Cranes and crops: investigating the viability of blue cranes in agricultural lands of the Western Cape
The Western Cape population of Blue Cranes (Anthropoides paradiseus) is of great importance as the largest and most stable population throughout its range. This species is strongly associated with agricultural lands in the Western Cape, and therefore may come into conflict with farmers who perceive them as damaging to crops. Blue Cranes are suspected to be locally nomadic, but little information has been collated to support this and they are also relatively understudied in terms of demographic parameters. This project investigates the viability of the Blue Crane population in three ways: exploring farmer attitudes towards cranes in two regions of the Western Cape (Swartland and Overberg) using 40 semi-structured interviews, generating estimates of survival using Mark-Recapture methods and exploring movement patterns using a long-term data set of resightings of marked individuals. These three components all add important aspects to the overarch ing goal of achieving a better understanding of threats to Blue Cranes in the Western Cape, and thus the population's long-term viability. Perceptions of cranes differed widely between regions: farmers in the Swartland perceived cranes to be particularly damaging to the feed crop sweet lupin (65% of farmers reported some level of damage by cranes), and 40% of these farmers perceived cranes as more problematic than other common bird pests. Farmers in the Overberg did not perceive cranes as highly damaging, although there was concern about cranes eating feed at sheep troughs. Survival was age-structured: individuals in their first year had a survival of 0.6, those in their second and third years that of 0.87 and adult individuals (4+) that of 0.72. The adult survival estimate is suspected to be underestimated due to ring loss. Resightings of colour-ringed cranes suggest that movements in the Western Cape were localized, with an average displacement of 24.6 km from their natal point. Only 3.8% of marked individuals were resighted in both the Overberg and the Swartland regions, indicating that movement between these regions was low and regional fidelity was high. There was significant movement within the Overberg however, and 90% of movements of >10 km were made within this region. This species therefore appears to be resident to locally nomadic in nature. Evidence for natal philopatry was also found: 57% of adults returned at least once to the area where they were ringed as juveniles. These results highlight the need for location-specific management solutions to crop-damage by cranes, and contribute to the understanding of basic demographics for this vulnerable species.
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Percy FitzPatrick Institute of African Ornithology