“Here comes the sun”, over zonnebadende vogels

Sunbathing in birds is a well described, but still mysterious and rarely quantified phenomenon. Sunbathing in birds is not an easy target to study since it is observed occasionally and often unexpected. During 1998-2005 I made notes about sunbathing birds I met by chance. If possible information about species, date, time, duration and posture of the bird was sampled. Sunbathing was observed exclusively during moments of sharp sunshine from March through August with a peak occurrence in July (Fig. 1). The species most observed was Blackbird (Table 1), but this figure may be strongly biased by the relative abundance and tameness of the species. Most observed postures are lateral sunning and spread wing postures. However the sample size for other species than Blackbird are small, larger birds were only observed to sun with spread wings and not laterally. In Balckbirds a sunbath took on average 264 seconds (5-1263). Duration in the course of the year followed frequency (Fig. 1 and 2) with July being the month with the longest sunbaths. A preference existed to sunbath in the morning from 9-11 o'clock (Fig. 3) and on days with high temperatures (Table 2). Days with a maximum temperature of more than 30°C are rare in Holland, but represent 21% of days on which sunbathing was observed. Often sunbathing occurred simultaneously in birds that were within visual contact. So it might be that seeing a sunning bird may provoke others to do so. On the other hand also birds were sunning simultaneously without visual contact which indicates that conditions may have been suitable that day. It is unclear why birds are sunning. Mentioned are factors like production of vitamine D, thermoregulation, getting rid of parasites, feather maintenance or simply for pleasure. None of these factors is fully satisfactory as an explanation. It is however without doubt that birds like to sunbath.