PARENTAL BEHAVIOR 



1285 



many others). In other cases, incubation 

 may start earlier, when only half the clutch 

 has been laid (northern phalarope, Tin- 

 bergen, 1935; blackbird, Messmer and 

 Mcssmer, 1956; yellow-headed blackbird, 

 Fautin, 1941; etc.). Still other species are 

 reported to begin incubating from the lay- 

 ing of the first egg. This is usually reported 

 for water-birds in which both sexes share in 

 incubation, such as gulls (Tinbergen, 1953; 

 Barth, 1955; Ytreberg, 1956), terns (Hardy, 

 1957), herons (Verwey, 1930; Allen and 

 Mangels, 1940), plovers (Rittinghaus, 

 1956), etc. There are, however, also occa- 

 sional reports of songbirds in which incu- 

 bation begins with the laying of the 1st egg 

 {e.g., Amadon, 1944b; Simmons, 1954). 



Field observers report that birds some- 

 times sit on the nest as if incubating, even 

 before any eggs have been laid (Roberts, 

 1940; Simmons, 1955b; von Pfeffer-Hiilse- 

 mann, 1955) . This cannot be regarded as re- 

 liable evidence of incubation behavior before 

 egg-laying, because the behavior of such 

 l)irds may lack several important compo- 

 nents on true incubation behavior (Simmons, 

 1955b) .The great crested grebe, for example, 

 when sitting on an empty nest-platform be- 

 fore the eggs appear, does not fluff out the 

 feathers on the ventral side of the body as 

 does a sitting bird, nor does it show the 

 characteristic settling-down movement 

 (Simmons, 1955b). The male European jay 

 often sits in the nest during nest-building, 

 although in this species only the female sits 

 on the eggs (Goodwin, 1951). There are, 

 however, reliable experimental demonstra- 

 tions of incubation behavior before egg-lay- 

 ing in gulls and pigeons. Tinbergen (1953) 

 and Ytreberg (1956) have found that her- 

 ring gulls and black-headed gulls will sit on 

 eggs placed in the nest shortly before the 

 laying of the bird's own eggs. Poulsen 

 ( 1953 » placed 2 eggs in the finished nest of 

 each of 10 pairs of pigeons which had not 

 yet started to lay. All the birds immedi- 

 ately began to incubate. Lehrman (1958a) 

 found that, when pairs of ring doves were 

 placed in cages containing a nest bowl and 

 nesting material, and tested 7 days later, all 

 of the birds were ready to sit on eggs, al- 

 though most of them had not yet laid eggs 

 of theii- own. 



The foregoing paragraphs undoubtedly 

 give the impression that there are sharp, 

 well defined, easily observed interspecific 

 differences in the time of onset of incuba- 

 tion behavior, and that it is quite easy to 

 tell when incubation behavior starts. Un- 

 fortunately, such is far from being the case. 

 There are extraordinary difficulties in the 

 way of determining when incul)ation be- 

 havior actually starts, and these difficulties 

 have the effect of merging and blurring the 

 distinctions which sometimes seem so easy 

 on the basis of casual field observation 

 (Heinroth, 1922; Nice, 1954). The onset of 

 incubation behavior is often not abrupt, but 

 quite gradual. For example Gurr (1954) re- 

 ports that the blackbird begins to incubate 

 with the 1st egg, but the proportion of time 

 during which the bird is on the egg increases 

 from about 7 per cent to about 90 per cent 

 during the egg-laying period, with an incre- 

 ment on each day. Kendeigh (1952) studied 

 the incubation behavior of the house wren 

 by means of a device which recorded tem- 

 perature changes of the eggs, so that he had 

 records of the time actually spent warming 

 the eggs. He showed that the number of pe- 

 riods during which the bird sits on the eggs 

 increases gradually and progressively from 

 about 10 on the first day of egg-laying to 

 about 35 on the last day of the laying of the 

 5- or 6-egg clutch. 



Even when the bird is reported to be sit- 

 ting from the 1st egg, and where observation 

 seems to indicate that the bird is in fact at- 

 tending to the egg, analysis of the actual in- 

 cubation period of the egg frequently reveals 

 that the so-called "incubation" occurring 

 during the early stages of egg-laying is ac- 

 tually not providing much heat for the eggs. 

 Barth (1955) found that, although the 2nd 

 egg of the common gull is laid, on the av- 

 erage, 46 hours after the first, and the 3rd 

 egg 47 hours after the second, the 2nd egg 

 hatches only 4 hours after the first, and the 

 3rd egg only 21 hours later, indicating that, 

 although the birds are seen to be incubating 

 from the 1st egg, incubation is probably not 

 very effective until the last egg is laid. Simi- 

 larly, Paludan (1951) found that the 3rd 

 eggs of clutches of herring gulls contained 

 embryos larger than 2nd eggs of the same 

 age, and that embryos in 1st egs-s (again of 



