Spring trends

Over the past 30 years phenology has provided biologists with clear evidence that spring is arriving earlier.

Trees have been coming into leaf sooner. Migrant birds are arriving earlier with swallows now a week ahead of their dates in 1970. Frogspawn is being spotted before Christmas in the south-west, while comma and holly blue butterflies have been sighted as early as March.

Analysis of spring results 1998-2005

The UKPN now has 8 years of spring results, in addition to the historical data contained in the database.

The bumble bee was once again our most recorded event. The warm start to the year made for a flurry of very early sightings in January, following hard on the heels of the first frogspawn sighted before Christmas in Cornwall.

A closer look at temperatures

January and February 2004 were warmer than 2003, February in particular was 1.3˚C warmer than 2003. However March and April were cooler, March being 1˚C cooler than 2003 (though still over 1˚C warmer than 2001, our phenological benchmark). This demonstrated the importance of March temperatures for the timing of many spring events as despite a warmer January and February, 2004 event dates were later than 2003’s.

So what effect did this have on phenology?

Birds The timing of the arrival of migrant birds was much the same as 2004, and 3.7 days earlier than 2001. Nesting was on average a day later than in 2003 but still over 8 days earlier than 2001.
Insects We know that of all the different species groups, insects show the most response to warmer temperatures. Although 5.1 days later than in 2003 they were first seen on average 13.6 days earlier than 2001.
Amphibians 1.7 days later than in 2003, but still 8 days earlier than 2001.
Flowering and budburst were within a day of 2003 records while first leaf was 1.6 days later, but again all were earlier than 2001 by between 11 and 12 days.
In comparison to 2003 we enjoyed almost 2 extra lawn-mowing free days, on average, but still our first cut dates in 2004 were over 10 days earlier than 2001.

In summary...

Whilst the colder temperatures in late February and March made for a start-stop spring in 2004, and events were just under 2 days later than 2003, on average, all monthly temperatures and event dates were higher and earlier respectively, than in 2001.

The variation of timing between species remains disturbingly evident, with all taxa changing at very different rates relative to 2001; plants about 10 days earlier, insects 2 weeks earlier and bird activity (migration and breeding) about a week earlier.

Click here for a table of results for the last 5 years.

Spring 2005

We had our first report of frogspawn from Pembrokeshire in November 2004, followed by the now familiar early sighting from the Lizard, Cornwall, on 16th December. Snowdrops, lesser celandine and rooks nesting were all recorded before Christmas. We also had reports of over wintering house martins and swallows and even a turtle dove from the BTO, and sightings of bumblebees foraging to feed young from a number of recorders.

Bursting into bud

Records from Jean Combes in Surrey are positive proof that the leafing of trees has advanced over the 1980s and '90s. Horse chestnut shows the greatest advance of 12 days, oak 10 days and ash six.

Ash before oak

Ash before oak,
We're in for a soak.
Oak before ash,
We're in for a splash.

There is little evidence to support the 'ash before oak' rhyme even when records as far back as the 18th century are analysed. The leafing of ash has changed more slowly than oak the buds of which are now bursting at least a week earlier than they did in Marsham's time. The exceptionally warm spring in 2002 accentuated this difference - more

Butterflies like it hot

The Butterfly Monitoring Scheme has shown that the emergence of butterflies in early summer varies with temperature. The ringlet is an excellent example. Its appearance has become progressively earlier in recent years, for every 1°C rise it can be seen a week earlier.

The BMS has kindly provided us with the following summaries of 2001, 2002 and more recently 2005.

In terms of butterfly abundance, records indicate that 2001 was one of the poorest years for butterflies in general in the United Kingdom since the BMS started in 1976. This can be at least partly attributed to poor weather, especially the very wet autumn, winter and early spring of 2000/1. 2002 was another poor year for butterflies in general, although slightly better than 2001. It ranked fifth lowest of the 27 years of the BMS. Exceptions to the overall trend were the spring flying butterflies that benefited from warm and sunny weather between February and April 2002. The skipper species (grizzled and dingy) and orange tip all occurred in good numbers, as did the spring generation of small white, small copper and holly blue. The spring species that over winter as adults, the peacock and brimstone, also had good abundance in 2002 compared to previous years. The generally cool and wet weather in summer 2002 had a detrimental effect on numbers of butterflies that fly at this time.

2005 seemed to be very mixed weather-wise with some hot sunny weather interspersed with periods of cooler wet weather. Most common garden butterflies do not appear to have fared particularly well and species such as peacock and small tortoiseshell actually seem to have done badly in the summer. However holly blues did well in both generations in many areas. At least some specialist species, especially some of the blues appeared to have a generally good year. Butterfly numbers seemed to drop off rapidly as we approached the end of the season and mostly did not seem to have benefited from the good September weather we had. Migrants were generally poor in 2005 with mostly low numbers of red admirals and painted ladies being reported and only a few clouded yellows, though higher numbers of red admirals were reported in October.

Early birds

Migratory birds are also responding to climate change. Records from coastal bird observatories from 1960-96 show that two woodland species - chiffchaff and blackcap - appear to be arriving in Britain earlier. Their response to temperature rise is about two to three days per 1°C. If you are interested in birds, take a look at the British Trust for Ornithology’s Birdtrack website to view the pattern of arrival of migrants in spring 2005

Interestingly our average dates for first sightings of migrant birds in spring 2002 and 2003 coincided beautifully with peak arrival dates recorded by the British Trust for Ornithology’s Migration Watch project. See British Trust for Ornithology graphs for turtle dove, cuckoo and swallow compared to UKPN first seen dates.