Look for Lichens
Now that the leaves have fallen from the trees it is much easier to see lichens growing on the branches and trunks. They survive throughout the year but thrive in the damper and lighter conditions of winter and spring. They can be found on rocks and walls as well as trees, the species and coverage varying according to many factors including aspect, the acidity of the rock or tree bark and its smoothness. They are often quite easy to find on trees with slightly alkaline bark such as elder, sycamore or maple. Each one consists of a species of alga growing together with a fungus. The alga (a simple plant) photosynthesises, providing the fungus with sugars, and is itself protected from drying out and sometimes from high light intensity, by the fungus. In The Voyage of the Beagle, Chapter XVI, Darwin described how lichen was one of the few living things able to survive the harsh climate of the coastal mountains in Chile.
As well as lichens, look for algae on the trees. At this time of year the base of some trees is bright green because of the growth of Desmoccocus olivaceum, a single-celled green alga. Under the microscope the algal cells are often in groups of 4. Also look for tree trunks with apparent red stains on the trunk caused by another alga, Trentepohlia classified as a green alga but with a dominant red pigment. It often forms the algal partner in lichens.
There are an estimated 27,000-36,000 species of marine and freshwater algae worldwide of which about 5000 live in freshwater or on land in the UK . Those which survive on land are found in damp places or during the wetter months of the year. Some appear after heavy rainfall in summer.
This time of year is very difficult for many birds. The cold weather and reduced food supply means many die, especially the smaller birds whose comparatively large surface area compared to volume means that they lose heat faster than larger species. In The Origin of Species by Means of Natural Selection, Darwin wrote about the effect of hard winters on the mortality rate of birds.
"I estimated (chiefly from the greatly reduced numbers of nests in the spring) that the winter of 1854–5 destroyed four-fifths of the birds in my own grounds; and this is a tremendous destruction, when we remember that ten per cent is an extraordinarily severe mortality from epidemics with man. The action of climate seems at first sight to be quite independent of the struggle for existence; but in so far as climate chiefly acts in reducing food, it brings on the most severe struggle between the individuals, whether of the same or of distinct species, which subsist on the same kind of food. Even when climate, for instance, extreme cold, acts directly, it will be the least vigorous individuals, or those which have got least food through the advancing winter, which will suffer most."