Evidence concerning the Function of Nocturnal Call Notes of Migratory Birds

Abstract

Nearly all birds which migrate at night give characteristic flight calls, but only recently has any attempt been made to investigate these quantitatively.Ball (1952) and Vleugel (1954Vleugel ( , 1960) ) relied on the unaided ear to register the notes of migrants aloft.Graber and Cochran (1959, 1960) have successfully recorded call notes with a parabola pointed skyward.A tape recorder automatically monitored a 1% minute portion of each lo-minute interval through the night.The data collected in these studies have been used to interpret the volume of migration, its species composition, and the conditions under which it takes place.The function of night calls has not been a major concern of these workers.There has been no lack of speculation concerning the significance of night calls, however.Some suggest that the calls are not adaptive.Ball ( 1952) supposed that the dawn surge of call notes of thrushes might be inspired by hunger and light from the oncoming dawn.Hudson (1923) postulated that the calls were an expression of fear in the unfamiliar night, an idea shared by a number of others.But it seems unlikely that a behavior pattern which is so frequent and characteristic is functionless.A frequently suggested function of the night calls is that they serve to hold flocks together.A corollary to this is the suggestion that young benefit from the experience of adults by travelling in their company (Brewster, 1886).Another related idea implies that the calls are mutually stimulating, each call urging flight partners onward (Tyler, 1916; others).Lowery and Newman (1955) suggest that the calls may serve an ' Lechosounding" function, facilitating the landing process.This suggestion, while perhaps reasonable, is not considered here for lack of evidence.The evidence presented here is based on analysis of the calls of caged birds exhibiting migratory restlessness and calls given by free flying migrants.Both lines of evidence clearly indicate that for at least some passerine species the calls are communicatory. METHODS AND MATERIALS Most of the captiveBobolinks (Dolichonyx oryzivorus) used in this study were taken as adults near Kenmare, North Dakota, following the breeding seasons of 1959 and 1960.Additional birds of unknown breeding origin were taken from a transient flock at Gainesville, Florida, in early May, 1960.Three birds were hand reared in June, 1960, after being taken as nestlings near Ithaca, New York.All these birds were held in individual cages and maintained on a diet of Purina game bird startena.Each bird was maintained in an individual holding cage with two perches.Those perches were attached to the lever arm of a microswitch so that every new perching movement was recorded on an Esterline-Angus chart.Occasionally the bird might trip the switch twice in landing, but at slow chart speeds this showed as a single deflection.The purpose of this monitoring was to ascertain when an individual bird was nightactive and might provide suitable material for orientation experiments in the cylindrical orientation test apparatus (see Hamilton, 1962).But in addition, these holding cages provided considerable information on the temporal pattern of activity.These data were heavily supplemented by direct observation, either of the fearless hand-reared birds in the holding cages or of wild-caught birds in the orientation apparatus.In the latter case observations were made from below and the bird was unaware of the observer' s presence.