BRANTA — Trevor Tinlin
To what extent are thrushes irruptive? A comparison of Turdus spp. breeding in northern Europe using several variables which distinguish irruptive from regular migrants
Institution: University of Birmingham, UK
Supervisors: Graham Martin, Jim Reynolds
Details: MSc 2011 (Completed)
Subject Keywords: Irruptive migrant; regular migrant; migration; continuum; thrush; Turdus; Falsterbo; EURING
Species Keywords: Common Blackbird; Fieldfare; Redwing; Song Thrush; Mistle Thrush; Bohemian Waxwing; Western Yellow Wagtail
Irruptive migration can be dramatic when it takes the form of a large-scale invasion by a species after an absence of many years. The irruptions of Bohemian Waxwing Bombycilla garrulus into the British Isles are a classic example. This form of migration has been difficult to study in the past but, in recent years, advances have been made in our understanding of irruptive migration and an explanatory framework has been developed. Several variables which distinguish many irruptive migrants from regular migrants have been identified and this has contributed to the idea of a continuum of migratory behaviours between regular and irruptive migrants.
In this study I have analysed the migratory variables for a closely-related group of thrush Turdus spp. (Common Blackbird Turdus merula, Song Thrush T. philomelos, Mistle Thrush T. viscivorus, Fieldfare T. pilaris and Redwing T. iliacus) which breed in northern Europe because some, particularly Fieldfares and Redwings, show unusual aspects to their migratory behaviour which have irruptive characteristics. I have analysed each variable – proportion migrating, timing, distance and direction, plus breeding and wintering area fidelity – individually and in combination, using a data set of systematic counts from Falsterbo (Sweden) and a data set of ringing recoveries from the EURING database. I used Western Yellow Wagtail Motacilla flava and Bohemian Waxwing in some of the analyses to represent regular and irruptive migrants respectively. I used simple statistical tests on variability between species for each variable.
The results support timing and proportion migrating as variables which can be used to distinguish regular and irruptive migrants at the extreme ends of the spectrum, with thrush spp. falling in between. However, statistical analysis did not find many significant differences between the thrush spp., except in the case of wintering area fidelity. A simple method of ranking each species for each variable and combining the ranks revealed an interesting result which supported the idea of a continuum, with the thrush spp. taking up positions along the continuum between regular and irruptive migrants.
Overall, this study has provided some evidence to support the distinctions between regular and irruptive migrants but does not provide evidence of a continuum of migratory behaviour within the thrush spp.; such a continuum may exist, but it will require more sophisticated handling of data and statistical techniques in order to tease any differences apart. The theoretical framework may need strengthening to provide a view on how the separate variables should be combined in any future analysis.