Home > Migration Watch > About Migration > Recent changes

Recent changes

There has been a lot of recent interest in changes in climate, the phenomenon of ‘global warming’ and how this might impact on bird populations.  It’s not only birds that have been studied; butterflies, flowers and amphibians have all received attention.  The timing of biological events is termed phenology.

Bird Observatories log books, county bird reports and detailed personal notes offer us a chance to look at the changing pattern of bird migration.  Using long-term climatic data such as monthly air temperature we are able to see to what extent the climate can explain the observed changes in timing of migration.  Tim Sparks of the Centre for Ecology and Hydrology is fanatical about the subject and has spent a lot of time extracting information from a wide range of sources.  In a recent paper Tim looked at family bird notes, bird reports and observatory data and was able to show a clear pattern for the earlier arrival of migrants in recent decades and the strong influence of temperature along migration routes.  Tim suggests that the arrival of the Swallow advanced by about 1.6-1.8 days per 1°C increase in temperature. When you relate Willow Warbler arrival dates to March temperature you find that Willow Warblers arrive between 1.3-2.3 days earlier per 1°C increase in temperature. Further reading If you have an interest in recording all aspects of nature then click here to visit the UK Phenology Network website.

		Chris Mason is another phenology enthusiast and has published a paper using records from the Leicestershire and Rutland Ornithological Society annual bird reports for the period 1942-91.  Chris showed that Chiffchaff, Sand Martin, Blackcap and Sedge Warbler showed a strong tendency towards earlier arrival over the period, while Tree Pipit, Cuckoo, Whinchat, Whitethroat and Garden Warbler showed a trend towards later arrival. Further reading
		Whinchat

Humphrey Crick of the BTO’s Nest Record Unit has a keen interest in climate change and has used the extensive nest record data set to investigate the timing of egg laying over a 25-year period 1971 to 1995.  Humphrey together with his colleagues at BTO analysed an amazing 74,258 records covering 65 species!  They found a trend towards earlier laying dates for 20 species, and only one species laying much later (Stock Dove).  They found that the 20 species were laying 8.8 days earlier on average than they used to (range 4-17 days).  The species showing the trend towards earlier laying covered a broad range of species and nesting times: Oystercatcher, Curlew, Redshank, Dipper, Wren, Long-tailed Tit, Nuthatch, Starling, Tree Pipit, Redstart, Whitethroat, Blackcap, Wood Warbler, Chiffchaff, Willow Warbler, Magpie, Carrion Crow, Chaffinch, Greenfinch and Corn Bunting.

So why are birds nesting earlier?  Birds are more likely to nest early if temperatures rise earlier in the year.  There is evidence that laying date is related to temperatures in March and April.  Food availability plays an important role in determining the timing of laying; there must be enough food available at the right time to feed the chicks.  In years of high spring temperatures average flowering and leafing dates may also be advanced which will have an effect on the abundance of insect supplies for birds.  Nesting earlier in the season may actually benefit birds because it would give more time for juveniles to get into good condition before the winter so may improve the chances of surviving from one year to the next.  It is crucial however, that there is no mis-match in the timing of chicks hatching and food supply.
Further reading