58 Bird -Lore 



These pictures are simply imaginary lines drawn on a map of the world^. 

 from place to place, wherever the heat (temperature) of the air or its weight 

 (pressure) are the same. 



They may be called lines of equal temperature and lines of equal pressure,, 

 but the words isotherms and isobars are shorter, and just as easy to remember 

 when once we know what they mean. 



Now, since the weather is constantly changing, the pictures of it are differ- 

 ent from day to day; but it is possible, by studying the pictures which have 

 been made of the weather every day for a year, or ten years or twenty-five years, 

 to make a picture which shows what the weather usually is like on any particu- 

 lar day. The chart given below shows what the weather usually looks like 

 when it is heated up to 35° F., and where such weather may be found on the 

 fifteenth of each month from January to June. 



The isotherm of 35° F. is often called the isotherm of spring, for when the 

 thermometer stands at 35° F. we know it is time co look for spring. 



Plants and animals do not need thermometers and barometers and weather- 

 vanes to tell them when spring is coming. They are themselves so sensitive to- 

 changes in the weather that many of them, at least, might be called "living 

 thermometers." 



It is much easier to learn the plants and animals when we know their 

 habits with respect to the weather. Birds, for example, have quite different 

 habits at different seasons of the year. Some birds live all the year round in 

 the cold North, others in the hot South, others in places where it is only moder- 

 ately cold and warm. Many birds, however, have a wonderful habit of travel- 

 ing thousands of miles each spring and fall, in order to nest in the North and to. 

 spend the winter in the South. This habit is called migration. We do not know 

 just how much the weather has to do with it, but we do know that when the 

 spring isotherm reaches us, migrating birds begin to come too, just as surely 

 as the snow melts or the grass grows. 



During February and March, you will see birds which have come down 

 from British America or the northernmost part of the United States to spend 

 the winter in your vicinity (winter residents); birds which live all the year 

 round with you (permanent residents); a few birds, perhaps, like the Pine 

 Grosbeak or Snowy Owl, which come South from the cold North only occasion- 

 ally or for a short time (winter visitors) ; and the earliest of the birds which 

 make long-distance journeys every year (spring and fall migrants). Learn 

 one of each of these groups of birds, if possible, and watch their movements 

 closely when the isotherm of 35° F. reaches you. As it grows warmer 

 than 35° F., find out where the different birds go. 



There is so much that is worth knowing about the weather and its effect 

 upon all things, living or dead, that there is really no place to stop when once 

 we have started to study it. 



Perhaps the most interesting fact about the weather is that there is so 

 much that has never yet been found out about it. 



