BTO Shortest Day Survey

Long-standing Garden BirdWatchers may recall the Shortest Day Survey that we carried out in December 2004, in association with The Today Programme on BBC Radio 4. We reported the general findings at the time but since then we have carried out a more detailed examination of the observations. The two research papers detailing our findings have just been published, one in the online early version of Bird Study and one in the online early version of the Journal of Ornithology. The findings are summarised below:

Ockendon, Davis, Toms & Mukherjee (2009). Eye size and the time of arrival at garden feeding stations in winter. Journal of Ornithology. CLICK HERE TO ACCESS THIS PAPER

The work, led by the BTO’s Nancy Ockendon, has revealed that birds with larger eyes (relative to their body size) start arriving at garden feeding stations sooner after sunrise than birds with smaller eyes. This suggests that the time at which garden birds begin to forage on winter mornings may be limited by their visual capabilities. It has always been assumed that small birds need to start feeding as soon as possible after sunrise in order to replenish energy reserves depleted overnight. Our findings are important, therefore, because they suggest that such birds may not be able to begin feeding as early as they would like; instead they are held back by not being able to see well-enough to start feeding. Birds which have large eyes relative to their body size, such as Robin, can start feeding earlier and are among the first to arrive at feeding stations.

Ockendon, Davis, Miyar & Toms (2009). Urbanization and time of arrival of common birds at garden feeding stations. Bird Study. CLICK HERE TO ACCESS THIS PAPER

The seemingly more surprising finding was that there were clear differences in arrival times between urban and rural populations. Why should urban populations of a particular species arrive later after dawn than their rural counterparts? Interestingly, there are a number of factors which may operate differently between the two habitats. Urban areas are well known for the degree of light pollution associated with them, in the form of street and security lighting. If this light pollution was influencing the urban populations then you would expect to see earlier emergence in urban areas rather than later emergence. Another factor operating in urban areas is heat pollution, with waste heat escaping from factories, residential properties and other buildings. This waste heat can increase the temperature in urban areas by as much as 8°C, particularly in our larger cities, creating what is known as an ‘urban heat island effect’. With higher overnight temperatures in urban areas, it seems likely that small birds roosting in these areas would need to use a smaller amount of their fat reserves than would be the case for birds roosting within the wider countryside. With fewer reserves lost overnight, urban populations could afford to be more leisurely come the morning, the need to replenish reserves less urgent than for birds that had spent a colder night elsewhere.