Thursday, 7 February 2013

Session 4 - 7th February 2013

Following our fieldtrip to Pennington Flash last week, this session began by recapping the birds we saw and checking them on a list.  A total of 42 species were seen, although not all of them by everyone, and this was a good count considering the very windy weather conditions.

We then went on to discuss the birds we had seen during the week and Martyn told the class that he had a few birds he'd like positively identifying from some blurry record shots he took in North Wales last weekend - it was agreed to do this at the end of the session.

Bonaparte's Gull
Our first discussion today centred around last week's homework - to identify a gull from a photograph.  Peter had given us some good clues about this bird, but nevertheless it was still quite difficult to pin it down.

The main clue was that it wasn't a British bird and a book with European birds (such as the Collins bible) would be need to ID it.

I personally to'ed and fro'ed between a Slender-billed Gull and Bonaparte's Gull as seemed to have most of the features of both - except that was for the dark spot just behind the eye.

So in the end the bird proved to be a vagrant Bonaparte's Gull in adult winter plumage with the remnants of it's dark hood showing as the spot.  Having said that the bird's legs did seem very long for this Gull.

Harris's Hawk in Flight
For our 'Birds in History' section, we were given a sheet on the Harris's Hawk which recollected Laura's story about the young birds which, when threatened, stand on each other in the nest with the youngest on top forming a pole-like structure.  This is thought to be the inspiration for the Native Americans' totem pole!

We also had a quick look at a handout from the Yorkshire Post on the 'White Nun' Smews which are increasing number in Yorkshire due to snow and ice in the Low Countries.

Dartford Warbler

With spring around the corner, the main topic of this session was the identification of Warblers as they will soon be migrating here as the year progresses.  Excluding rarities, we can easily see twelve species of Warbler in Britain, ten of which are regular summer visitors and two of which are resident.  The two resident birds are the Dartford Warbler and Cetti's Warbler.

A useful acronym for remembering the four migrant genera is PALS.

P  - Phylloscopus (3 species) - Chiffchaff, Wood and Willow Warbler
- Acrocephalus (2 species) - Reed and Sedge Warbler
L  - Locustella (1 species) - Grasshopper Warbler
S  - Sylvia -  (4 species) - Blackcap, Garden, Common Whitethroat and Lesser Whitethroat

Each genera has a favoured habitat, such as marshes, woodland, scrub etc and this helps in their identification. Failure to identify warblers is mainly due to lack of field experience - once you've learnt their songs and perhaps seen them, they're gone - they're only around for half the year at best as they come in the spring to breed and remain until August.

The most common summer visiting warbler is the Willow Warbler which is often confused with its 'twin species', the Chiffchaff.  As these birds look virtually identical, their song is the most reliable form of identification, the differences being very subtle.  Both will have arrived and be in full song by mid-April and sometimes earlier.  They are trans-Saharan birds which some here to breed and so they sing their hearts out to get a mate and establish a territory.  Chiffchaffs will be in song in the middle of March and the birds you hear will be those which have just arrived

Siberian Chiffchaff
There are no leaves on the trees when they first arrive, and this makes them easy to see April being the best time due to the lack of vegetation.  After they have mated they stop singing and the leaves start to appear, making viewing a lot more difficult.  Willow Warblers moult in Autumn and leave by September. Some Chiffchaffs overwinter in Britain and we currently have a Siberian Chiffchaff along with two 'normal' Chiffchaffs at Wince Brook in Middleton.

Whereas Willow Warbler and Chiffchaffs are 'lookalikes', some birds are 'soundalikes', such as the Reed Warbler and Sedge Warbler and the Blackcap and Garden Warbler.  The Common Whitethroat is a rusty-tinged bird that wants to be seen, whereas the Lesser Whitethroat is a greyer bird which doesn't want to be seen.  More on these next time.

We continued the session with Peter's fascinating account of why birds migrate, which is now presented here in summary form.

The most recent Ice Age, which had lasted for about 70,000 years, came to an end some 10,000 years ago. There have been five Ice Ages in the past, each separated by an interglacial period and bird migration started at the end of the last Ice Age. At the peak of the last Ice Age about 15,000 years ago, the mean temperature in Britain was 15° C lower than it is today.  As the mean temperature in Britain today is about 8° C, that means the temperature was about -7° C on average.  In Mediterranean Europe it was 10° lower and in the tropics it was 5° lower than today.

At the end of the last Ice Age, Spain and Portugal, France and south east England were tundra habitats with low temperatures, no trees and a short growing season. The Ice Cap was about 1 mile thick in the northern parts of Britain and so there really was not much life about. Any heat from the sun was reflected back into space off the ice by the albedo effect.

When the Ice Age finally came to an end, huge volumes of water were produced by the melting ice and the UK became an island being cut off from France by the water which became the English Channel.  The temperature rose and life started to be reborn after 70,000 years of Arctic conditions.  With the revegetation of the land came insects and after thousands of years the UK was eventually completely forested.  It is noted in the Domesday Book that in 1066 the whole of the country was covered in trees.

Meanwhile most life was concentrated in what is now Africa, which was a 'Garden of Eden' 5000 years ago. The Sahara Desert area was much bigger than it is today, so big that it acted as a barrier to movement of living things. But as the global water levels rose, the Sahara shrank in size.  Previously there had been nothing much north of the Sahara, but as it receded it became less of an obstacle and new territories and lands were opened up for living things, including birds.

In the highly populated area south of the Sahara there was competition for food and also many predators. So some birds started pioneer northerly movements to find food elsewhere which they did in abundance north of the Sahara, as well as there being no predators.  There was also the added benefit of longer daylight hours for hunting insects and foraging for food and all these factors led to birds having bigger and healthier broods.  The birds which moved north did better than those that stayed put and so natural selection favoured the migrants, who passed on their behaviour through their genes to their offspring.

After breeding, some birds then returned home and so this was the start of the traditional migratory routes which many birds continue every year.  The natural ancestral home of many birds we call British (such as Swallows) is Africa.  It must benefit the species that have this strategy or else they would not survive or do it. Any bird migration takes place to exploit seasonal (albeit temporary) benefits.

To conclude, a wise and famous man (I think it was Peter) once said 'A bird will only undertake a migratory journey if the benefits of the journey outweigh the risks of staying put  - it does, so they do it'.

For homework we were given a sheet with three 'Mystery Migrants' to identify and we finished off the session by identifying the birds on some of Martyn's blurry photos.  The one he was most pleased about was the Purple Sandpiper, the topmost bird in the shot below, as this was a 'lifer' for him.

Mixed flock of Turnstones, Dunlin and a Purple Sandpiper  - (c) Martyn Jones

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