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May, 1917 WILD DUCKS IN A CITY PARK 87 foreground. The flock resting upon the water is composed mainly of Canvas- hacks (Marila valisineria), a deep water species that feeds but little upon land. These birds arrive about October of each year. After the close of the shoot- ing season they begin to scatter, for, with general protection elsewhere there is no longer need of congregation within this sanctuary; so that the ducks cease to be a conspicuous feature of Lake Merritt some time before their departure for distant breeding grounds. OaklaJ?d, Californ.ia, Febr,ary 14, 1917. Fig. 35. SAME FLOCK AS IN FIGS. 33 AND 34. TI[EBE IS A LIMIT TO TIIE TRUSTFUL- NESS O EVEN. VERY TAME ?ArILD DUCKS AND PItLEGMATIC MUDlIENS. FACTORS INVOLVED IN THE NESTING HABIT OF BIRDS By CLARENCE HAMILTON KENNEDY ?VITH TgVELVE DRA?VINGS BY THE AUTHOR NE OF the most interesting series of problems in ornithology is that con- nected with thc high development of the nesting habit in birds. But little has been done to correlate bird anatomy and nesting habits, or even to fig- nre out the causes leading to the great diversity of nests which birds build, for when the present writer wished to look up some obscure points on the nesting hal)its in this group, great was his surprise that in a group so thoroughly worked, so little had been done in the study of nidification other than the mere collection of data on the nesting habits of individual species. No group in the animal king- do?n has been so thoroughly worked as that of the birds, yet the attempts to fig-