RELATIONS WITHIN A SPECIES 



345 



Mississippi, and Atlantic Flyvvays in reference to the 

 general areas covered. However, the flyways are not 

 clear cut; they sometimes cross and merge with one 

 another. The regularity of migrant arrival has 

 drawn much attention. Some birds appear to be 

 quite predictable in this respect, because they gen- 

 erally arrive within a day of a particular average 

 time: however, other species vary a few weeks in both 

 departure and arrival dates. 



DANGERS 



Migration is one of the most hazardous activities 

 of a species. Migrants are highly susceptible to 

 predation. At no other time are they as likely to be 

 so completely in the open and have as little access to 

 protective retreats. In fact, certain predators appear 

 to capitalize on this situation by traveling with the 

 migrants or by shifting their prey species to the 

 migrants in their area. Migrants are also susceptible 

 to environmental extremes, especially freak weather 

 changes. Nonseasonal storms can eliminate large 

 numbers because the animals cannot find suitable 

 shelter. 



Birds have some unique difficulties. Night mi- 

 grants not infrequently crash into man-made objects. 

 For example, encounters with lighthouses have 

 caused thousands of birds to be killed in a single 

 night. Of the natural dangers, weather appears to 

 be the greatest. Rain can watersoak wings and lead 

 to drowning in the ocean. Snow or ice storms may 

 bury thousands in a frozen substrate. Wind may 

 blow them from their course, perhaps far to sea; and 

 even if they survive the storm, the birds may be 

 unable to return to land. Also, such windblown in- 

 dividuals might arrive in an unfavorable land en- 

 vironment and die because they can find neither suit- 

 able food nor shelter. 



HOMING MECHANISM 



Localities sought by migrants probably are not 

 reached by a universal mechanism. This consti- 

 tutes one of the few real facts in a set of phenomena 

 that are barely beginning to be understood. A few 

 examples will emphasize the state of present knowl- 

 edge about how animals locate places they return to 

 year after year. 



Silver salmon {Onchorhynchus kisutch) migrate be- 

 tween their fresh-water hatching grounds and the 



ocean. The means of finding the ocean probably is a 

 relatively simple mechanism, involving either passive 

 reaction to the effects of downstream water flow or 

 some orientation down a stream. Once the fish gets 

 into the ocean it leaves the vicinity of the stream, but 

 where it travels from there is virtually unknown. 

 For this reason, there is no basis for theories as to how 

 the site of the breeding stream is reached again. 

 However, at the point of return, the sense of smell 

 may be used to identify the proper stream. 



The western North American newts, salamanders 

 of the genus Tancha, may utilize two different ways 

 to locate migration areas, one mechanism functioning 

 in the breeding travel and the other in the landward 

 one. The breeding ponds or streams are located 

 from some distance by the sense of smell. However, 

 some experimental individuals found their breeding 

 stream three years after being placed three miles 

 away and on the other side of mountains! It appears 

 that the distance and topography involved in that in- 

 vestigation would have prevented identification by 

 smell; and memory of the smell or appearance of 

 landmarks also is not likely, because the animals 

 were transported into what must have been an un- 

 known area. In the opposite movement from breed- 

 ing waters to underground retreats on land, the sense 

 of sight is used to seek dark horizons. This behavior 

 pattern causes the animals to locate protected sub- 

 strates. The dark horizons are most likely to be 

 produced by woods or similar habitats. On the 

 other hand, light horizons generally are toward 

 open areas, or places of greater temperature extremes 

 and lower humidities than the dark habitats. 



Memory, in conjunction with sight and smell, may 

 be the mechanism of the deerlike mammals, but 

 memory alone may serve migrating whales. Deer 

 have interdigital and tarsal glands on their legs. 

 The secretions of these glands may persist long 

 enough to "mark" the migration trail for later smell 

 perception, but sight recognition of familiar features 

 of the migration trail might also be of help. Migrat- 

 ing whales often lift their heads in a manner that sug- 

 gests they view the shoreline. For this reason, it is 

 assumed that they use knowledge of shoreline to- 

 pography to navigate to the breeding grounds. 



Birds long have represented the great unknown of 

 migration mechanisms. For some time, memory has 

 been used to explain the method of species that 

 follow well-marked physiographic features. These 

 birds follow large rivers, mountain ranges, and 



