At this time of year a large component of the flying fauna of southern Africa forsake their natal areas and the Austral winter to track the sun, travelling north into the equatorial fringes of the colossal Congo basin.
In the late afternoon of May 3 the grey rain-drenched skies across Arusha opened to allow warm sunlight to reach the soaking earth. I stole away from this infernal computer to climb a nearby hill that reaches southward, like a great green whale back, from Mount Meru almost into the centre of this burgeoning little city. Immediately it was apparent that far more mobile life forms were also responding to the sudden return of the sun. Velvet green, cobalt blue and wine red the so-called White-fronted Bee-eaters, dancing above the uppermost twigs of the tall trees on this shamba hill, were being especially vociferous. Swapping lofty perches with characteristic nasal laughing cries they were ambushing large orange and red butterflies, darter dragonflies and spider-hunting wasps that were passing over the hill in a north-easterly direction.
Looking up I wondered somewhat absently whether many of the butterflies might be migrating. Then other matters took my attention and I too drifted off. Some minutes later however, whilst walking down the sunny western side of a low cape-apple hedge, a Painted Lady butterfly flew up at my feet. My first in Arusha in months.
The last lovely spangled individual of this most cosmopolitan butterfly species was seen in December, though they had been frequent all around Arusha between June and November. This coincidence with the appearance of the Painted Lady butterfly in my native Europe, where the population is sustained each spring entirely by annual migration northward out of Africa, seemed potentially quite significant.
A few metres farther and another Painted Lady flew up from its basking place at the foot of the hedge. It chased around with the first for a few moments before both returned to their respective sunning spots. In a very small area on this western evening edge of the hill there was in total five Painted Ladys. I thought back to the bee-eaters, and then to an African Migrant butterfly, the first that has ever entered our house, on this very afternoon. The pieces surely matched. Several insect species were actively migrating northward.
At least two of the bee-eaters’ butterfly victims, and several of the other butterflies that were passing overhead unscathed, were also known migrants, either Tiger-Monarchs (of the Danaidae family) or the famous Monarch-mimic of the Nymphalidae the Danaid Egg-Fly. The Danaid butterflies are particularly foul-tasting, manufacturing toxic compounds from their food plants whilst living in the caterpillar realm. Can bee-eaters digest such toxic materials, carefully avoided by most predators, or do they learn to visually separate the flying mimic from the flying model, quite an achievement if true. And are the marvellous rainbow colours of the bee-eater family some kind of acknowledgement that they can do just that?
In the same area as the butterflies I watched a basking hoverfly, Eristalinus taeniops, the “band-eyed drone” a honey bee mimic widespread in the Old World tropics, that may be poised to reach northern Europe from outposts in Iberia. I speculated whether this wee scrap of life sitting on a Cordia leaf in the evening sun, was also a migrant. Possibly it too was moving equatorwards on this very day, once a rat-tailed maggot from some rain puddle in the southern tropic of Mozambique, whence it had first emerged to track the sun, somehow sensing the intoxicating air of freedom!
By May the bigger intercontinental fare – the Eurasian bird migrants - may have all but gone from the latitude of Tanzania. Nevertheless for those nature lovers who can get out and about, even within a city, the spell-binding wonder of visible migration thankfully continues.