Vol.XVIlI 



1919 



] C-L-ELAST), Birds of the Pilliga Scrub, N.S.W. 275 



this expectation is one likely to be realized within limits is seen 

 by figures representing outward and inward journeys over the 

 same or similar country. In several cases, as regards various 

 species, I have found the numbers on the two journeys to 

 approximate very closely. In the present instance rather 

 striking parallelisms resulted when the figures for the outward 

 and return journeys were compared. Thus, 193 Myzanthce were 

 counted going to Baradine and 187 on the return journey by a 

 slightly longer route ; the figures for Strnthidea cinerea were 

 respectively iii and 83, and for Corcorax melanorhamphiis 32 and 

 30 ; whilst for Rhipidura motaciUoides they were 25 each way. 

 The two journej's make a check on each other, and if the numbers 

 recorded err gravely in showing the true bird population, much 

 more discordant results than these should have been obtained 

 when considering birds more or less universally distributed 

 throughout the area traversed (as shown by their being recorded 

 in so many stages of the journey). Birds, of course, only met 

 with in limited areas, and then perhaps in numbers (in this case, 

 for example, Fairy Martins and Artaniiis superciliostis and .-1. 

 personatits), cannot be taken as checks for this purpose. 



The area of observation is a difficult matter to decide, and varies 

 with the means of progression, the kind of country, and the species 

 of bird. It is the distance travelled multiplied by the distance 

 the view extends on each side of the track. In open country this 

 latter may be extensive for large birds such as Hawks — perhaps 

 a quarter or even half a mile on each side — w^hilst for small birds 

 in cover it may be a chain or less. One may perhaps make a 

 general conservative estimate of 10 chains to include mixed types 

 of country where forest is offset by more open plains. This 

 would give the lateral range of vision as a quarter of a mile, so 

 that the distance travelled divided by 4 will give the area over 

 which observation has been made. On this basis, the number 

 of individual birds of each species per acre or per square mile 

 may be considered fairly exact for the larger and medium-sized 

 birds, but a great under-estimation for small birds or birds 

 keeping hard to cover. Taking the above estimate as a standard, 

 liowever, each species can be dealt with on its merits, according 

 to its habits, the nature of the country, &c. The numbers of 

 l)irds seen may, for convenience, be estimated per square mile 

 on the above basis, and then individual species may have, if 

 necessary, a multiplying factor added to show roughly the 

 modification necessar}^ in each case. For instance, if the distance 

 the birds can be seen on each side of the track be estimated at 

 only half of the above 10 chains, this factor will be 2. 



In the tables I kept * I have recorded the species of birds seen 

 whilst travelling, the numbers met with on each stage of the 

 journey (indicating general or local distribution of a species), 

 the totals for the outward and return journeys (which show the 

 reliability of the method), the grand totals, and the estimated 

 * Pressure od space has prevented their inckision. 



