7/21/2019 MIGRATION - Streamline

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UNIT 23: MIGRATION

One of the greatest mysteries of nature is the instinct to migrate. Every year miions of creatures fee the nee! to move for one reason oranother. Most of us have seen the arriva or !e"arture of migrating foc#s of $ir!s. Migration% ho&ever% is not confine! to $ir!s% $ut can $e

seen in reties 'for e(am"e turtes% frogs)% insects '$utterfies% ocusts)% fish 'ees% samon% tunny ) an! mammas 'rein!eer% seas%emmings% &haes% $ats). Many of these creatures succee! in navigating over ong !istances. *o& e(acty they manage to !o this sti

remains a mysteries. There are severa "ossi$iities. They may navigate $y using one or more of the foo&ing:

+ The sun.2 The stars.

3 The Earth,s magnetic fie!. '-hen a sma $ar magnet is attache! to a "igeon% it is una$e to navigate.)

A sense of sme./ Geogra"hica features. '0ir!s fying from North Africa to 1rance seem to foo& coastines an! vaeys.)

hanges in tem"erature. '4amon can !etect a change in &ater tem"erature as sma as 5.536.)

7 4oun!. '-haes an! $ats seem to use sonar.)E("eriments suggests that these navigationa a$iities are "arty instinctive. In one famous e("eriment a young sea$ir! from the isan! of

4#o#hom% off the -esh coast% &as ta#en across the Atantic $y "ane to 0oston% /+55 #m a&ay. It &as reease!% an! &as $ac# in its nest

t&eve an! a haf !ays ater.The Arctic Tern

This sea$ir! ho!s the recor! for ong8!istance migration. Arctic Tern $ree! in Northern ana!a% Greenan!% Northern Euro"e% 4i$eria an!

Aas#a. In ate August they set off on a +7%/55 #m 9ourney &hich ta#es them south% "ast the -est coasts of Euro"e an! Africa to the ti" of4outhern Africa '+%555 #m in 5 !ays). They then fy roun! to the In!ian Ocean an! !o&n to Antarctica% &here they s"en! the Antarctic

summer. On the &ay $ac# they sometimes ma#e a com"ete circuit of Antarctica $efore returning to their $ree!ing8groun!s. The roun!

tri" is over 3/%555 #m in eight months '25 #m a !ay &hen they are fying). The Arctic Tern sees more hours of !ayight than any other

creature% $ecause it e("eriences t&o summers a year% one in the Arctic region% an! one in the Antarctic. These regions have amostconstant !ayight in summer. One tern% &hich &as ringe! in Nor&ay as a chic#% !ie! in e(acty the same "ace% t&enty8seven years ater.

;resuma$y% it ha! ma!e the 9ourney t&enty8seven times.The Euro"ean 1resh&ater Ee

Euro"ean 1resh&ater Ee% &hich oo# i#e sna#es $ut are reay fish% $egin an! en! their ives in the 4argasso 4ea% southeast of 0ermu!a.

As eggs an! arvae they !rift for three years to&ar!s Euro"e% changing $oth sha"e an! coour as they reach the fresh8&ater estuaries ofEuro"ean rivers. They s"en! the ne(t nine to nineteen years in rivers% streams% a#es an! "on!s. As they a""roach o! age they seem to

have an une("aine! com"usion to return to the 4argasso 4ea to $ree!. Many ees &hich have foun! their &ay into "on!s an! a#es come

out of the &ater an! trave overan!% gi!ing through !am" grass. -hen they reach the sea% they ma#e their &ay to the 4argasso% &herethey $ree! an! !ie. No ees ma#e the 9ourney t&ice. The ee has an acute sense of sme% &hich is use! for navigation in oca &aters% $ut

inherite! memory seems the ony e("anation for their migration to the 4argasso.

The