The Size of Softbills by Weight


Sizes of birds are invariably given by length because this has been the only practical way. While it gives some idea of size it runs up against the problem of bulky feathers and, in particular, tail length. Also, the length can be measured in several different ways (from top of head, tip of beak, etc.). For example, the size of the same small bird (a Solitaire) varied from 6.5"/ 16.5 cm to 8.75"/ 22 cm in several different field guides.

A more accurate and useful method is weight. It should not be an overwhelming task to eventually compile a weight list of the relatively few kinds of birds kept in captivity. Of course, there is variation to be found. Some factors to be considered are:


Indoors or out: cage or aviary Migrating



Subspecies/ geographic origin Season/ climate

Well fed/ starving: established/ newly imported (these categories often coincide).

Birds are almost always heavier when kept out of doors, especially in winter (in northern climes, anyway), as they are in the breeding season or at migrating time, even if they are not going anywhere. A small bird can easily increase in weight by five percent after eating, and a hungry, newly imported Miniver ate 32 percent of its weight in mealworms per day, for several weeks.

Birds of prey, with their feast-orfamine eating pattern, normally experience a much wider fluctuation than small passerines. A Screech Owl (Otus asia) that was found in midwinter, frozen and starving, promptly gained 44 percent or, if you like, started at 69.S percent of its usual weight. This was 30.5 percent down and almost at the fatal limit for Shamas (see below). The anomaly lies in whether you use the higher or lower weight to calculate the percentage.

Even larger gains are common in fisheaters. An immature Bald Eagle regularly increased by 10 percent each


meal (that was all it was givenl). A Great Blue Heron (Ardea herodius), lighter than the eagle, ate even more. Thin as a coathanger, it was found frozen to a branch beside a dead companion. An enterprising Humane Society officer brought it to me, branch and all! For the rest of the winter, this four foot bird regularly ate between 20 and 25 percent of its weight in fish per meal and still found the time to stab at my eyes every time it was fed. An injured, hungry Kingfisher (Megaceryle alcyon) gulped down 40 percent of its weight in fish, all at once! So for consistency, all these birds should be weighed before

meals. .

In a number of cases, birds can be sexed by weight (see table) and these numbers will grow with the weighing of more species.

Also, weight is a useful guide in grouping birds to be housed together. This is a notoriously difficult problem for the average softbill-keeper with limited space. It will be found that birds can be kept together with less risk if they are the same weight than if they look the same apparent size.

The tables can be used as a health guide by comparing the weight of a bird to the expected average and for many years I have used this as an aid to prognosis. Often, weight loss - the traditional "going light" - is the sole sympton of impending trouble. Before a sick bird dies, it invariably loses weight and this is especially rapid in the final few days. For example, on the day it died, a softbill chick lost 13 percent of its previous day's weight. Birds that are below a certain minimum weight when obtained cannot be expected to survive. Shamas, which averaged 31g - and showed the greatest variation of all the birds in this study - did not survive if they were one third or more below normal, i.e. under 20.Sg for males or 19g for females.

After taking the above into account, birds in this study - all softbills - have been quite consistent and random weighings have been remarkably close.

Abbreviations used in the table: b = breeding condition

i =indoors

m =molting

o = outdoors

Weights are of normal, healthy adults, unless noted otherwise. •