Verreauxs’ Eagle Aquila verreauxii at the Walter Sisulu National Botanical Garden, Roodepoort: Prospects for their long-term persistence

Verreauxs’ Eagle Aquila verreauxii has a distribution similar to that of the rock hyrax Procavia capensis and is widespread in southern Africa, being absent from the drier regions of the Kalahari, the plains of southern Mozambique and north-eastern KwaZulu-Natal and mountainous regions of Lesotho (Davies & Allan 1997; Simmons 2005). Populations are thus dictated by hyrax population fluctuations, with breeding suppressed in years of low hyrax numbers (Davies & Allan 1997).

Walter Sisulu National Botanical Garden, Roodepoort, has been a territory for a pair of Verreauxs’ Eagle for many years. The breeding pair is of iconic significance at the garden and they are a popular attraction for tourists and visitors. Because of urban development in the region, and resultant reduction in natural prey items in the vicinity of the nest site, there is concern over the persistence of these birds at the garden. Observations over 16 years (1993-2005) indicate that of 15 breeding attempts (no laying in one year) 11 successful fledgings occurred (BEPR unpubl. data). This consistency in breeding activity, with few irregularities, suggests that they are unlikely to vacate the area. However, there is concern that the effects of urban change in the region of the garden may lead to future pressure for food resources of breeding birds, and it is suggested that the pair be strategically monitored to assess and monitor their persistence at the garden.

It is not known whether the home range of the pair presently breeding in the Walter Sisulu National Botanical Garden at Roodepoort is marginal; and whether territorial hunting grounds are impacted on by increased human disturbance and development. Observations recording hunting success and records of prey returned to the nest site in breeding years suggest that habitat and hunting grounds have indeed become reduced in recent years, resulting in an increase in foraging range. Increased hunting distance then has implications on long-term breeding prospects and the viability of the territory in support a breeding pair. Previously, the pair has been supplemented with prey since hunting success is thought to have declined. However, the pair has been successful in returning with a variety of prey items. This prey diversification may, however, be a response to reductions in preferable prey items and it is suggested that prey diversification has enhanced their survival at the garden. Supplementation, however, was stopped in 2009 to investigate whether the eagles would be able to sustain themselves and a growing chick during their breeding cycle. Without feeding fledging of the nestling was successful suggesting there was sufficient food for the birds.

The breeding pair of Verreauxs’ Eagle at the Walter Sisulu National Botanical Garden offers the ideal opportunity to conduct research on the biology of an apex predator breeding in an urbanised and human-altered environment. Data collected on the biology of these birds are important for our understanding of complex ecological interactions present in our natural world. In addition, a greater understanding and knowledge is important for management decision-making processes, and the overall maintenance of the integrity of the ecological functioning of the garden. For many years volunteers have been involved in the collection of breeding and feeding data of the eagles. This has occurred on an ad hoc basis and has contributed somewhat to our understanding the biology of these birds. However, the value of observers has been underutilised, mainly because of the inefficient and misinformed manner in which data collection has occurred. By refining observation and data collection techniques we would like to propose a revision of the overall research objectives of the Black Eagle Project Roodekrans programme. This will involve a two tiered research focus on data collection through, 1) observations of breeding, feeding and behaviour, and 2) data collection on the breeding biology of these birds through active monitoring of the nestling each year. Each of these points is elaborated on in more detail below.

OBJECTIVES

It is proposed to conduct research in three-yearly intervals, with the intention of revising objectives and aims as new information becomes available. The objectives of the study during 2006-2008 have been revised and are now indicated as follows. Specifically we will attempt to address a number of overall questions that will contribute to our understanding of Verreauxs’ Eagles in general and in particular the current breeding pair at the garden.

1. What is the post-fledgling survival rate and how far do juveniles disperse from the natal area subsequent to fledging?

Data on the survival of fledgling Verreauxs’ Eagles are few; in addition very little is known of the survival and dispersal of fledglings from the breeding pair at Walter Sisulu National Botanical Garden or elsewhere. No more than 400 Verreauxs’ Eagles have been ringed in southern Africa (Oatley et al. 1998). Of those recovered, most have been rehabilitated and/or relocated birds (8 of 19), and few have survived longer than a year (Oatley et al. 1998). This exemplifies the lack of knowledge on this relatively common yet little known raptor species. Answering these questions is important in management decisions and maintaining the long-term persistence of this pair in the region. With the input of numerous raptor biologists, methods of tracking the fledgling have been investigated.

1.1 GPS tracking

In 2007 a GPS backpack-tracking device, similar to that used on vultures, was tested on an adult Verreauxs’ Eagle at the National Zoological Garden of South Africa in Pretoria (Transponder ID 00-0020-2E53; Ring number: G-18545). Fitting occurred on 26 March 2007 under the supervision of Sarah Chabangu (Bird Curator) and Dr Paul Bartels (expert veterinarian advisor). The device was removed on 26 April 2007 with no apparent harm to the bird. If successful, a similar device would then have be attached to a wild nestling Verreauxs’ Eagle just before fledging. After much consideration, despite the study on the zoo bird being successful, it was decided not to attach a similar backpack to a wild bird. It was felt that the device needed to be significantly lighter. Efforts are therefore ongoing in an attempt to source funding to attach a lighter solar powered satellite transmitter to a wild bird. Such devices now weigh approximately 1.5 g, are solar powered and provide real time data on bird movements (Kenward 2001). They have been used successfully for tracking smaller species such as the Eurasian Hobby Falco subbuteo (mass = c. 230 g; Strandberg et al. 2009) to the Red-footed Falcon F. vespertinus Linnaeus 1766 (mass = c. 135g).

1.2 Patagial Tags

Patagial Tags have been used for a number of years as a method for successfully tracking movements of vultures (and other raptors) (Botha 2007). An advantage of this technique is that a greater number of recoveries and resightings are recorded (when compared to ringing), providing valuable information on movements and vulture biology. This technique will therefore be employed to assist in monitoring the post-fledgling movements of juveniles recruited every year at the Walter Sisulu National Botanical Garden. In this instance, the fitting of patagial tags will be supervised and carried out by raptor biologists experienced in the process (e.g. Andre Botha, Endangered Wildlife Trust). In addition, we will draw on the expertise of researchers in Cape Town (i.e. Lucia Rodrigues, Dr Andrew Jenkins) who have, for the past few years, been successfully attaching patagial tags to Verreauxs’ Eagle nestlings in the Cape Town area (see Figure 1). This component of the project will be long-term since it will rely on re-sightings and the reporting of recovered rings or recaptured birds. A wide network of amateur birding enthusiasts in the field will record sightings. In addition, the nestling will be fitted with a metal ring (Safring G-ring; 26mm Incoloy) applied to the tarsus (tarsometatarsus). The application of uniquely marked metal rings has been applied for many years and is an ethically accepted technique used in the study of birds (See http://www.birdingafrica.com/verreauxs_eagles.htm for details of patagial tags on Verreauxs’ Eagle in Cape Town).

The outcome of this aspect of the project will be the accumulation of detailed data on the movements and post-fledgling period of breeding Verreauxs’ Eagle at the WSNBG.

2. What are the effects of urbanisation and development on the long-term prospects for the persistence of breeding Verreauxs’ Eagle at the Walter Sisulu National Botanical Garden?

This aspect of the project has been addressed by an MSc student (see details below under Progress) and involved summarising all feeding and breeding data collected by volunteers of the BEPR. This report will be made available on completion of the project.

1.1 Behavioural monitoring of WSNBG Verreauxs’ Eagles.

Information collected to date by the volunteers of the BEPR has been useful in understanding and monitoring the breeding Verreauxs’ Eagle at the WSNBG. However, for the information to be of sound scientific value the methods of data collection need to be refined. Plans are currently in progress to address this issue and a standard format data sheet and training programme with volunteers will occur in the near future.
This method of data collection has very little impact in disturbing the eagles on the nest, if any. As a result data collection will be ongoing. These data will be valuable in interpreting data collected on bird activity at the nest (i.e. interactions of parents with different sex nestlings, diet of different sex nestlings, behaviour of each parent towards different sex nestling etc).

METHODS

Nestling ringing will be carried out by an experienced climber and bird-handler (Luke Verburgt). For the application of the patagial tag the relevant expertise will be present to apply the tag (see details of procedures above under objectives; details of project participants to follow).
In addition, the project will operate through the input of a number of experts in their respective fields; members of the team being as follows (contact details of primary project members given in Appendix 1 below);
Craig Symes - University of the Witwatersrand (Academic advisor and project co-ordinator)
Libby Woodcock - Black Eagle Project Roodekrans (Walter Sisulu National Botanical Garden liaison and project co-ordination, public relations and fund raising)
Luke Verburgt – University of Pretoria (rock climbing expertise)
Andre Botha – Endangered Wildlife Trust, Birds of Prey Working Group (fitting of patagial tag)
Paul Bartels – Pretoria Zoological Gardens (Advisory and field veterinarian).

Additional experts have been and will be consulted further during the period of the project for additional professional advice:
Zephné Bernitz (Veterinarian and southern African raptor expert, Middelburg)
Ben Hoffman (Africa Calling – Raptor Breeding and Rehabilitation Centre, Pietermaritzburg)

RISK ASSESSMENT AND PROTOCOL

The high profile of the Verreauxs’ Eagles at Walter Sisulu National Botanical Garden is recognised. As such, all necessary precautions will be made in ensuring the welfare and well being of the Verreauxs’ Eagles at the garden.
The senior member of the ringing team is a qualified A-permit ringers (Safring) and has a successful record of handling and ringing numerous raptor species. Someone experienced in the process will conduct Patagial tag fitment; i.e. Andre Botha who has a vast amount of experience in fitting patagial tags to vultures.
Luke Verburgt is an experienced mountain climber and member of the Mountain Club of South Africa. He has experience in fitting vultures with rings and has during 2006-2008 accessed the nest at the WSNBG to fit the nestling with a ring. In Cape Town nestlings have been successfully removed from the nest for tagging before being returned. This technique has proved successful and no ill harm is received by the nestling. If a patagial tag is to be attached to the nestling we would envisage removing it from the nest for the process of attaching the tag.

MOTIVATION

Progress to date:

During 2006-2008 the nestling was successfully ringed at the garden. Information on breeding attempts in these years are summarised below. Each of these nestling was sexed, providing valuable long-term information on Verreauxs’ Eagle biology. Therefore, during the ringing and tagging procedure efforts will be made to collect a small blood sample (<50µl) for genetic sexing. This sample will also be housed at the Wildlife Biological Resource Centre - Biobank Africa (Director; Dr Paul Bartels). Knowing the sex of each nestling will provide valuable information on the breeding cycle of Verreauxs’ Eagle.

An MSc (coursework) student (Tamara Kruger) has recently completed a project entitled, “Long term prospects for the persistence of breeding Verreauxs’ Eagle (Aquila verreauxii) at the Walter Sisulu National Botanical Garden, Johannesburg”. This project investigates habitat changes within the vicinity of the nest site during the past three decades and summarizes breeding and feeding data of the breeding pair at the garden since 1993. This project is near completion and will provide valuable information for future decision making processes in the garden concerning the breeding Verreauxs’ Eagle. The outcome of this project will hopefully advise on future data collection techniques concerning breeding and feeding of Verreauxs’ Eagle at the garden.

Table 1. Verreauxs’ Eagle breeding data (2006-2008) (Libby Woodcock) 

SUMMARY

The overall aim of this project is to engage with concerned citizens on the wellbeing and persistence of Verreauxs’ Eagle at the Walter Sisulu National Botanical Garden, Johannesburg. Through the co-ordinated efforts of scientists and volunteers we hope to provide valuable information on Verreauxs’ Eagles that will assist in long-term decision making processes concerning development in the immediate vicinity of the garden. This liaison will engage all relevant stakeholders in formulating a working strategy for the greater understanding of Verreauxs’ Eagles at the WSNBG.

Craig Symes
School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, e-mail: craig.symes@wits.ac.za

REFERENCES
Botha, A. 1007. A review of colour-marking techniques used on vultures in southern Africa. Vulture News 56:                  52-63.

Davies, R.A.G. & Allan, D.G. 1997. Black Eagle Aquila verreauxii. Pp. 175-177. In: The atlas of southern African                 birds - Vol. 1.Non-passerines, (eds). Harrison, J.A., Allan, D.G., Underhill, L.G., Herremans, M., Tree,

A.J., Parker, V. & Brown, C.J. BirdLife South Africa, Johannesburg.

Kenward, R.E. 2001. A Manual of Wildlife Radio Tagging (Biological Techniques). Academic Press, London
.
Oatley, T.B., Oschadleus, H.D., Navarro, R.A. and Underhill, L.G. 1998. Review of ring recoveries of birds of prey                in southern Africa: 1948-1998. Endangered Wildlife Trust, Johannesburg.

Gargett, V. 1990. The Black Eagle. Acorn Books and Russell Friedman, Johannesburg
.
Simmons, R.E. 2005. Verreaux’s Eagle Aquila verreauxii. Pp. 531-532. In: Roberts’ - Birds of southern Africa.               7th edn. Hockey, P.A.R., Dean, W.R.J. and Ryan, P.G. (eds). Trustees of the John Voelcker Bird Book               Fund, Cape Town.

Strandberg, R., Klaassen, R.H.G., Hake, M., Olofsson, P. and Alerstam, T. 2009. Converging migration routes
            of Eurasian hobbies Falco subbuteo crossing the African equatorial rain forest. Proc. R. Soc. B 276:             727- 733.

APPENDIX 1. Contact details for significant research project participants.

Additional professional input.