Helmut C. Mueller

THE BAL-CHATRI: A TRAP FOR THE BIRDS OF PREY

fledged were produced by four double-brooded females. One double-brooded fe,nale produced at least six fledglings in 1957. 27. The species endures and overcomes high mortality of nests by virtue of its high breeding potential. 28. Adult ,nales sang full songs as late as August 6. Adults did not un,dergo postnuptial molt during nesting. 29. Fledging occurred as late as August 17-18. 30. Adults apparently migrate south fro,n Oklahoma before molting. There are no September or later records of adults for the state. Juveniles have been recorded in September, female-like birds in OctobeT. 31. Differences exist in the breeding behavior of the Indigo and Painted Buntings.

The Evolution of Reversed Sexual Dimorphism in Size

Females are larger than males in most species of Falconiformes. This dimorphism can be great: the average weight of a female can be almost twice that of a male (Mueller et al., 1979; Newton and Marquiss, 1982). The first hypothesis on the functions of reversed sexual dimorphism (RSD) was probably advanced by Frederick II in the thirteenth century (Wood and Fyfe, 1943). Many hypotheses have been proposed in recent years; most have been supported only by theory or selected anecdotal evidence. As noted by Andersson and Norberg (1981), most hypotheses on RSD are extremely difficult (if not impossible) to test experimentally. The comparative method (Lack, 1968) appears to be simultaneously the best and easiest method of testing these hypotheses.

Hawks Select Odd Prey

Hawks were offered one color of mouse for ten consecutive captures and then a choice between that and another color. The birds usually selected the odd, or unusual, color. This behavior is probably adaptive because odd prey in nature would be unfit.

THE PERIODIC INVASIONS OF GOSHAWKS

ABSTRACT.--We watched for Goshawk migration each autumn in the years 1950-74. Two major invasions, composed largely of adults, were seen, one in 1962 and 1963 and the other in 1972 and 1973. The 1972 southward movement was probably the greatest in history. Comparisons of in- terannual fluctuations in age and sex ratios with those derived from a model population strongly suggest that major invasions of Goshawks indicate a massive reproductive failure and a precipitous decline in population, approaching 70% for 1972-73. We suggest that the magnitude of an invasion is determined by the synchrony of the decline of prey populations in time and space, and also by the size of the Goshawk population. We hypothesize that agonistic interactions between Goshawks play an important role in producing both invasions and massive reproductive failure. Our calculations suggest that the decline in population in 1972-73 was sufficiently great that recovery will take more than 12 years of optimal reproduction. We therefore predict that several 10-year cycles will pass before we witness another major invasion of Goshawks.--Department of Zoology and Curriculum in Ecology, University of North Carolina, Chapel Hill, North Carolina 27514 and Cedar Grove Ornithological Station, Route 1, Cedar Grove, Wisconsin 53013. Accepted 14 January 1976.

PREY PREFERENCES IN THE SHARP-SHINNED HAWK: THE ROLES OF SEX, EXPERIENCE, AND MOTIVATION

SHARP-SHINNED HAWKS (Accipter striatus) show considerable sexual dimorphism in size. In our sample 899 males had a mean weight of 100.00 g and 1,009 females a mean individual weight of 170.0 g. Evidence from stomach contents indicates differences in the prey taken by each sex (Storer, 1966). This paper offers experimental evidence for sex and age differences in prey selection and also suggests that hunger plays a role in the reaction to prey.

Naive Ducklings Show Different Cardiac Response To Hawk Than To Goose Models

Naive mallard ducklings were exposed to overflights of a silhouette of either a hawk or a goose on one day and the other configuration on the next day. An audio record of the heart rate was recorded utilizing a small transducer. Most of the ducklings (14 of 20) showed a greater variance in heart rate in response to the hawk than to the goose (p<0.01 ). These results indicate that the ducklings without prior, specific experience can differentiate between a goose and a hawk and show a greater emotional response to the latter. This constitutes evidence for the recognition of configurational stimulus without prior, pertinent experience. The use of cardiac responses as a measure of emotionality or fear is discussed, as are the merits of various measures of changes in heart rate. We conclude that variance in heart rate is an excellent measure of emotional response to a stimulus.

The relationship of hunger to predatory behaviour in hawks (Falco sparverius and Buteo platypterus)

Abstract The relationship of hunger (as measured by food consumption) to predatory behaviour in two species of falconiform birds was investigated in the laboratory. The tendency to kill prey was correlated with hunger, and only hungry birds killed prey. I suggest that there is no predatory instinct or drive which operates independently of hunger and offer other rationales for the occasional excessive killing by some vertebrate predators. Falco sparverius exhibited a circadian rhythm of hunger and killing by some vertebrate predators. Falco sparverius exhibited a circadian rhythm of hunger and killing tendency, with a peak in the late afternoon. Buteo platypterus possibly showed a slight peak in the morning.

Observations on Migrating Saw-Whet Owls

Saw-whet Owls are rarely observed during lnigration because of their nocturnal habits and the secluded habitat in which they spend the day. As early as 1911 Taverner and Swales presented evidence which indicated that the Saw-whet Owl (Aegolius acadica) migrates in great numbers through southern Ontario. However, the Sawwhet Owl is generally regarded as a permanent resident in Minnesota (Roberts, 1930), Michigan (Wood, 1951), and Wisconsin (Groinroe, 1963). Bent (1938) states that the owl is usually considered a permanent resident and that the movements of this owl are too irregular to be considered true migration. This paper offers evidence which indicates that the Saw-whet Owl is a fairly common fall migrant on the western shore of Lake Michigan. In addition, data on weights, measurements, age ratios, and annual variations in the magnitude of migration are presented. The data for this paper were obtained in a study of bird migration conducted at the Cedar Grove Ornithological Station. The station is located on the west shore of Lake Michigan about 64 km north of Milwaukee, Wisconsin. A detailed description of the area can be found in Mueller and Berger (1966). A portion of the study involved the use of mist-nets to gain an index of the nmnbers of migrants in the area. For some years mist-nets have been left up overnight at Cedar Grove to avoid the work of setting and removing the nets each day. We caught our first Saw-whet Owl in 1956, and by the end of the autmnn of 1961 we had trapped 45 individuals, all of them in the months of September and October. In the autumn of 1962 we expanded our netting program, setting up a number of larger-mesh nets primarily for taking hawks. This almost doubled the net area of previous years. In addition, the netting period was extended well into November in 1962, 1963, and 1964. The dimensions, mesh size, and placement of some of the nets were varied somewhat in the years 1962, 1963, and 1964 (Table 1), but these manipulations did not appear to markedly affect the catch of owls. The locations, mesh size, and dimensions for 374 m • of the nets remained unchanged throughout the three years. The nets were placed in lanes cut through dense brush or along the edges of a clearing. Nets of 36, 61, and 106 mm mesh were employed. The bottoms of nets ranged from 15 cm to I m off the ground, and the tops from 3.1 to 5.5 m high. At least 313 m a of the nets were up by 31 August in 1962, 1963, and 1964. All nets were up by 1 October at the latest in all three years. The nets remained in place until 17 November in 1962, 3 December in 1963, and 20 November in 1964. The nets were furled because of severe weather on only nine nights during the three autumns. Nets were visited until well after dark and at least once again later in the evening. The last net visit was usually made

The Development of Prey Recognition and Predatory Behaviour in the American Kestrel Falco Sparverius

Nine hand-reared American kestrels were exposed to a series of objects, stationary and moving, ranging from a tissue-paper ball, a crude tissue-paper mouse, through stuffed and dead mice, and finally live mice. Only one bird responded consistently to paper models, and only to moving mouse-models. All birds attacked live mice, including five that had not responded to models or dead mice. Two of the five inexperienced birds attacked their first live mouse in less than 10 sec. Attacks on paper models were desultory and disoriented and are believed to represent play rather than predatory behaviour. Attacks on live mice were intense, rapid, sustained and well-oriented, with the mouse being grasped by the thorax, neck or head and biting directed to the head or neck. Eight of nine birds attacked their first mouse (live, dead, or stuffed) expertly and all nine birds achieved consistent, expert perfonnance in less than six trials. The inference is drawn that experience plays a minor role in the development of recognition, capturing and killing of mice by kestrels.

Some Observations and Comments on the Periodic Invasions of Goshawks

THE periodic invasions of the Goshawk (Accipiter gentilis) into the northern United States are well known. A summary of pertinent data and an analysis of the phenomenon have been published by Keith (1963). The relative paucity of quantitative observations on Goshawk population cycles prompted this publication, which summarizes approximately comparable observations of migration, including at least one invasion, for 15 consecutive years, and provides information on the age structure of the migrant population.