Leaded versus Unleaded Doves

The doves flew in fast, aiming their flight pattern at the edge of the waterhole. Puffs of powder-fine dust marked the landings, each bird hurriedly dipping its beak into the cool, thirst-quenching liquid. Just as speedily as they arrived, their take-off was hasty, almost frenzied as they headed for the safety of the surrounding bush, flying fast and low to avoid predators. Watching this spectacle of drink and flee, my guest, an ornithologist of international standing, began musing about what we could learn from these birds. I listened as he advanced his hypothesis.It was based on his findings that adult doves in Cape Town, South Africa contain on average seven times more lead in their body tissues than the same species living in rural farmland only 50 kilometres from the city. These significantly higher concentrations were believed to reflect a relatively higher degree of lead in the urban air, presumably as a result of motor vehicle exhaust emissions. The rate at which doves accumulate this insidious poison in their bodies was not known however. The Professor continued ….. “if we catch doves in Etosha, keeping half of them here and half in Cape Town, we may be able to measure the build-up of lead in their muscles respectively”. His reasoning was logical and most original I thought. The method we ultimately agreed on was far from simple!

It involved trapping a large number of these fast flyers by means of nets suspended from poles near to the water’s edge. Known to ornithologists as ‘mist nets’, their fine mesh effectively camouflages them from approaching birds. I had used this method successfully in the Namib Desert to mark various bird species with numbered metal leg bands. The Etosha capture would be different – vastly different! In the Namib the likelihood of encountering potentially dangerous animals at a waterhole was as remote as the desert itself. Also, in the desert, I had hidden the nets amongst vegetation in a dry riverbed. At Etosha, the possibility of a hazardous situation developing when nets were placed close to a waterhole was very real. Elephants come to quench their massive thirsts at a pace that exceeds any scientist’s ability to furl the nets. Moreover, lions approach to drink on silent pads that don’t betray their presence until the feline form emerges from the undergrowth surrounding the drinking place. My proposal to counter this was to set the nets around a waterhole located on open ground, giving me time to remove them from the path of approaching big game. The Professor was understandably not concerned with these finer details. He wanted young doves that were growing and therefore developing their powerful breast muscles. This emergent tissue would be the ideal medium in which lead was able to deposit. The manner whereby I provided the doves was secondary.

My increasing concern for the consequences that this capture held for me was temporarily interrupted by an exclamation of excitement from the Professor who had spotted a Lanner Falcon mounting its attack on the drinking doves. Coming in high, with the Sun behind to obscure its approach, the raptor stooped on its unsuspecting prey, wingtips folded against its tail, reaching a velocity in excess of 200 kilometres an hour. Too late! The dove’s sensed danger, scrambling frantically into the air as the Lanner’s talons hit an unfortunate bird, killing it instantly in an explosion of feathers. The Professor’s words for this contact were “the falcon is binding with its prey”.  We had witnessed ancient aerial acrobatics of the highest order. Following this thrill the learned Professor’s enthusiasm knew no bounds. We, in reality meaning “I”, would provide the doves, whereafter they were to be divided into two groups and housed at Okaukuejo Rest Camp in Etosha and in Cape Town. The air they subsequently breathed would carry with it any traces of lead, depositing the heavy metal in their growing bodies.

With permission obtained to trap a total of 60 doves in Etosha and keep them captive, I set up mist nets at a waterhole that was away from tourist routes. My hopes that doves would not detect the finely meshed nets in the open were dashed. The slightest breeze caused the nets to billow, warning alert drinkers to divert and land safely away from my trap. Clearly this was not going to produce 60 captive doves in the time required. With rising apprehension I relocated my equipment to a waterhole surrounded by fairly dense vegetation. Then I sat and waited for the doves (and imagined elephants and lions) to come. Birds flew into the nets in numbers that I could barely handle. I cursed and removed everything from protesting Redeyed Bulbuls to irritated Pied Barbets that nipped me painfully with pincer-like beaks. Also many doves enmeshed themselves, but my catch had to be limited to juveniles whose age was estimated according to the state of moult in their primary wing feathers. The Professor required very young birds that had fledged four to seven weeks previously. It took many hundreds of doves and two months of tense waiting to secure the catch before the experiment could begin. Thirty young doves were kept in an aviary in my Okaukuejo garden and the remaining 30 were transported to Cape Town where they started to breathe the air from the city in a cage at the university’s campus. Both cages were large enough to permit the doves to fly and so exercise their flight muscles. Subsequently, on a given date at two-monthly intervals, I euthanised three doves, taken randomly, at Okaukuejo and the Professor did the same in Cape Town. These sacrificial birds were assayed for lead, using atomic absorption spectrophotometry, a method that entailed dissecting the muscle tissue whilst immersed under distilled water to prevent any possible contamination by lead in the laboratory.

The fact that we reached this stage proves I had survived the dove catching. Were there any dramatic developments at the thickly vegetated waterhole? The only memorable one was when a warthog enmeshed itself in my nets, quickly destroying one section with indignant squeals and snorts before it trotted off in apparent triumph, filaments of netting trailing from its muscular body. No elephant herd attacked my ambush for doves and no lions encircled me whilst I was engrossed in removing netted birds.

The most alarming result came from the doves kept in Cape Town. Their pectoral muscles, which power flight, had accumulated from two to five times more lead concentration in the space of a year than their counterparts housed at Okaukuejo. Lead is toxic and is a ubiquitous element in modern societies, occurring in practically all living organisms that inhabit industrialized areas, including humans. Given the virtual absence of “heavy” industry in Cape Town at that time, it is likely that the source of lead came primarily from automobile exhausts. The analyses were sensitive enough to detect traces of lead, albeit minimal, in the doves kept at Okaukuejo.  Thus, even the relatively small amount of traffic in Etosha during the 1970s, when this experiment took place, left its legacy in the wildlife. Traffic in both Etosha and Cape Town has increased by several magnitudes during the ensuing 30 years, with concomitant increases in fuel consumption. Nevertheless, the increasing introduction of unleaded petrol since then bodes well for our health. Ultimately, elimination of leaded fuel may make both doves and people breathe more easily.

Text by Dr. Hu Berry
Published in “Flamingo”, September 2005

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