Woodland management: The contrasting fortunes of two sympatric thrush species.

1. Woodland management: The contrasting fortunes of two sympatric thrush species. Mark T Edwards and D Charles DeemingSchool of Life Sciences, Riseholme Campus, University of Lincoln. Introduction Methods Data were compiled using territory maps (Fig.1), On each map each territory was numbered and its area measured using ImageJ (5). The proportion of coppice was then calculated in the same way for each region of the wood in each year. Data were analysed firstly for the whole wood and then for each region using Mann Whitney U tests, ANCOVA and either Pearson product moment or Spearman rank correlation. The proportion of each region occupied was determined by calculating the maximum number of territories possible (area of the region divided by the mean territory size for each species in each year) then was used to calculate the percentage of occupancy for each species in each year. Results Population trends: When the data were analysed as two regions blackbirds found to have more territories than song thrushes (Fig. 2) in each of the ash and semi natural (p<0.001). Song thrush territory numbers showed a negative correlation with year (P<0.001) in each of the regions while blackbird territories increased in number in the ash. Proportion of habitat occupied: The two species occupied differing amounts of each area at different times (Fig.3). ANCOVA showed that the year was significant (p<0.05) and the species and region were highly significant (p<0.01). All interactions were either significant or highly significant. The effects of coppicing: In each region the proportion of coppice increased steadily from 1976 until the early 1990s, after 1991 very little new coppice was added. Blackbird occupation correlated with coppice in the ash (p<0.001) and negatively correlated with coppice in the semi natural (p<0.05). Therefore blackbird occupation increased with coppice in the ash but decreased with coppice in the semi natural. Song thrush occupation decreased with coppice in the ash (p<0.01) Discussion Across the whole wood the local situation for these two species looks to be in line with national trends (6,7). Provision of different habitat types through the use of coppice appears not to be helping the song thrush. There was no evidence that the blackbird had displaced the song thrush but it is feasible that once the blackbird reaches its carrying capacity it may be difficult for the song thrush with its more specific ecological requirements(8,9) to recolonise an area. One of the most interesting outcomes of this study was the sharp increase in both territory number and size for blackbirds in the ash region, this happened in the early 1990s at around the time that the coppicing reached its peak and coincided with sharp declines in song thrush occupation in both regions. Woodland management through coppicing may be beneficial to blackbirds, particularly in ash but it is of no help and may even be detrimental to the song thrush. The song thrush (Turdusphilomelus) has undergone a significant national decline over recent years whilst the sympatric blackbird (Turdusmerula) has not fared so badly. These closely related species are ecologically similar but not thought to compete directly (1). Causal links for declines are unclear but considerable circumstantial evidence implicates pesticides, farming intensification and habitat loss (2,3,4). Woodland is an important habitat for both species and woodland management offers the opportunity to engineer optimal habitats for these species. This study compared habitat occupancy in both species at Treswell Wood, Nottinghamshire. The habitat is split into two regions (Ash and semi natural) and there is differing management of coppice cycles between the regions. The BTO have run the Common Bird Census within the wood and have produced maps (Fig 1.) showing the number and size of territories for each species in each year since 1976. Fig 2. No of territories per species per year for each region. Acknowledgements Thanks to: Chris Du Feu and John Clark of the Treswell ringing group. Carl Soulsbury for feedback. Nottinghamshire Wildlife Trust, owners of Treswell Wood. Fig 1.Example of a Treswell territory map Further information Read the full thesis at: https://ulincoln.academia.edu/MarkEdwards Fig 3. Percentage utilisation by two species in each region of Treswell Wood. References (5). (6). (7). (8). (9). Rasband, W.S., ImageJ, U. S. National Institutes of Health, Bethesda, Maryland, USA, http://imagej.nih.gov/ij/, 1997-2012. Baker, H., Stroud, D.A., Aebischer, N.J., Cranswick, P.A., Gregory, R.D., McSorley, C.A., Noble, D.G., and Rehfisch, M.M. (2006) Population estimates of birds in Great Britain and the United Kingdom. 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