Analysis of Annual Totals for Honey-buzzards in UK

The years analysed are 1996-2010 with a gap in 1997 where information is not to hand. It would be useful to extend the series further, back to at least 1990. The sources are not identical through the period: 1996-2000 totals basically come from the telephone-based Birdline systems and 2002-2010 from the web-based Birdguides system with 2001 using both sources approximately equally. The Birdguides database system came into being in November 2000. Clearly it is useful to plot the counts against year to see what trends emerge. Five charts have been produced to date as reproduced here.

Chart A is a simple plot of annual total against year without any transformations; Chart B shows a break-down of the count by season as spring (April-June), summer (July) and autumn (August-November); Chart C is an attempt to put a smooth line between the annual counts, including the two exceptionally high counts in 2000 and 2008; Chart D shows an 8-year moving average of the counts, the number 8 being chosen to include just one peak year in each average; Chart E shows a 3-year moving average of the counts with the peak years 2000 and 2008 ignored. The count for 2008 is subject to revision when the analysis is completed.

Chart A shows the two very sharp peaks in 2000 and 2008 when conditions were favourable for visible migration to be detected. Leaving these two years aside though, there does seem to be an increase in annual counts from 1996-2001 through to recent years 2006-2010.

Chart B breaks down the annual total by season. In midsummer (July for Honey-buzzard) migrant totals are very low with much higher numbers in spring, April-June, and autumn, August-November. Unremarkably numbers in autumn are higher than those in spring as such totals will include juveniles of the year. What is remarkable though is the consistency in the figures with counts in spring (of adults obviously) varying only from 88-117 over the 10-year period 2001-2010. Such stability would be associated with a species breeding in the UK, not with a drift migrant where numbers will vary widely from year to year, according to weather conditions. It would be expected that only a small fraction of the total number of birds moving will actually be seen as this is a broad-front migrant in the UK with no pressure points at which to make an intensive study. If say 10% of birds were detected in spring this would indicate a UK population of 500 pairs. Numbers in autumn vary much more. Firstly the number of juveniles migrating depends very much on the success of the breeding season. Secondly the weaker flying juveniles are more likely to become visible in poor weather conditions. From 2000-2010 the lowest autumn counts are the 102-104 in 2004 and 2005, suggesting productivity was poor in these years. The highest counts of 782-948 in 2000 and 2008 respectively indicate both high productivity and poor weather for migration. It is interesting though that other counts range rather consistently from 116-197, indicating a reproducible autumn movement more typical of a UK breeding bird than of a drift migrant.

Charts C-E are attempts to smooth out year to year fluctuations to be able to obtain a better gauge of the long-term trend. Chart C shows that smooth curve fitting is difficult with the two large peaks but still shows an upwards trend. Charts D and E both use moving averages, an 8-year one in chart D so that a peak figure is included in each year and a 3-year one in chart E, excluding the two peak years of 2000 and 2008. Both show an increase, particularly the 3-year series.

There are a number of qualifications. The total for 2008 is not final yet. The data was collected by different techniques between 1996-2000 and 2002-2010 with 2001 a hybrid year. It would be very useful to extend the series back to the early 1990s and include the missing year 1997.

Further work is therefore needed. The analysis does though at this stage indicate that the trend in numbers is clearly upwards and the seasonal pattern and variation are as expected for a UK breeding bird without the erratic figures expected for a passage migrant.

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