Urbanisation - light pollution vs urban heat island

Light pollution and the urban heat island are both associated with urbanisation. Since blogging about these I've been wondering how studies can be sure they're really seeing the effect of one or the other on phenology.

During winter birds need to get up early to start feeding, as they use their energy up overnight, and need to eat or they'll die of starvation. Ockendon et al. (2009) separated the urban heat island from effects of light pollution by hypothesis testing. In urban areas, they could get up later if it's warmer because they won't have used as much energy up over night staying warm, or they could get up earlier and start feeding sooner because there are artificial sources of light. They found birds appearing at garden feeders later in the a.m. in urban areas in Britain than in rural ones - pointing towards the urban heat island. They also found a statistically significant difference between the response of species, something you might expect if you've been following this blog. They thought the Eurasian collared dove might be getting up to feed earlier because of competition from feral pigeons (Fig. 1).

Fig. 1.

Time between first light and a bird being seen at a feeder (time relative to start; TRS) for the ten most commonly seen species. From Ockendon et al. (2009)

They used modelling to show that urbanisation, defined as the percentage of the surrounding 1 km square classified as urban/suburban, is statistically significant as a predictor variable for time of first bird arrival. However, they did not separate the effects of light pollution and temperature...