Peregrines and Powerful Owls in Namadgi and Tidbinbilla
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PEREGRINES AND POWERFUL OWLS IN NAMADGI AND TIDBINBILLA a report for the NATIONAL ESTATES GRANTS PROGRAM by Jerry Olsen and Rainer Rehwinkel Applied Ecology Research Group University of Canberra August 1995 1 Introduction 2 1 Chapter 1 - A Synopsis of the Biology of Peregrine Falcons 3 Chapter 2 - Peregrine Falcons in Namadgi and Tidbinbilla 28 Chapter 3 - Powerful Owls in Namadgi and Tidbinbilla 56 Appendix 1 - Updated Reports on Peregrine Falcons in the Murrumbidgee River Corridor, Googong Foreshores and ACT Forests 89 Appendix 2 - Raptor Conservation 98 1 INTRODUCTION The Peregrine Falcon Falco peregrinus is an indicator species that give us important information on environmental quality. Peregrines are highly sensitive to environmental pollutants which drastically reduced their numbers elsewhere in the world. They nest on cliffs and cliffs are permanent structures. Successive pairs of Peregrines will breed on a cliff for many hundreds of years if it has suitable ledges, available prey and no disturbance. These suitable cliffs are uncommon in the ACT. Of the nine pairs known in the ACT before the current study, one was on a building and has since been abandoned, two were in quarries and four were in Namadgi (Olsen 1992, Olsen and Olsen 1989). The Powerful Owl Ninox strenua is also indicator species because of its reliance on tall forest and the arboreal mammals that live in these forests (Garnett 1992). This forest is also limited in the ACT. In Chapter 1 of this report we present a synopsis of the biology of Peregrine Falcons as a basis for understanding and managing the species in the ACT. In Chapter 2 we discuss methods and results of the current study on Peregrines in Namadgi and Tidbinbilla and recommendations for management. In Chapter 3 we discuss methods and results of a current study on Powerful Owls in Namadgi and Tidbinbilla and make recommendations for management. In Appendix 1 we present updated reports for other Peregrine Falcon nests on lands managed by the ACT Parks and Conservation Service and ACT Forests to further help manage the species in the ACT. Finally, in Appendix 2 we cover some basic principles of raptor conservation to provide a basis for using the results of the Namadgi and Tidbinbilla studies. 2 CHAPTER 1 - A SYNOPSIS OF PEREGRINE FALCON BIOLOGY Brief description The Peregrine Falcons Falco peregrinus macropus is a crow-sized (female wts. 790 - 1100 grams, average 920 grams n=12) to magpie-sized (male wts. 538 - 675 grams, average 610 grams n=4) falcon with pointed wings, (span about 850 mm), quick wing beats, blue-grey back, characteristic black helmet and white bib, and black barring on the breast. Juveniles are browner with dark teardrops on the breast (Olsen, Crome & Olsen 1993). Distribution Peregrines are the most widely distributed bird in the world, occurring on all continents except Antarctica and on many islands, though they occur sparingly in many places because of pesticides. They occur throughout Australia mainly associated with cliffs (Emison and Weatherly 1993). Food and Hunting Falcons are predominantly meat-eaters and Peregrines are bird eaters. Falcons are designed for high-speed hunting and very few birds can outfly them. This characteristic more than any other has drawn humans to falcons - to study them, and use them as symbols and in falconry to observe and control this rapid pursuit of quarry (Ratcliffe 1993). Much that has been written on hunting falcons is based on falconry and much of this is about Peregrines. They are described as hunters that "wait on" high overhead until a bird flushes below them. Then they fold their wings and hurtle earthward at breakneck speed. Wild falcons certainly do stoop at birds flushed by humans, machines, birds or mammals. But is this the Peregrine's most common way of hunting? The short answer is no. Kenward (1990) points out that raptors with a choice of hunting techniques seem to use the method that costs the least energy. 3 Below, we examine some of the techniques and strategies used by hunting falcons. Hunting is difficult to study so there are few quantified data for Australian falcons. We believe it is important to understand which techniques a Peregrine can and cannot use so we can understand their feeding ecology in Namadgi and Tidbinbilla. Search and Attack Techniques Fox (1977 p.127) defined hunting as "when a bird of prey searches for, locates and attacks prey". He outlined seven search techniques that raptors use to locate their prey: (a) Still hunting (b) Fast contour-hugging flight (c) Soaring and prospecting (d) Slow quartering (e) Stalking (f) Listening (g) Flushing from cover We summarise these techniques below and apply them to Peregrines. 1. Still hunting A raptor using this technique perches for a period of time at a suitable vantage point, like a tree, telephone pole or cliff, and searches the surrounding area for prey. This is the most common search technique used by Australian falcons and probably by all raptors. Fox (1977) notes three main criteria for a suitable still hunting perch: (i) a good all-round view of an area well populated with prey species (ii) concealment from prey by foliage or broken background, for example, prey would be unlikely to see a raptor perched between the prey and the sun 4 (iii) elevation so the raptor can slip off the perch into a glide attack without warning the prey with wingbeats. Peregrines in the ACT sometimes launch their attacks from trees but they also launch their attacks from a perch on their nest cliff or a cliff in their territory (pers. obs.). Peregrines perching on cliffs are more difficult to see than Peregrines perched in trees. 2. Fast contour-hugging flight The raptor flies rapidly a few metres above the ground or tree canopy searching for prey which is surprised by the sudden appearance of the raptor. Fast contour-hugging flight is a highly active form of searching that uses considerable energy compared to still hunting. Peregrines using fast contour hugging flight cover huge areas because they fly at speed in straight lines or in broad arcs. 3. Soaring and prospecting Raptors remain airborne at height circling and searching the area beneath them. This form of searching is particularly well developed in raptors searching for mice or invertebrates - in Australian falcons, the Australian Kestrel Falco cenchroides and Brown Falcon Falco berigora. These raptors may search an area, then swing into a hovering position which provides them with a stationary platform they use to carefully scrutinise the ground below them. This technique is difficult for Peregrines to use. Peregrines don't hover and bird prey is likely to freeze if they see a falcon above them. Mediated flushing agents are needed to frighten starlings or other birds into the air for Peregrines to capture and this creates particular problems for Peregrines hunting over heavily forested areas of Namadgi and Tidbinbilla. Prey must fly above the canopy or over open roads, clearings or dams and be caught before it can dive under the canopy or water where it is safe from Peregrine attack. 5 Often a soaring raptor is displaying to keep conspecifics out of its territory. It is, therefore, making itself conspicuous and it is unlikely to capture prey. Some observers incorrectly see this soaring as hunting (pers. obs.). 4. Slow quartering Raptors fly low over grassland searching and listening for prey. The ground is thoroughly examined for rodents, birds and/or insects by quartering back and forth over one area. This technique requires a buoyant, light wing loading as found in harriers, Barn Owls and other rodent eaters. Peregrines, with their heavier wing loading, can seldom use slow quartering efficiently. 5. Stalking Most falcons seldom hunt prey by stalking it on foot. However, Brown Falcons do hunt locusts on foot and commonly search for yabbies and other invertebrates. They also hunt snakes by stalking and flying as the snake strikes at the falcons out-stretched wings. Peregrines have shorter legs and are not known to use this technique. They seldom catch prey from the ground (pers. obs.). 6. Listening Owls, harriers and accipiters commonly listen for prey to locate it but falcons are rarely recorded using this technique. Fox (1977) believed that New Zealand Falcons listened for calls of birds then located them. There is no evidence that Peregrines use this technique. 7. Flushing from Cover Accipiters commonly enter cover like bushes or hunt for prey under tree canopies but falcons seldom do. Peregrines seldom enter cover and may have 6 difficulty hunting within the vegetation that characterises Namadgi and Tidbinbilla. ATTACK TECHNIQUES Fox (1977) lists four attack techniques: (a) Direct Flying Attack (b) Tail Chasing (c) Glide Attack (d) Stooping a) Direct flying attack The raptor flies directly at sighted prey. This is the technique most commonly used by Peregrines and they often make repeated direct flying attacks at passing flocks of parrots or pigeons. If they fail to capture quarry they often return to the perch and wait for another passing flock (Cade 1982). b) Tail chasing The raptor chases quarry for a long distance in an attempt to overhaul it. Peregrines use this technique to catch Galahs Cacatua roseicapilla, Feral Pigeons Columba livia, Common Starlings Sturnus vulgaris or other flock birds that fly in the open and can escape high into the sky instead of diving for cover (Olsen 1974; Cade 1982). c) Glide attack This attack is used against unsuspecting but potentially mobile prey. It is usually launched from a perch and is designed to get the Peregrine to the prey without the prey being forewarned. The falcon slips off the perch with wings partly folded and glides, without wingbeats, in a straight line towards the prey.