The influence of mist and rain on the reproductive success of the Blue Swallow Hirundo atrocaerulea (1995-96)

Due to the locality of the Blue Swallow Natural Heritage Site it appears that the birds have no alternative but to forage in the mist. The birds cease foraging as soon as it starts raining. Eight eggs failed to hatch after a week in which rainfall occurred for 60% and mist for 85% of the time. Later in the season a further six eggs failed to hatch after a week in which rainfall occurred for 18% and mist occurred for 65% of the time. The only mortality of two nestling Blue Swallows occurred at the end of a week in which rainfall occurred for 65% and mist was recorded for 85% of the time. At a nest containing three eggs, during the presence of mist (16 degrees C wet / 19 degrees C dry) the female Blue Swallow spent a mean of 52 min h(-1) foraging and therefore only a mean of 8 min h(-1) incubating. In the absence of mist (21 degrees C wet / 26 degrees C dry) the female Blue Swallow spent a mean of 25 min h(-1) foraging and therefore 35 min h(-1) incubating. Rainfall did not occur during the monitoring of this nest. It appears that the mist greatly reduces the foraging efficiency of the birds and that rainfall duration impacts on the birds by limiting the available foraging time.