The temperature of 10°C is often taken to indicate the start of spring conditions, suitable for rapid plant growth and insect activity. The spread of the 10°C isotherm from south to north through Europe is shown in Figure 14.3, based on the average figures over many years. This particular isotherm takes more than three months to spread northeastwards through the whole continent, beginning in the southwest in February-March and not reaching the northernmost areas until July. It also takes almost as long to spread from the lowest to the highest parts of mountain areas, such as the Alps. It gives a good indication of the timing of spring conditions, suitable for bird breeding, in different parts of the continent.
The problem of using particular temperature values, like this, as a signal of spring's arrival is that they take no account of previous temperatures and daylengths which may also have influenced plant and insect development. The alternative is therefore to use some biological measure of the timing of spring, and for this purpose data have been assembled on the date of first apple flowering (Figure 14.4). This measure takes more than two months to progress from the southwest to the northern limits of the apple in southern Scandinavia. Whatever measures are taken, however, they indicate that conditions become suitable for bird breeding in the northernmost parts of Europe at least three months later than in the south. The spread is even greater in North America, which covers a greater span of latitude than Europe.
The rates at which most bird species move towards higher latitudes in spring seems to be associated with the dates that their particular foods become available at successive latitudes. Migrating birds need food not only for daily maintenance, but also to fuel successive stages of their journeys (Chapter 5). There would be no advantage in migrating birds getting far ahead of their food supplies, for they would then only lose body reserves, and might even have to turn back (as has sometimes been recorded, Chapter 4). For each species, therefore, the timing of spring arrival at particular latitudes generally coincides with the re-appearance of appropriate food supplies, and numbers of birds appear in breeding areas as soon as their local survival becomes likely.
Because some types of food become available at lower temperatures than others, different species arrive at particular localities in a fairly consistent sequence from year to year. Thus, species which depend on spring thaw to release their food supplies (such as some waterfowl and waders) arrive earlier than those that depend on aerial insects (such as midges), and earlier still than those that depend on larval insects from later-developing leaves (such as some warblers). Nectar-feeders depend on the opening of flowers, and the spring migration of the Ruby-throated Hummingbird Archilochus colubris through much of North America is nearly synchronous with peak flowering of Jewelweed Impatiens biflora, which is an important source of nectar at that time of year (Bertin 1982).
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