112 |Birds in the hand
Fig. 4.7In many species, such as passerines at middle and higher latitudes, the level
of fat reserves can be assessed visually from live birds because the amount of fat
accumulated beneath the skin within the tracheal pit and on the abdomen are directly
proportional to the total body fat. In this Great Tit, the feathers have been parted; the
fat is then visible as a yellow–pink mass beneath the skin; its quantity can then be
scored by reference to the fixed points formed by the two arms of the furculum. Top
left, score 1, fat just visible in the bottom of the pit. Top right, score 2, pit one third
full. Below left, score 4, the pit is just full. Below right, score 5, fat bulges beyond the
level of the furculum and onto the pectoral muscle. (Photo: Andrew G. Gosler)
(Gosler et al. 1998). In this system, visible fat is assessed in the tracheal pit
(between the halves of the wishbone) on a scale (0–5) where zero represents no
fat, and “5” represents fat filling and bulging out of the pit. It has been shown in
several species that fat in these discrete deposits reflects the total fat load in the
body. Moreover, it can be seen simply by blowing the feathers aside.