Potential use of bird tables and feeders to monitor wild bird populations and their behaviour

Hugh D. Loxdale, FLS, MBE 

In a recent article (Loxdale, 2022) published in the June issue of the Linnean Society of London’s house journal, The Linnean, I suggested that bird feeding stations (BFS’s) may be used in a broadly analogous manner to insect traps. Such traps are widely used to collect insects, especially using light (usually with tungsten filament or ultraviolet light bulbs), and especially night flying moths (Fry & Waring, 2001; Harrington, 2014; Fox, 2021). The Rothamsted Insect Survey, based at Rothamsted Research in Harpenden, Hertfordshire, UK, involves a nationwide network of tungsten light traps (Williams’ design; Williams, 1948) and has now been operating for many decades (Harrington, 2014). As such, it has provided important long term information on spatial and temporal dynamic changes in UK moth populations, including in relation to changes in agricultural practices (e.g. intensification) and climate change (Woiwod & Harrington, 1994; Bell et al., 2019; Fox, 2021). The network is supported by other scientific organisations or amateurs, the samples collected in each trap being regularly sent to Rothamsted, where the moths (as well as some other insects of agricultural or medical importance) are identified and the data so gained collated and analysed.

In this light, it may be possible to use BFS’s in a similar manner in order to acquire long-term data on bird populations, certainly concerning those species that are behaviourally-inclined to visit these feeders. As mentioned in my article, of the 38 species of bird seen at our bird table and associated feeders (several, including Niger seeds, mixed seeds, peanuts and fat balls) in my wife’s and my garden in North Devon, 17 species (~ 45%) were found over a period of 6 months (late June 2021 to mid January, 2022) to have visited the station. Interestingly, no more species have been seen than the original 17. This strongly suggests that, as with moth traps with their radius of attraction, BFS’s also have their radius of attraction. In the case of moth traps, this is around 30-50 metres (Bell et al., 2019). With birds visiting BFS’s, it may be more of course, perhaps 500 metres or so.

Most of the birds seen were either seed eaters or, like tits, omnivorous, with a diet comprising both insects and seeds. Clearly, different types of seeds plus fat balls tends to attract different bird species, but on lumping all the data collected together, an overall tally can be produced, from which a species richness curve can be produced, i.e. a graph showing the number of species as a function of time (Scheiner et al., 2022). Of the birds visiting the feeders and recorded, most will be local resident species, but occasionally, as my wife and I found, a migrant may appear, in our case, the North American House Finch, Haemorhous mexicanus, which visited our feeders for quite a few months prior to disappearing in October 2021 (Loxdale, 2022).

Compared to the entire UK avian fauna (McInerny et al., 2018), our sample from the edge of Exmoor only represents a small percentage of the total (~ 6%), and certain birds with specific feeding preferences (e.g. marine/aquatic species) are unlikely to visit and even if they did wish to (i.e. Herring gulls), are too big to enter the roofed-bird table or hang from the different feeders. Having said that, Jackdaws (Coloeus monedula) are adept at hanging on the feeders containing fat balls and peanuts and hence we had to eventually buy feeders with guards so that only the smaller bird species could feed. Another interesting point is that some species, such as the Bullfinch (Pyrrhula pyrrhula), which we regularly saw in our garden and the surrounding area, never visited our feeders, but I have seen photographs on the world-wide-web showing that they will indeed do so, or some individuals will, presumably by watching other birds like Chaffinches (Fringilla coelebs) and Goldfinches (Carduelis carduelis) and following their example. That we haven’t seen such behaviour suggests that it has indeed to be learnt by example and presently, is not yet that common in our neck of the woods. Perhaps if one individuals starts to feed thus, others will follow.

One can imagine that if a network of uniform BFS’s were established nationwide and people were prepared to record the birds visiting everyday over – say – a 6 month period or longer, then useful data on long-term trends in a range of predominantly seed eating birds could be gathered. This could be presented graphically as pie diagrams or histograms and statistics performed on the data to determine if traps vary significantly in terms of biodiversity from one another over a latitudinal or longitudinal transect per given period of time. Such data would provide direct empirical evidence of dynamic trends in British bird populations, certainly for those species that regularly visit such stations. Some surprises in terms of new species might be observed and maybe new trends in bird behaviour would be seen too....as with the Bullfinch and its slow acceptance of bird feeders as a means of getting a free meal.

The RSPB Big Garden Birdwatch provides a ‘broad-brush’ assessment of bird population trends over time, as does the BTO’s surveys, but with bird tables and feeders, a more precise ‘fine-grained’ indication of what our common garden birds do and how their dynamics change with time, especially the seasons (i.e. in relation to the establishment and relinquishing of territories during the breeding season) may be ascertained. For example, do the young birds readily copy the example of their parents and older siblings from previous breeding cycles or seasons....and if so, how quickly do they learn? This would provide novel information of bird behaviour, more especially their learning abilities, and hence, indirectly, their adaptation and ultimately, evolution. All such accumulated data is of considerable importance in monitoring biodiversity in relation to conservation programmes and modern day concerns about anthropogenic-induced habitat and climate change.

References

Bell, J. R., Botham, M. S., Henrys, P. A., Leech, D. I., Pearce-Higgins, J. W., Shortall C. R., Brereton, T. M., Pickup, J. & Thackeray, S. J. (2019). Spatial and habitat variation in aphid, butterfly, moth and bird phenologies over the last half century. Global Change Biology 25: 1982–1994. doi: 10.1111/gcb.14592.

Fox, R. (2021). Moths on the move. The Biologist 68 (3): 22-25.

Fry, R. & Waring, P. A. (2001). Guide to Moth Traps and Their Use (2nd ed.). London: Amateur Entomologists’ Society.

Harrington, R. (2014). The Rothamsted Insect Survey strikes gold. Antenna 38 (3): 159–166.

Loxdale, H.D. (2022). Insights from watching wild birds at bird feeders: biodiversity, behaviour and warning colouration. The Linnean 38 (1), June issue: 26-33.

McInerny, C. J., Musgrove, A. J., Stoddart, A., Harrop, A. H. J., Dudley, S. P. & the British Ornithologists’ Union Records Committee (BOURC) (2018). A Checklist of Birds of Britain (9th ed.). Ibis 160: 190–240. doi:10.1111/ibi.12536.

Scheiner, S. M., Cox, S. B., Willig, M., Mittelbach, G. G., Osenberg, C. & Kaspari, M. (2000). Species richness, species-area curves and Simpson's paradox. Evolutionary Ecology Research 2: 791– 802. https://lter.kbs.msu.edu/pub/2669

Williams, C. B. (1948). The Rothamsted light trap. Proceedings of the Royal Entomological Society of London A 23: 80–85. doi.org/10.1111/j.1365-3032.1940.tb00593.x

Woiwod, I.P. & Harrington, R. (1994) Flying in the face of change: The Rothamsted Insect Survey. In long-term experiments in agricultural and ecological sciences (Leigh, R. & Johnston, A., eds.), CAB International: Wallingford, UK, pp. 321-342.