Urbanisation (again)

I found a study by Dominoni et al. (2013) which looked at the reproductive phenology of blackbirds (Turdus merula) (Fig. 1) and how this is affected by artificial light. They found that birds affected by night light developed their reproductive system and moulted earlier than those which were not exposed to light. I was interested in this because they used experimental conditions to determine the effect of artificial light, so we can be sure that it wasn't temperature having the effect.

Fig. 1. Male blackbird. Source: http://shropshirebirder.co.uk/blackbird.html

I was thinking there are probably many unaccounted for factors in studies the effects of urbanisation on phenology. Wherever there is a city there will be increased temperatures, light pollution, sources of food, pollution... I'm struggling to find papers that actually separate these effects.

For instance I found this paper by Schoech and Bowman (2008) looking at the reproductive phenology of Florida Scrub-Jays (Aphelocoma coerulescens) (Fig. 2). Those birds from suburban habitats breed sooner in the year. I wrote about how this is makes animals more competitive in a previous post. The authors measured plasma levels of protein, etc. in birds and found it to be higher in the suburban birds vs the rural birds. They inferred from this that earlier breeding is caused by the availability of protein. I think it's good evidence for differences in diet between the two populations. And, they've collected data showing that breeding phenology varies between the two populations. Their results are consistent with their hypothesis that food availability would cause earlier breeding. But, I don't think that necessarily proves the connection because any field study like this will be too confounded.

Fig. 2. Florida Scrub-Jay. Such a beautiful bird! Source: http://barbrichphotography.blogspot.co.uk/2011/05/backyard-birding.html

In fact, the authors found that leutanizing hormone was also elevated in birds. Leutanising hormone is part of the reproductive physiology of birds, the cycle of which is connected to day length (Sharp et al. 1998). Maybe light pollution could have something to do with it, too?