Daniel D. Berger

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 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.

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

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.

The Daily Rhythm of Hawk Migration at Cedar Grove, Wisconsin

TABOR (1956) has shown that captive Common Buzzards and Roughlegged Hawks show activity rhythms under various experimental light regimes. Laboratory experiments by Mueller (1972) have shown that the American Kestrel or Sparrow Hawk has a circadian rhythm in hunger and predatory behavior that peaks in the late afternoon, while the Broadwinged Hawk shows no peak, or perhaps a slight peak in the morning. Subjective impressions gained from many autumns of watching migration suggested to us that accipiters were more commonly seen in the morning while buteos and falcons occurred later in the afternoon. We also felt that all species, except perhaps falcons, were easier to lure into traps in the morning, suggesting that hunger is at a peak early in the day. In this paper we subject these impressions to rigorous analysis.

Age Differences in Wing Loading and Other Aerodynamic Characteristics of Red-tailed Hawks

Abstract We examined age differences in wing loading, aspect ratio, wing span, and tail area in a sample of 117 Red-tailed Hawks (Buteo jamaicensis) captured at the Cedar Grove Ornithological Station, Wisconsin, during 1979–1987. Adults had significantly wider wings, lower aspect ratios, shorter tails, and smaller tail surface areas than juveniles. Red-tailed Hawks showed fewer age differences in aerodynamic characteristics than Sharp-shinned Hawks (Accipiter striatus), probably because of differences between the two species in the pursuit and capture of prey. Sharp-shinned Hawks take birds from above ground or after a brief chase, often in dense vegetation. Sharp-shinned Hawks require more aerial agility (ability to make rapid twists and turns) than is necessary for Red-tailed Hawks, which capture prey on the ground, usually after a glide or flight from an elevated perch.

Temporal changes in size of Sharp-shinned Hawks during fall migration at Cedar Grove, Wisconsin

Abstract We measured wing chord, tail length and mass and estimated levels of subcutaneous fat deposits of Sharp-shinned Hawks (Accipiter striatus) trapped at Cedar Grove, Wisconsin. Wing chord, tail length, and mass all showed a sudden increase beginning about 10 October in adults and juveniles and in males and females despite the fact that adults migrate about 20 d later than juveniles and males migrate later than females. Migrants captured at Cedar Grove late in the season did not differ significantly in wing chord or tail length from migrants captured at the Goshutes in eastern Nevada. We suggest that an increasing proportion of the birds captured at Cedar Grove after 10 October are of western origin because winds are more westerly in October than in September. The greater mass of Cedar Grove birds may be because of greater availability of prey than in the Goshutes or because the lower mass of birds in the Goshutes is a facultative adaptation for easier passage through the arid west.

Age and sex differences in wing loading and other aerodynamic characteristics of Merlins

Abstract We examined age and sex differences in wing loading, aspect ratio, and wing span in a sample of 208 Merlins (Falco columbarius) captured at Cedar Grove, Wisconsin, during fall migration, 1978–1993. We also examined differences in tail loading of 166 of these Merlins. Adult males had significantly greater mass and wing loading than juvenile males. Adult females differed significantly from juvenile females only in mass. Females were significantly greater than males in every measure except aspect ratio. There were no apparent age differences in tail area or flight surface loading, but females had greater values in both. Merlins show fewer age differences in aerodynamic characteristics than Sharp-shinned Hawks (Accipiter striatus), probably because of differences in how the two species pursue and capture avian prey. Merlins usually capture prey in the air, sometimes after multiple stoops and at high flight speeds. In contrast, Sharp-shinned Hawks take birds from their perch or after a brief chase, often in dense vegetation, at relatively low flight speeds. Slower flight speeds require larger control surfaces and can explain the increased age differences in wing and tail areas in Sharp-shinned Hawks.

AGE AND SEX DIFFERENCES IN THE SIZE OF PREY OF THE SHARP-SHINNED HAWK

Abstract We trapped Sharp-shinned Hawks (Accipiter striatus) during fall migration, using House Sparrows (Passer domesticus), European Starlings (Sturnus vulgaris), and Ringed Turtle Doves (Streptopelia risoria) as lures. Adults and males initiated attacks on sparrows more frequently than juveniles and females. Adult females initiated attacks on starlings and doves more often than adult males, but there was no such difference between the sexes in juveniles. Females actually struck all three species of lure more frequently than males, and juvenile females struck lures more frequently than adult females. There was no age difference in the incidence of strikes by males. Larger (longer-winged) juvenile males and females attacked starlings more often than smaller juveniles, but there were no size differences in strikes on prey in any age and sex group of hawks, suggesting that sexual dimorphism in size is not a result of selection for differences in prey size. Females struck disproportionately larger lures than males, presumably because they are less efficient at capturing smaller prey. Juveniles attacked doves as frequently as starlings, and there was little age difference in the incidence of strikes, suggesting that inexperience in judging the size of prey is an inadequate explanation for the many attacks that fail to result in strikes.