plains with only a little semi-natural habitat or in heterogeneous farmland landscapes with abundant field borders and semi-natural grasslands. The effect of farm management on species richness interacted with landscape structure, such that there was a positive relationship between organic farming and diversity only in homogeneous landscapes. This pattern was mainly dependent on the species richness of passerine birds, in particular those that were invertebrate feeders. Species richness of non-passerines was positively related to organic farming independent of the landscape context. Bird abundance was positively related to landscape heterogeneity but not to farm management. This was mainly because the abundance of passerines, particularly invertebrate feeders, was positively related to landscape heterogeneity. We suggest that invertebrate feeders particularly benefit from organic farming because of improved foraging conditions through increased invertebrate abundances in otherwise depauperate homogeneous landscapes. Although many seed-eaters also benefit from increased insect abundance, they may also utilize crop seed resources in homogeneous landscapes and conventional farms. The occurrence of an interactive effect of organic farming and landscape heterogeneity on bird diversity will have consequences for the optimal allocation of resources to restore the diversity of farmland birds.

The potential for indirect effects between co-flowering plants via shared pollinators depends on resource abundance, accessibility and relatedness

Co-flowering plant species commonly share flower visitors, and thus have the potential to influence each others pollination. In this study we analysed 750 quantitative plant-pollinator networks from 28 studies representing diverse biomes worldwide. We show that the potential for one plant species to influence another indirectly via shared pollinators was greater for plants whose resources were more abundant (higher floral unit number and nectar sugar content) and more accessible. The potential indirect influence was also stronger between phylogenetically closer plant species and was independent of plant geographic origin (native vs. non-native). The positive effect of nectar sugar content and phylogenetic proximity was much more accentuated for bees than for other groups. Consequently, the impact of these factors depends on the pollination mode of plants, e.g. bee or fly pollinated. Our findings may help predict which plant species have the greatest importance in the functioning of plant-pollination networks.