Jutta Leyrer

Stochastic atmospheric assistance and the use of emergency staging sites by migrants

Numerous animals move vast distances through media with stochastic dynamic properties. Avian migrants must cope with variable wind speeds and directions en route, which potentially jeopardize fine-tuned migration routes and itineraries. We show how unpredictable winds affect flight times and the use of an intermediate staging site by red knots (Calidris canutus canutus) migrating from west Africa to the central north Siberian breeding areas via the German Wadden Sea. A dynamic migration model incorporating wind conditions during flight shows that flight durations between Mauritania and the Wadden Sea vary between 2 and 8 days. The number of birds counted at the only known intermediate staging site on the French Atlantic coast was strongly positively correlated with simulated flight times. In addition, particularly light-weight birds occurred at this location. These independent results support the idea that stochastic wind conditions are the main driver of the use of this intermediate stopover site as an emergency staging area. Because of the ubiquity of stochastically varying media, we expect such emergency habitats to exist in many other migratory systems, both airborne and oceanic. Our model provides a tool to quantify the effect of winds and currents en route.

Densities of individually marked migrants away from the marking site to estimate population sizes: a test with three wader populations

Capsule Population estimates based on the mark–resighting method can be a useful alternative to population‐wide counts. Aims To investigate whether the mark–resighting method can be used as an alternative to counts to estimate the size of wader populations. Methods Individual colour‐marking and subsequent resightings allowed accurate estimates of annual survival for three populations of waders, on which basis we could estimate the actual number of marked birds alive. Densities of marked birds were determined on sites away (2000–4300 km) from the ringing locations expecting marked birds to be randomly distributed among non‐marked conspecifics. Population sizes are estimated by combining these densities with the number of marked birds alive. Results We found indications that the distribution of marked birds was indeed random in the locations away from the site of marking. The estimated population size of Red Knot Calidris canutus canutus was in accordance with the most recent estimates based on counts. Our estimate of the Calidris c. islandica population was somewhat lower, and that of the Bar‐tailed Godwit Limosa lapponica taymyrensis population was considerably lower than the latest estimates based on counts. Conclusion Population estimates based on the mark–resighting method can be a useful alternative for, or addition to, population‐wide counts, as long as the assumption of random distribution of marked birds at the reading sites is taken into account. We conclude that the Afro‐Siberian Bar‐tailed Godwit population has recently decreased in size or has been substantially overestimated during the counts.