Avian biodiversity reaches its most spectacular levels in forested habitats at tropical latitudes. Often perceived as climatically benign and among the least challenging of habitats for birds to occupy (compared, for example, to deserts), life in tropical and subtropical forests can in fact be physiologically demanding. High air temperatures combined with oppressive humidity create stifling conditions, as anyone who has tramped through the forests of northern KwaZulu-Natal in 38-degree heat on a humid day can confirm. For birds living in open habitats along forest edges, these conditions are often combined with intense solar radiation while foraging.
The physiological challenges of very hot, humid weather arise from the fact that in these conditions birds effectively lose their primary defence against heat stress: evaporative heat evaporatively cooling. Dissipating through behaviours such as panting can take place only if the surrounding air is dry enough to absorb water vapour expelled by the bird. During periods of very high humidity, the gradient required for birds to offload heat is no longer there. Instead, the heat produced by activity or gained from the environment is retained in the birds' tissues, causing the body temperature to increase.
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