18 DISEASES OF DOMESTICATED BIRDS 
These barbs each carry two rows of barbules which interlock with 
those of adjoining barbs. The feather is elevated by a small muscle 
located in the skin at the root of the quill. 
The comb and wattles of fowls and the caruncle of turkeys are 
developed from the skin. 
INCUBATING PERIODS 
The length of time necessary for hatching eggs by natural incu- 
bation shows some variation. The normal period for the common 
hen is 21 days but this period may be shortened to 19 or 20 days 
by warm weather and unusually constant brooding by the hen. On 
the other hand, if the eggs have been abandoned and allowed to cool 
for a time, hatching may be delayed for several days. These varia- 
tions doubtless occur in the case of other birds. The incubation 
periods of various birds, subject to variations as explained are as 
follows. 
TasBLe I.— Incupatine PEriops 
MAGI <sieesatedete ete vhcs2n eb hars Qi belated eles 21 days 
PRGSSaNt  ovwcwedies ae ectaweeweroeawe 25 days 
TD UGK sec aite saeiees decree antec ste aes aos ee soe 28 days 
Peatowl 2 sie vetarn oi awe ayes dais 28 days 
Guinea-fowl ....... 0... ce cece ee eee 25 days 
GOOS6.: cca Sia scdcokee eek nee Stee es 30 days 
DUrkGy™ uct. wiacee loci a atone s eran 28 days 
Pige0n. wiii.as veel gia neweee ee ees s 18 days 
PULSE, TEMPERATURE AND RESPIRATION 
Loer has made an extensive series of observations on the tempera- 
ture, frequency of pulse and respiration in domestic birds. His re-. 
sults are summarized as follows. 
Frequency of heart beat as determined by digital palpation after 
opening the thoracic cavity was 128 to 140 per minute in the hen 
and 141 to 149 per minute in the pigeon. The heart beat of the 
duck as determined by auscultation varied from 150 to 180 per 
minute. 
