Darker Cape residents live in shadier neighbourhoods‚ say scientists

Black sparrowhawks of a darker hue prefer shadier areas of the Cape Peninsula‚ scientists have discovered. They also have different eating times‚ preferring to dine when the weather is cloudy.

The findings emerge in a paper by two experts at the University of Cape Town’s FitzPatrick Institute of African Ornithology‚ who studied the behaviour of six male black sparrowhawks over three April-October breeding seasons.

Solar-powered GPS loggers costing around R18‚000 each were harnessed to three birds with white plumage and another three with dark plumage. Data from the 14g devices showed that the dark birds reduced their hunting activities as the light level increased.

Writing in Nature‚ Gareth Tate and Arjun Amar said their findings may help to explain why three-quarters of black sparrowhawks in the Cape are dark compared with less than 20% in the north-east of the country. “This variation is most closely correlated with ambient light levels during the winter period.”

The scientists say the camouflage provided by darker plumage also affected the birds’ choice of habitat. The darker birds “displayed a significantly higher probability of using habitats with denser tree canopy cover”.

In common with nearly one in three raptor species‚ black sparrowhawks are “colour polymorphic”‚ which means there is variation within a population. They attack prey — mainly doves in the Cape — from above after short ambush flights‚ and Tate and Amar said the camouflage offered by dark winter clouds was probably a key factor influencing the Cape population.

They also found that mixed pairs — a dark male and a white female — produced more offspring than pairs of the same colour‚ and said this may be because the parents’ ability to hunt under different light conditions meant chicks received more food.

The ornithologists said they assumed that when the tagged males were away from their core territories they were hunting. “However‚ we recognise that they may have also been ... avoiding begging females and chicks.”