. Bulletin of the Department of Agriculture. Agriculture; Agriculture. 42 BULLETIN 185. U. S, DEPARTMENT OF AGEICULTUEE. warbler is a good example of this usual habit. Some summer warblers that return to the Great Slave Lake region to breed after spending the wmter in Central and South America amve at their nesting gromrds when the average daily temperature is about 47° F. Accord- ing to the movements of the Canada goose, these summer warblers might be expected to pass up the Mississippi Valley and on to their. Fig. 19.—Isoclironal migration lines of the red-eyed vireo ( Vircosylva oUvace
. Bulletin of the Department of Agriculture. Agriculture; Agriculture. 42 BULLETIN 185. U. S, DEPARTMENT OF AGEICULTUEE. warbler is a good example of this usual habit. Some summer warblers that return to the Great Slave Lake region to breed after spending the wmter in Central and South America amve at their nesting gromrds when the average daily temperature is about 47° F. Accord- ing to the movements of the Canada goose, these summer warblers might be expected to pass up the Mississippi Valley and on to their. Fig. 19.—Isoclironal migration lines of the red-eyed vireo ( Vircosylva oUvacea). An example of a recently extended migration route. The birds which are to nest in New England advance along the Atlantic coast in approximately a straight line and at a fairly uniform speed, while those which are to nest in Washington advance up the Mississippi Valley at about the same speed until eastern Nebraska is reached, when they turn sharply to the northwest and more than double their speed as they journey along this recently extended route to the far northwest. (See p. 37 and also fig. 18.) summer homes at the same time as the northward-moving tempera- ture of 47° F. But if this were so they would never leave the United States, for the average temperature of the coldest month of the year at New Orleans is 54° F. As a matter of fact, the summer warblers of Great Slave Lake are probably too weU content with the warm, humid, msect-laden au' of the south to brave the arctic blasts before. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original United States. Dept. of Agriculture. [Washington, D. C. ?] : The Dept. : Supt. of Docs. , G. P. O.
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