An all-new, in-depth look into the diet of a familiar face: the blue tit

Worldwide, there are many species of small birds that live in trees and mainly eat invertebrates. However, finding out exactly what invertebrates they are eating has always been tricky and is not well known, even for very common and well-studied species like the Blue Tit, which cheerfully hops around the gardens of millions of people throughout Europe. The reason for this is that the birds themselves are small, their prey is even smaller and difficult to identify in the field, and the predator-prey interaction is often happening very fast and high in the canopy, making observation almost impossible.

Our study set out to find out what blue tits were eating early in the spring when branches are bare. Photo credit to Gergana Daskalova

So, how has identification of prey been achieved in the past? For static and accessible chicks in nestboxes this has proven easier, with the use of neck collars and cameras in the nest giving good coverage. For adults, the only way has been euthanising the birds and dissecting the gut and gizzard contents, and this is not ideal – it is very destructive, excludes soft bodied dietary items from being observed and only allows identification to a low resolution. In this study, we show how large-scale faecal metabarcoding (genetically identifying prey contained within faecal samples) can overcome these challenges, identifying prey to excellent resolution across time and space in a completely non-invasive and non-destructive way. In this post, I am going to discuss the biological and natural history advances that this study has provided.

Firstly, it is interesting to note how diverse the diet of Scottish blue tits is in early spring. We identified over 400 different invertebrate prey items from nearly 800 samples at a time of year when invertebrate resources are low because leaves are not yet out. The fact that most of these were rare, with almost half only recorded from a single sample each, suggests that blue tits have a wide dietary range and are opportunistically taking advantage of many food resources as and when they find them rather than solely focussing on particular items. This is supported by there being an average of five, and up to twenty, different prey items per sample. Some prey items were favoured, with 15 species present in over fifty samples each, the most common of which (the micromoth Argyresthia goedartella) present in over a third of samples. Most of these common species, including the caterpillar of this species, were associated with resources available in early spring, such as birch catkins, but I will return to this later. Moths in general were both the commonest and most species rich prey items, with at least one of over 100 identified species present in three quarters of samples. Other common prey orders included aphids, flies, beetles, gall wasps and spiders.

Collecting faecal samples throughout early spring from a transect of 40 (now increased to 44) field sites (which cover 2° of latitude, 400m in elevation and many different woodland types) allowed us to assess how blue tit diet changed along environmental gradients and as spring progressed. Dietary variation could either be due to local resource availability or preferences, and seasonal variation in diet has implications for time-sensitive life events (such as breeding or moving) while spatial variation could affect population density or productivity. We show that dietary richness increases as spring advances but does not change significantly with geography or habitat, whereas dietary composition exhibits significant turnover along temporal and spatial gradients. Such insights into how a generalist insectivores diet varies over space and time are very rare indeed. The high resolution of the prey data also allowed us to show that moths increase in prevalence in the diet with increased latitude and elevation and aphids show a steep increase in prevalence in the diet as spring progresses, presumably as they emerge to feed on growing buds and fresh leaves.

One particularly intriguing dietary occurrence is winter moth, a species whose caterpillars are known to be a crucially important food source for feeding nestlings later in the spring, but whose presence in the diet this early in spring was unexpected. As their occurrence in the diet increased throughout the sampling period, from a 2% chance thirty days before egg laying to a 17% chance at the point of egg laying, we assume that the life stage being preyed upon is early instar caterpillars. We also forward the possibility that feeding on these caterpillars provides a direct cue to the birds to start laying eggs, as how the birds time their breeding to coincide with the late spring winter moth caterpillar peak is an area of intense research and speculation. Although we cannot provide any direct evidence of this, it would provide a very direct and probably very reliable cue and offers an interesting new avenue for further work.

A winter moth caterpillar (Operophtera brumata). Photo credit to Gergana Daskalova

As mentioned previously, much of what we knew about blue tit diet was based upon microscope identification of physical prey remains encountered within the guts of euthanised birds, with the most comprehensive studies being carried out in southern English oak woods in the middle of the last century. One of my personal highlights was the ability to compare our genetic prey dataset from Scotland today with those physical datasets from England 60 years ago. Rather incredibly, some of the common taxa identified were identical, including the springtail Entomobrya nivalis, gall wasps of the genus Andricus, and several species of fly larvae feeding on emerging tree buds. Not only does this validate our method and emphasise the widespread nature of these dietary items, it is also testament to the immense skill and patience of those with microscopes and tiny body parts to identify!

However, many of the other species that we identified were previously unknown dietary items. This is due in part to the extra identification resolution that we were able to utilise with these new techniques allied with our larger sample size, but also in part due to the differing habitats studied. Half of the ten commonest taxa we identified were exclusively or primarily associated with birch trees, which is by far the commonest tree across our study region but is far less common further south in England. This could highlight an adaptability of insectivorous birds to flexibly adjust diet according to local food sources and raises the possibility that one reason we didn’t find a significant effect of habitat on diet is due to the widespread nature of birch across our sites. It will therefore prove interesting to contrast diet with regions with differing locally abundant trees if this method gains more widespread use.

More specifically, many of the commonest dietary items found in this study, particularly the commonest moth caterpillars (Figure below), are associated with birch (and alder) catkins. This explains previous observations of tits foraging on catkins at this time of year, but rather than eating pollen or plant material as was hypothesised, it appears that they are opening the catkins to prey on the caterpillars living and feeding inside them. Equally, our dietary results are consistent with observations of tits feeding around birch and sycamore buds on freshly emerging aphids, with three out of four of the commonest aphid species associated with these. The ability of the results from this method to tie in behavioural observations, physical studies, and reflect the local habitat creates an accurate and believable picture of the diet of blue tits across Scotland at this time of year and validates metabarcoding as a technique to better understand the diets of insectivorous birds.

The prevalence of taxa across samples (A – inset identifies the 10 commonest species and the number of samples they are found in), the percentage of samples including different orders (B) and the species richness of different orders (C).

To find out more, read the full paper here:


Dr Jack Shutt,

Postdoctoral Research Associate, Manchester Metropolitan University.

The Phenoweb project video

We track tree, invertebrate and bird phenology, and to do so, we collect data from 44 sites along a 220 km long transect in Scotland. To find our more about our fieldwork and how that links to our research objects, you can watch the Phenoweb video below. A huge thank you to Gergana for putting this video together.

The 2017 field season in review

This year we had four new sites. We also increased the number of nestboxes to 8 at many sites, which takes the total up to 334.

The first egg was on April 10th, making this our earliest year yet. A very noticeable pattern was that the egg laying was a fair bit earlier in the south than the north, which marks a reverse of trends we’ve seen in previous years. Trees also seemed to be a bit slower to come into leaf in the north this year. You may also be able to pick out a trend for later breeding as elevation increases and this is a pattern that we see every year and which must be associated with temperature.

The breeding season began quite well with the earliest sites fledging quite a few young. However, a spell of really foul weather around June 6th led to quite a lot of nest failure – due to heavy rains preventing the adults from foraging.

Across the entire transect 949 blue tit young were fledged.

By ringing the nestlings and adults we’re learning quite a bit about their movements. Very often we find it’s the same pair at a nest in each year. We’ve also found cases of birds moving between sites – sometimes up to 15 miles away from where they hatched.

We’ve had a great field season and are looking forward to analysing all the data we’ve collected!

Disclaimer: All blue tit adults and young were handled by licensed individuals with the appropriate permits.


My fieldwork highlights

As the field season comes to an end, I’ve been reflecting on the many great aspects of getting to be part of the Phenoweb project. Here are the moments that particularly stood out for me.

The routine. It can be daunting to start a new job, go to places you’ve never been before in search of specific trees, nestboxes and branches. Of course, we had maps and a very supportive team, but I still enjoyed the moment when we really had our routine down – we would arrive at a site and know exactly where we have to head, we would find the branch where we are mean to count invertebrates straight away.

The surprises. Of course, it was great to have some surprises, too! My personal favourite was when I dropped the plastic bag for branch beating and lifted it up only to find a beautiful fawn curled up among the bluebells. A truly fairytale moment. Another surprise was a really snowy day we had in late April. Remember that smooth routine we had? Well, we quickly got reminded of what finding the trees and branches was like in the very beginning of the field season, as that day in April, everything was covered in snow, hiding the string we use to flag trees and the current status of tree buds (as you couldn’t see them).

The new skills. I love the feeling of having learned something new, and there is plenty to learn along the transect – tree identification, bird handling and ringing, branch beating. I had never thought of the date as “day 113 of the year” before, but quickly we stopped thinking about Mondays and Tuesdays, with questions like “What day is it today?” immediately followed by a number.

The new places. Thanks to working on the transect, I got to see much more of Scotland than I had before. The north sites were my favourite – such beautiful scenery! An inspirational place to work.

The little stops along the way. During the field season, I learned a lot, I felt inspired and motivated to continue pursuing science, and I had fun. I also had the best fudge donuts of my life, ate delicious cake and scones, took in the sounds of spring as we quietly had lunch or chatted away on one topic or another. Fieldwork along the transect truly is about the journey and the stops along the way, and what a journey it’s been!

Little peaceful moments

By Gergana

Wildlife encounters along the transect

Over the last couple of months, we have seen many blue tits – as chicks begin to hatch, each site abounds in blue tits. Though of course blue tits are a lovely sight, we have been lucky to spot a lot of other wildlife along the transect, too. We get to spend most of the day outside, field notebook at hand, recording phenology and measures of bird fitness. Meanwhile, dippers are dashing up and down streams, sometimes even a tawny owl is looking down at us, and often it’s all happening in a magical fairytale-like setting of an woodland, carpeted in bluebells.

A particularly fairytale-like moment was when we walked over to a nestbox only to find a fawn curled up among the bluebells. Magic! Of course, we quietly checked the nestbox and quickly moved away to avoid disturbance, but we’ll definitely remember seeing the fawn (aka Bambi The Bluebell) for years to come!

Like out of a fairytale

One day, we got up even earlier than usual in hopes of seeing a capercaillie. Well, that didn’t happen, but nevertheless, we’ve had some pretty exciting bird sightings. There’s a tawny owl at two of our sites, and we spotted a long-eared owl near another. A pair of northern wheaters greets us as we drive down a small country road towards another another site.

Some of us on the field team saw black grouse for the first time, but regardless of whether we had seen them before or not, we all find seeing and hearing the grouse thrilling.

Then there are also the golden eagles – or so Gergana has heard, she didn’t spot any, thus no eagle photos.

Since the establishment of the transect in 2014, we’ve seen 141 bird species at our sites or directly between sites along the transect. Most of us on the team spend a lot of time working away on computers, so the field season presents a wonderful opportunity to get outside, collect data, and get inspired by nature in the meantime.

It’s peak season!

Our days are packed with branch beating and counting invertebrates, weighting and ringing chicks and looking up the occasional tree that is not in full leaf out yet (like all the aspens!). You can definitely tell it’s peak season – the spreadsheets and field notebooks are filling up, adult blue tits are dashing in and out of nestboxes feeding their chicks.

Blue tits are busy, and so are we! Here is a video of an adult blue tit coming in and out of the nest – if you listen closely, you can hear the blue tit song getting closer and closer.

We have been counting more and more caterpillars, and they are getting bigger. Every once in a while we spot particularly sparkly beetles, too. It feels really rewarding to go to bed after a jam-packed day of fieldwork, knowing that we’ve collected lots of data, spent time outside and worked smoothly as a team!

Disclaimer: All blue tit adults and chicks are handled by licensed individuals with ringing permits.

Fieldwork on the transect

Fieldwork is well underway. We have been walking through the Scottish woodlands as spring is gradually arriving all around us – tree buds opening, blue tits singing. Our days start early, sometimes with tea, more often with coffee. We are working in two teams – one in the south part of the transect, one in the north. The best part is when we meet in the middle and get to exchange stories about what happened during the day.

The first few days of trying to find the marked trees (think finding a needle in a haystack is hard, try a bright pink string in a brown-ish woodland…) are behind us – the trees and nestboxes are a familiar sight by now, and we’ve fallen into a nice routine. It’s actually a pretty special kind of repetitiveness to get to go to the same site, same trees and nestboxes week after week – there are so many little moments of natural wonder for which we have developed an appreciation.

Working on the transect is brilliant. I love the diversity of each site, and watching spring arrive day by day is a real treat. It’s so exciting to arrive at a site to check how the season has progressed and find that buds have burst on a new tree species or that there are new eggs in a nest.

Kat Keogan, Field assistant

For us, each day is a roadtrip. The destinations are the same, but each time we visit them, they are a tiny bit different – our job is to spot those differences and turn them into data on fitness (the survival of the blue tit chicks) and phenology. Phenology refers to the timing of life events, for example when trees first open up their buds, or when blue tits lay their first egg for the season.

There is still much more of the field season to come, so where will our roadtrips take us? Ultimately, to a better understanding of how the blue tit phenological optimum varies across space and time. Along the way, we’ve been to places in Scotland we hadn’t seen before, we’ve honed our tree identification and data collection skills, and we’re excited for what the rest of the field season has in store for us.


227km, 44 sites and 334 nestboxes – fieldwork begins!

Spring is just around the corner, the vans are packed, we are ready to kick off another field season along the Phenoweb transect. We will visit sites every two days through the spring, recording various phenological and fitness measures. The project was set-up to explain spatiotemporal variation in blue tit (Cyanistes caeruleus) breeding phenology and fitness and it provides the empirical backbone to this proposal.

The transect stretches 227km from Edinburgh (55.98°N) to Dornoch (57.89°N). In addition to a latitudinal gradient, the transect incorporates an elevation range from 0 – 450m, a temperature range of ~3.5°C and a variety of woodland habitats. At each site we have six to eight 26mm hole nestboxes, which exclude larger passerines such as great tits, and two thermochron i-buttons, which record accurate hourly temperatures.

We are excited to observe and record the arrival of spring in Scottish woodlands and will be sharing more of our experiences along the transect as the season progresses.