AUSTRALIAN 80 BIRD WATCHER AUSTRALIAN BIRD WATCHER 1985, II , 80-91 The Black Falcon Falco subniger.· A Summary of Information and Comparison with the Brown Falcon Falco berigora By GREGORY V. CZECHURA, Queensland Museum, Gregory Terrace, Fortitude Valley, Queensland 4006 and STEPHEN J.S. DEBUS, P.O. Box 1015, Armidale, N.S.W. 2350 Summary The literature concerning field recogmhon, body weight, diet, feeding/hunting behaviour, courtship and breeding biology of the Black Falcon Falco subniger is reviewed and supplemented with field observations. Some comparisons are made with the Brown Falcon Falco berigora. Generally, the (larger) Black Falcon tends towards faster and more aerial hunting methods and avian prey, and the Brown Falcon tends towards slower and more stationary searching methods and terrestrial prey, with greater emphasis on reptiles. In bodily dimensions, hunting behaviour and diet, the Black Falcon resembles the Lanner Falcon Falco biarmicus and especially the Lagger Falcon Falco jugger rather than other Australian falcons. In its adaptation to an arid environment (breeding strategy, diet and hunting tactics), the Black Falcon appears to differ somewhat from the Grey Falcon Falco hypoleucos. Data on the biology of the Black Falcon are limited and require further investigation. Introduction Calaby (1976) stated that not much is known about the habits of the Black Falcon Falco subniger, and claimed that confusion with dark-phase Brown Falcons Falco berigora has cast doubt on statements made about it. Cade (1982) implied that little is known about its hunting habits, breeding behaviour and nesting ecology. The purpose of this paper is to review what is known about the Black Falcon in order to identify some aspects of its behaviour and ecology that differentiate it from the Brown Falcon; also to draw together data scattered in the literature, and to place recent work (Cupper & Cupper 1981, Baker-Gabb 1984a, Hollands 1984) within the context of Cade's treatment of the genus Falco. Field recognition The differences between the Black Falcon and the corvids on one hand, and the Brown Falcon on the other, have been exhaustively reviewed elsewhere (Olsen 1975, Pizzey 1980, Price-Jones 1983, Hollands 1984, Fraser 1985). Two points made by Olsen should be emphasised, viz. the kestrel-like flight action of Black Falcons and the 'step' in the outer edge of their tail tip resulting from the two outermost tail feathers being shorter than the rest. This latter feature is illustrated, but not mentioned, in Slater (1970) and is clearly shown in a photograph in Hollands (1984); it is visible only when the tail is fanned or partly so. As most observations tend to be made of birds in flight, some of the more useful identifying flight characteristics of the Black Falcon versus the Brown Falcon are summarised herein (Table 1, Figures 1 and 2). These characteristics are based on the above reviews, other guides (Condon 1966, Slater 1970, Storr & Johnstone 1979) and our own field observations. VOL. 11 (3) SEPTEMBER 1985 Black Falcon: Summary of Information 81 Table 1 Selected flight characteristics of Black Falcons Falco subniger and Brown Falcons Falco berigora Characteristic Black Falcon Brown Falcon Wing tip Pointed (Figure la) Rounded (Figure 2a) 'Fingers' in wing tip Not obvious Usually obvious Tail Narrow and tapered Broad, no 'step'; usually when furled, giving rounded when soaring; streamlined appearance; appears tail-heavy squared when soaring, with 'step' Wing attitude when soaring Slightly drooped Upswept, with 'kink' or gliding (Figure 1 b) (Figure 2b,c) Trailing margin of wings Usually straight Usually curved when soaring (Figure Ia) (Figure 2a) Figure 1. Diagrammatic view of Black Falcon Falco subniger: (a) flight silhouette; (b) wing attitude when gliding or soaring. Figure 2. Diagrammatic view of Brown Falcon Falco berigora: (a) flight sillhouette; (b, c) wing attitude when gliding or soaring. Drawing by G.V. Czechura In flight, the Black Falcon is direct and has a stiff, 'flicking' action like a Peregrine Falcon Falco peregrinus. Olsen (1975) noted that the wing tips are held down, thus forming an arc. In contrast, the flight of Brown Falcons is sometimes erratic, with much jinking and side-slipping during display. The flapping action of the Brown Falcon is of an overarm, 'rowing' type (Olsen 1975, Pizzey 1980). These differences are seen to best advantage during fast flights. Black Falcons at these times fly rapidly and directly with very quick wing beats. In contrast the fast flight of Brown Falcons, as when displaying or evicting intruders, becomes even more jerky and erratic: in particular, the downstroke becomes deeper and exaggerated, even being momentarily held at the bottom of the stroke (GVC pers. obs.). It should be noted that under some circumstances, both Black and Brown Falcons may assume similar or even reversed flight patterns. For example when not hunting, the Black Falcon's flapping action can be remarkably deep AUSTRALIAN 82 CZECHURA & DEBUS BIRD WATCHER and leisurely (McGilp 1934, Pizzey 1980), though not as laboured as a Brown Falcon's wing action (GVC and SD, pers. obs.). McGilp (1934) noted that the appearance of the Black Falcon in leisurely flight contrasts strongly with its speed and prowess when hunting. Reversal of the general trends of flight action is evident during display flights, when the Brown Falcon's wing beats can be quick and shallow (Baker-Gabb 1982, Appendix 1) and the Black Falcon's can be deep and slow (Hollands 1984). Such exceptions to the general rules on the flight actions of these two falcons could be sources of confusion for the unwary observer. Body Weight Baker-Gabb (1984b) gave the weight of female Black Falcons as 879-1000 g, averaging 940 g (n=2) and males as 620-710 g, averaging 664 g (n=5); a female in Hall (1974) weighed 843 g and Olsen & Olsen (1984) stated females to average close to 950 g. These data suggest that the 'females' in Brown & Amadon (1968) and Price-Jones (1983) that fall within the above range for males, may have been mis-sexed; one such 'female' from Price-Jones was actually a male (P. and J. Olsen in !itt.). Further examination of such anomalous specimens and data is warranted, since one would expect the Black Falcon to be as strongly sexually dimorphic in size as the other swift, largely bird-eating falcons (Newton 1979). Furthermore, in all observations of pairs in the field, pairs have always exhibited sexual dimorphism as great or greater than that of Brown Falcons (D. Baker-Gabb in !itt.). Using Baker-Gabb's morphometric data and the formula in Cade (1982, Table 4) the Black Falcon's wing loading is 0.38 glsq. em for males and 0.43 glsq. em for females, rather than Cade's figures of0.34 glsq. em for both sexes (based on an underestimate of body weights). This places the female Black Falcon in the heavily wing-loaded class of falcons (Olsen & Olsen 1984), especially since the formula slightly underestimates actual wing loading (Cade 1982). Though slightly heavier than the Peregrine Falcon, the Black Falcon has a lower wing loading and therefore greater manoeuvrability (Australian Peregrine: 0.50 and 0.57 g/sq. em, calculated as above). The Black Falcon is somewhat larger and considerably more heavily built than the Brown Falcon, with a higher wing loading (male Brown Falcons 417- 510 g, av. 474 g, females 560-730 g, av. 625 g, wing loading 0.34 and 0.37 g/sq. em; sources as above). Interestingly, the Black Falcon has a similar wing loading to Australia's other (smaller) desert falcon, the Grey Falcon Falco hypoleucos: male 0.30 g/sq. em, female 0.43 g/sq. em (Cade 1982, Czechura & Debus 1985). Diet The Black Falcon takes a variety of prey, including insects, reptiles, birds, mammals and carrion (Brown & Amadon 1968, Cupper & Cupper 1981 , Price-Jones 1983, Hollands 1984). This falcon frequently preys on birds, taking some up to the size of adult Pacific Black Ducks Anas superciliosa and Swamphens Porphyria porphyria (Bedggood 1979), 1059 g and 802 g respectively (Baker-Gabb 1982). McGilp (1934) recorded attacks on cockatoos, pigeons, Galahs Cacatua roseicapilla and ducks. Hall (1974) recorded probable Galah remains in stomach contents. Bennett (1 885) found that Little Button-quail Turnix velox and 'young' passerines such as a Richard's Pipit Anthus novaeseelandiae. songlarks Cinclorhamphus spp. and chats Ephthianura spp. were common prey, but he also recorded predation by this VOL. 11 (3) SEPTEMBER 1985 Black Falcon: Summary of Information 83 Black Falcon Falco subniger in flight Plate 20 Photo: David Hollands Brown Falcon Falco berigora in flight Plate 21 Photo: Photo: J. & L. Cupper AUSTRALIAN 84 CZECHURA & DEBUS BIRD WATCHER falcon on Letter-winged Kites Elanus scriptus. Quail occur frequently in the diet of Black Falcons in a number of areas (Berney 1905, Batey 1907, Cameron 1932, Shanks 1952, Austin 1953, Czechura 1971, 1979, Haddon 1976). Austin (1953) recorded Australian Magpie-lark Grallina cyanoleuca and Common Skylark Alauda arvensis as prey. Rabbits Oryctolagus cuniculus, at least in the southern and central parts of Australia, form a prominent part of the Black Falcon's breeding diet (Cupper & Cupper 1981 , Baker-Gabb 1984a, Hollands 1984), thus confirming the general observations of Bennett (1885), McGilp (1934) and other naturalists. Unfortunately, the upper weight limit has apparently not been recorded for rabbits taken by Black Falcons; only immatures have been recorded as prey (Baker-Gabb 1984a). Rodents and other small mammals are also taken (Keartland in North 1912, Le Souef 1944, Cupper & Cupper 1981 , Price-Jones 1983, Hollands 1984) as are insects such as grasshoppers (Haddon 1976, Hollands 1984) and dragonflies (GVC pers. obs.). Though a smaller and more lightly built raptor (see above), the Brown Falcon does manage to take prey within the range and 'manner' of Black Falcons.