122 BULLETIN 195, UNITED STATES NATIONAL MUSEUM 



sets. Baldwin (Baldwin and Bowen, 1928) found that of 21 pairs 

 that had two broods that he had under observation, the average was 

 6 eggs for the first and 5.5 eggs for the second brood. Of 19 pairs 

 known to have but one brood the average number of eggs laid was 

 6.3 per female. 



Birds may be classed as determinate with respect to egg production 

 when they lay a definite number of eggs in a set and indeterminate 

 when they can be induced to continue laying by egg removal. Cole 

 (1930) found that if eggs, presumably from the same female house 

 wren, were removed daily, the bird layed an unusually large number of 

 eggs. Detailed measurements he made revealed that the length of the 

 eggs increased in general to a certain point, then rested, then increased 

 to a second high point, then rested, and for a third time increased to 

 a high point ; following this there was a downward trend. Thus there 

 appeared to be four cycles, separated by rest periods. It is suggested 

 that the process of incubation may react on endocrine glands to cause 

 cessation of egg production. 



The eggs vary from short-rounded-ovate to oval in shape. The 

 ground color is white, usually with a vinaceous tinge. They are 

 thickly speckled with minute dots of brownish red or cinnamon-brown, 

 which are often so dense as to conceal the ground color giving the 

 entire egg a uniform salmon-colored or reddish-brown cast. The 

 color is deepest at the rounded end, and in many eggs there is a wreath 

 of spots concentrated around this end of the egg. The eggs vary in 

 their long diameter from 0.58 to 0.70 inch and in their short diameter 

 from 0.46 to 0.53 inch. The average dimensions of 100 eggs are 

 0.64 by 0.50 inch. The measurements of 50 eggs in the United States 

 National Museum averaged 16.4 by 12.7 millimeters ; the eggs showing 

 the four extremes measure 18.3 by 13.2, 15.8 by 15.5, 14.7 by 12,2, and 

 16.3 by 11.7 millimeters. 



The house wren has two distinct breeding periods. The first, ac- 

 cording to Kendeigh (1940) , who has made very extensive observations 

 at the Baldwin Bird Research Laboratory, Cleveland, Ohio, begins in 

 the middle of May and lasts until the end of June ; the second begins 

 late in June and lasts to the middle of August. Egg-laying occurs 

 most regularly during the first 2 weeks of each period, but occasional 

 sets may be deposited at any time. One egg is laid each day during the 

 egg laying period until the full complement of eggs is completed. 

 Certain observers have credited the house wren with as many as three 

 broods in a season, but these cases are unusual, and it is extremely 

 doubtful whether three broods are ever successfully reared. 



Young. — The incubation period of the house wren is 13 days. The 

 egg temperatures in the nest of the house wren according to Kendeigh 

 fluctuate between 33.9° C. and 36.9° C. The temperature of 35° C. 

 may be considered the temperature at which incubation to hatching is 



