Countless connections in complex communities. Bird–arthropod food webs for boreal passerines unravelled via faecal metabarcoding

Finding out the exact food that insect-eating birds provide to their young has long been challenging. Finland is home to tens of thousands of insect species, many of which are small or can look similar. However, different insects may be important to different birds: they may be available at different times, only be found in specific places, or provide particular nutrients. This dissertation uses DNA-based methods to identify the eaten prey from the poo of nestlings of five songbirds: the crested tit (Lophophanes cristatus), willow tit (Poecile montanus), blue tit (Cyanistes caeruleus), great tit (Parus major), and pied flycatcher (Ficedula hypoleuca). The detail made possible by DNA revealed that the birds' diets covered hundreds of insect species, with relevant differences within and between bird species, as well as between years. This information is about more than just these birds however. Since both the insects’ availability and the birds’ breeding times are highly seasonal and dependent on temperatures, knowing the links and mechanisms in this food web is key to understanding the variable impacts of climate change on different species.

The various experiments described in this dissertation show the benefits of the method’s speed, resolution, and sensitivity, using samples that can be easily and harmlessly collected. However, the results also highlight that caution is needed when interpreting the findings from indirect observations, as small amounts of DNA can easily end up in the samples. Still, in a rapidly changing world, DNA-based methods are increasingly important for practically identifying and detecting species, as well as understanding the numerous interactions they may be involved in.