THE INCUBATION PERIOD. 43 



the mating of a male A with a female B will produce eggs which will incubate in 

 approximately 18 days, while the resultant period of a male B with a female A 

 will be perhaps 13 or 14 days. These values are given merely for illustrative 

 purposes. For the data, and the discussion of the phenomenon, the reader is 

 referred to Chapter XV, Volume II. 



NUMBER OF EGGS. 



Most species lay two eggs ; there are some, quite a number, that lay a single egg. Even 

 those that normally lay two eggs sometimes lay but one, so that we do not need any "muta- 

 tion" to get a species that lays one. I find my white-faced, or Wonga-Wonga, pigeons 

 (L. picata) produce only one egg. At least this has been found several times in succession 

 (possibly two eggs were laid in a few cases) . The passenger-pigeons (Ectopistes) laid one 

 egg in each of 23 cycles recorded. 



The editor finds the following list, and references to the literature, of one-egg- 

 laying species: 



The following pigeons lay but 1 egg: Dodo, Catenas, Ectopistes, Didunculus, Goura, 

 yellow-billed pigeon, bandtail pigeon, and some species of Carpophaga and Columba. 

 Generally 2 eggs are laid and in exceptional cases 3 (E. Evans, Birds, The Cambridge 

 Nat. Hist., Vol. IX, 1899, p. 328). Darwin states (Animals and Plants, II, p. 190) that 

 Ectopistes invariably lays 2 eggs in America, but only 1 in Lord Derby's menagerie. The 

 same fact is also asserted of the white-crowned pigeon. Le Souef (Emu, II, 1903, pp. 

 139159) recorded Ptilopus ewingi (rose-crowned fruit-pigeon) and Myristicivora spilo- 

 chroa (nutmeg pigeon) as pigeons that lay but 1 egg. He also found, in one case, but a 

 single egg from Lophophaps leucogaster (white-bellied plumed pigeon), but was uncertain 

 as to the normal number. (SS 10, Sh 8/13, SS 4, R19.) 



Among the records for two-egg-laying species a single egg was noted in 8 

 cycles. These were found in the following pairs: Geopelia humeralis, passenger- 

 ring-dove, mourning-dove-white ring, blond ring, homer, a pair of hybrids, and 

 twice in a pair of mourning-doves. 



Sometimes eggs may be lacking entirely, although the birds may unite, build 

 a nest, give the preliminary symptoms of egg-laying, and continue with the incu- 

 bation of an empty nest. This phenomenon occurred in the second cycle of a 

 crested and ring-dove pair and in 2 cycles of another rather similar pair. For 

 further details the reader is referred to the records given in Chapter VII, on 

 "Defective Cycles." 



TIME OF LAYING. 



The first egg is dropped in the afternoon. Table 4 gives the data for 180 eggs, 

 and there is no exception to the rule; it applies to Ectopistes and L. picata, which 

 produce but 1 egg in a clutch, as well as to those pairs that occasionally failed to lay 

 a second egg. The recorded times range from 3 h 30 m to 6 h 50 m , a period of approx- 

 imately 3 hours. Dividing the afternoon into the two periods of before and after 

 5 o'clock, we find that 68 eggs were laid in the first period and 69 in the second. 

 The more usual laying time for the first egg is thus around 5 p. m. 



The time of the second egg depends upon the species, and to a slight extent 

 upon the season. Some species invariably lay in the morning, while others 



