Pollen: a useful ornithological research tool?

At the turn of the last century, renowned American ornithologist Jed Burtt published an opinion piece in The Auk entitled Think Small, where he encouraged ornithologists to “adjust their investigative scale downwards”. In the 25 years since Burtt’s call to action, research into avian microbiology has increased, molecular techniques are routinely used to study taxonomy and phylogeny, and chemical analysis of bird feathers is revealing information on diet, habitat, and migration. However, there is another way in which ornithologists could benefit from “thinking small” – increasing the incorporation of pollen analysis into avian research. 

Pollen grains are tiny reproductive cells from seed-producing plants, measuring around 30 microns. They are protected from degradation by a tough outer layer known as the exine, which is sculptured in ways that allow grains to be identified, at least to Family or Genus, using a high-power microscope.

Figure 1. Examples of pollen grains that can be found associated with birds.

Traditionally, pollen analysis has been used principally to study past vegetation, climate, or environmental change, as well as within in forensic investigations. Recently, the potential to use pollen analysis within contemporary research or to support ecological monitoring, management and conservation has been recognised, but there has been no review of past insights and future possibilities specifically within ornithology. A careful review of literature of the last 75 years across all seven continents has demonstrated that pollen analysis is spatially widespread within ornithology and increasing over time.

Figure 2. Temporal trend in publication of bird-pollen research from 1950 to 2023.

Habitat use

Pollen found on birds, particularly on feathers, can be a useful way of determining habitat use. For example, Domínguez-Vázquez et al. (2021) assessed habitat use by the Black-polled Yellowthroat (Geothlypis speciosa) around Lake Cuitzeo, Mexico using pollen. They found birds were more terrestrial than had been expected for this water-associated species, suggesting regular movement away from the lakeside. Pollen profiles also differed between males and females, indicating spatial resource partitioning between the sexes. Analysis of fossil evidence can also allow the habitat used by extinct birds to be profiled. For example, analysing pollen associated with fossilised avian footprints of ancient Eocene birds on King George Island, Antarctica, allowed the vegetation of that location during the Eocene to be reconstructed, thus providing information on the habitat use of these ancient avian species (Poole et al., 2005; Mansilla et al., 2014). 

Migration

Avian migration is hard to study. Ringing recoveries are often so low that very little information can be elucidated and tracking devices are not always appropriate, especially for small species. Pollen on feathers of migratory birds, or within encrustations on the bill, can be a useful research tool. For example, Wood et al. (2014) for the short-distance migrants (Blackcap Sylvia atricapilla and Chiffchaff Phylloscopus collybita) returning to the UK in spring carried more Eucalyptus pollen and less Citrus pollen than sub-Saharan migrants (Garden Warbler Sylvia borin and Willow Warbler Phylloscopus trochilus), thus providing insights into differences in stopover habitats for these species.  

Diet

Pollen in faeces or pellets can be used to infer diet. This can involve simple baseline profiling or assessing change in diet for the same species across space and time. For example, Moe and Bjune (2009) identified pollen in faeces of Willow Grouse (Lagopus lagopus) in Norway to quantify seasonal shifts in diet, finding that trees were important immediately after snow-melt, before birds switched to use herbs as spring progressed. In an entirely opposite strategy, Austral Parrakeets (Enicognathus ferrugineus) in Argentina depend to such an extent on Nothafagus pollen as a protein source that they track seasonal flowering by moving altitudinally, thereby keeping their diet temporally consistent (Díaz and Kitzberger, 2006). 

Population dynamics

In the absence of direct data from bird censuses or ringing, there are very few proxies that enable modelling of past populations. Analysis of pollen retrieved from sediments associated with avian deposits has the potential to assist in understanding avian population dynamics, including long-term change. For example, on the Galapagos island of Genovesa, Conroy et al. (2015) used increases in Red Mangrove (Rhizophora mangle) pollen within sediment (linked to nutrient enrichment by guano) as a proxy for the number of breeding Red-footed Booby (Sula sula). This allowed change in seabird population dynamics to be modelled over a 400-year period. 

In conclusion, there are numerous examples in the literature of avian research insights that have been unlooked by pollen analysis, but also considerable untapped potential. In particular, the advances that would be afforded by identifying bird-associated pollen to species level using DNA metabarcoding are especially exciting, as this would allow pollen to be identified to species level. In this way, use of pollen as a proxy for habitat use, migratory routes, or diet, would become more detailed and thus more useful in both research and applied contexts. Hopefully more ornithologists will be inspired to include pollen analysis when they follow Burtt’s (1999) advice to “Think Small” and thus unlock new insights into avian biology.