ALTITUDE OF THE MIGRATION FLIGHT 59 



unremitting attention, do not supply us with more than a know- 

 ledge of the period of time in autumn or spring during which such 

 disturbances have talcen place. From this, however, we are not able 

 to form more than an approximate conclusion as to the actual 

 duration of the migration, since we can never determine whether 

 the first observed individuals of a species are, in reality, the inaugu- 

 rators of the migration at that particular time, or whether they 

 may not have been preceded, weeks before, by a vanguard, travelling, 

 according to the normal manner of the migration flight, at great 

 and impenetrable heights above. 



On the other hand, it would appear to bo extremely unsafe to 

 base, on observations of this nature, the line of arrival or the migra- 

 tion-front of a species, or to draw conclusions from them, as to the 

 velocity of the migration flight, as von iliddendorfl:" attempts to do. 

 For in the first place it cannot be determined whether one is dealing 

 with individuals whose spring migration is proceeding in a northerly 

 direction, and not with such as are pursuing a westerly course ; and, 

 further, one cannot establish with any degree of certainty, whether 

 the flrst observed individuals of a species are actually the breeding 

 birds belonging to the particular district of observation or not. 

 By means of data of this nature, one can never state with the neces- 

 sary degree of precision, as to when a species arrives at or passes 

 over a particular degree of latitude or longitude. In fact, such data 

 only yield information in regard to such disturbances in the normal 

 migration as may have occurred within the limits of the area under 

 observation. These disturbances, however, being, as has been already 

 intimated, solely determined by meteorological contingencies, might, 

 just as well, have occurred four hundred miles farther north or south, 

 east or west, or perhaps not have taken place at all, in which latter 

 case the migration would have proceeded on its normal course, and 

 the migrants, ti'avelling far beyond the range of human vision, 

 would have escaped the observer's notice altogether. We should 

 then note down the migration as having been a very poor one, whilst 

 all the time our feathered friends may be already building their 

 nests in the far north and the far east, or preening their feathers 

 in the warm sunshine of the south, and looking back with joyful 

 feelings to ajournoy happilj- and safely accomplished, thus verifying 

 the saying of our Heligoland shooters : ' Time gone b}', birds gone 

 by ' — which means, that if, during the proper migration period of 

 particular species, none of the birds have been seen in consequence 

 of so-called contrary winds, it is vain to look for any to appear after 

 the expiration of this time, however favourable the wind or the 

 weather may be. 



Before closing this chapter, I would mention another interesting 



