Impact of density and environmental factors on population fluctuations in a migratory passerine

Abstract

Summary 1.Populations of plants and animals typically fluctuate because of the combined effects of density-dependent and density-independent processes. The study of these processes is complicated by the fact that population sizes are typically not known exactly, because population counts are subject to sampling variance. Although the existence of sampling variance is broadly acknowledged, relatively few studies on time-series data have accounted for it, which can result in wrong inferences about population processes. 2. To increase our understanding of population dynamics, we analysed time series from six Central European populations of the migratory red-backed shrike Lanius collurio by simultaneously assessing the strength of density dependence, process and sampling variance. In addition, we evaluated hypotheses predicting effects of factors presumed to operate on the breeding grounds, at stopover sites in eastern Africa during fall and spring migration and in the wintering grounds in southern Africa. We used both simple and state-space formulations of the Gompertz equation to model population size. 3. Across populations and modelling approaches, we found consistent evidence for negative density-dependent population regulation. Further, process variance contributed substantially to variation in population size, while sampling variance did not. Environmental conditions in eastern and southern Africa appear to influence breeding population size, as rainfall in the Sahel during fall migration and in the south African wintering areas were positively related to population size in the following spring in four of six populations. In contrast, environmental conditions in the breeding grounds were not related to population size. 4. Our findings suggest negative density-dependent regulation of red-backed shrike breeding populations and are consistent with the long-standing hypothesis that conditions in the African staging and wintering areas influence population numbers of species breeding in Europe. 5. This study highlights the importance of jointly investigating density-dependent and densityindependent processes to improve our understanding of factors influencing population fluctuations in space and time.

Effects of summer weather on reproductive success of the Red-backed Shrike (Lanius collurio)

“…Although we show that populations' reproductive success are correlated with weather conditions in the breeding area, a recent meta-analysis found that fluctuations in population size were positively correlated with environmental conditions outside the breeding areas (Pasinelli et al 2011). This result supports the 'tubhypothesis' stating that population size is mainly limited by survival in the non-breeding season (Saether et al 2004).…”

Quantifying drivers of population dynamics for a migratory bird throughout the annual cycle

“…However, changes in population size are most responsive to habitat loss during the period in which populations are more severely limited [3]. Unfortunately, relatively few empirical studies provide quantitative evidence indicating whether population size of migratory species is more sensitive to the amount of breeding habitat or non-breeding habitat [7,34,35] and we are unaware of any study done at range-wide scales.…”

“…To date, most empirical research on this subject has focused on documenting whether local populations experience limiting factors during the breeding season and to a lesser degree during the non-breeding season [7,8]. However, because limiting factors often vary across a species range [9] and through time, it is generally not possible to extrapolate from these local studies to understand range-wide population dynamics [10].…”

An Overview of Bird Migration Studies in Egypt

“…In a 1969 paper R. Moreau, stated (notwithstanding the imposing two volumes of Nicoll and Meinertzhagen 1930) that Egypt south of Cairo and on both sides of the Nile Valley remained one of the worst-documented parts of Africa. The past years have seen a surge of interest in Egyptian ornithology and substantial areas of the country, hitherto unknown, have been surveyed.…”

“…The past years have seen a surge of interest in Egyptian ornithology and substantial areas of the country, hitherto unknown, have been surveyed. The book "The birds of Egypt" (Goodman et al 1989) aimed to synthesize the new information on the birds of Egypt and to some extent re-evaluate the material available to Nicoll and Meinertzhagen (1930).…”