12 
Nebraska Bird Review 
Twiest (unpub. b) found the same 
thing for Redwing nestlings. After 
day eight, when the tissue addition 
rate is reduced, the percent of de¬ 
fecation continues to be 100 percent. 
There is a good possibility that the 
rate of defecation is governed by 
several factors: the amount and type 
of food intake, the change in utili¬ 
zation of food by the digestive sys¬ 
tem, and the stimulus of activity 
caused by handling and perhaps by 
fright. Observations (author) on 
Black-billed Cuckoo {Coccyzus ery- 
thropthalmus) nestlings indicate 
that they defecate at the slighest 
touch of an intruder. This may be a 
protective mechanism. 
e. Eye opening. Eye opening in 
nidocolous birds is an important part 
of their development. Until this stage 
in development, they are dependent 
on the auditory and mechanical 
(touch) sensations. Table 7 shows 
that between days three and four, 
the eyes of Goldfinch nestlings are 
beginning to open. Between days four 
and five the eyes become half open 
and all are fully open by day seven. 
Walkinshaw (1939) reported the eyes 
of Goldfinches beginning to open on 
the average of 3.5 days of age. Burns 
(1921) reviewed the time of eye 
opening in several species of birds 
but did not state whether they were 
partly or fully open. Since the acqui¬ 
sition of sight changes the behavior 
of the nestling and its reaction to 
external stimuli, this should be of 
utmost importance to any biologist 
working with nestling birds. Fautin 
(1941) found the eyes of Yellow- 
headed Blackbirds opening by day 
three. Holcomb and Twiest (unpub. 
b) found that eye opening took place 
in the same interval for the different 
stages in Redwing nestlings as is 
shown above for the Goldfinches. 
f. Calculation of nestling age. 
When one reviews the tables on 
weight and other meassurements it 
is obvious that some of them would 
be of little value for calculating 
nestling age. Some characteristics 
such as wing, mandibular tomium, 
mandible tip to nostril opening, and 
some feather tracts have such a small 
mean increase each day that marked 
differences do not occur, and there 
is considerable overlap in range 
values from one day to the next. A 
character like toe span is difficult 
to measure and if one is not care¬ 
ful, the toes can be injured. Lack 
and Silva (1948) found that the 
European Robin (Erithacus ruhe- 
cula) showed a great range in the 
value of weights for nestlings. There 
was a great range in the weights of 
nestlings for each day. Table 1. Lack 
and Silva (op. cit.) found that the 
European Robin nestlings had pri¬ 
maries splitting open on the ends on 
day 7.5 although on the previous 
day (6.5) the weights of the 62 
young involved, varied between 9.5 
to 17.1 grams. This would lead one 
to believe that feather projection and 
fringing might be a better criteria 
of age than weight. Horwich (1966) 
used feather development as a means 
of aging young Mockingbirds {Mi- 
mus polyglottos) and found it quite 
reliable. The data presented by Fos¬ 
ter (1967) on the appearance 
and fringing of feathers in the 
Orange-crowned Warbler ( Vermi- 
vora celata) would be very helpful in 
determining the age of nestlings. 
Table 4 shows that if the alar tract 
is projecting, most of the birds would 
be at least three days old and if the 
capital tract is projecting most of 
the birds are at least six days old. 
Furthermore, if the alar tract is 
fringing, most nestlings will be at 
least six days old and if the capital 
tract is fringing they are at least 
seven days old. 
The total length would seem an¬ 
other likely characteristic for de¬ 
termining age because of the rapid 
