Sex differences in gannet migration and demography – new paper out!

During spring and summer the Bass Rock is home to around 75,000 pairs of  gannets making it the largest colony of northern gannets in the world. Deploying GPS loggers on adult and immature gannets in the spring and summer is giving us a great insight about where they forage during the breeding season but we know far less about where they go once they leave the colony in the autumn. Increasing our knowledge of where gannets spend the non-breeding season is crucial for conservation and determining protected areas at a time when pressures on the marine environment from climate change, renewable energy infrastructure and fisheries, are increasing.

A recent study, authored by Zoe Deakin from the University of Exeter, has tracked the migration of male and female gannets from the Bass Rock and Grassholm (SW Wales) using geolocators (small loggers that record light levels) to see whether the two sexes differ in their wintering destinations. It has also used data from the re-sighting of adults with colour rings to test for sex-specific differences in adult annual survival and determine future impacts on colony growth.

The gannets from Bass Rock and Grassholm have diverse migratory behaviours, wintering from the North Sea down to West Africa. There were differences between the sexes with 90 % of the females travelling the furthest distances to winter in the Canary Current Large Marine Ecosystem (CCLME) compared with 57 % of males. The CCLME (shaded area off West Africa in the figure below) is currently one of the most intensively fished areas of the planet and has the world’s highest levels of illegal, unreported and unregulated (IUU) fisheries (Agnew et al. 2009 – see full paper below for reference).

Centroids of locations in December of male (blue) and female (orange) northern gannets tracked with geolocators from Bass Rock (n = 30; 17 males and 13 females) and Grassholm (n=19; 13 males and 6 females). Colony location is marked with a solid black circle. Solid black lines mark borders of Large Marine Ecosystems. Shaded area represents the Canary Current Large Marine Ecosystem (CCLME). (Deakin et al. 2019)

Survival of females from Grassholm was slightly lower than for males but there was no difference in survival between the sexes from Bass Rock despite similarly high proportions of females from both colonies wintering in the CCLME. It is possible that high levels of bycatch in the CCLME could explain the lower survival of female gannets but it is unclear why there should be a difference between females from the two colonies.

Colour rings with unique codes allow easy identification of individual birds at the colony – photo Jude Lane

The paper goes on to demonstrate the importance of accounting for sex-specific differences in survival when predicting population trajectories and highlights the importance of monitoring the behaviour of seabirds year-round.

The paper “Sex differences in migration and demography of a wide-ranging seabird, the northern gannet” has been published in Marine Ecology Progress Series and is free for anyone to download so if you’d like to read the paper for yourself then you can find it here.


First paper published!

The first paper from research I have been involved with has been published!

Gannets, like many marine predators, can travel hundreds or thousands of kilometres to find food, but how they learn to locate prey patches in the open ocean has always remained a mystery.

During the first year of my PhD we tracked both adult and immature gannets from Bass rock using GPS devices. The tracks they provided have allowed us to reveal key differences in the way in which adults and immature birds forage.

Adults repeatedly targeted the same areas of the North Sea, areas associated with oceanic fronts where different bodies of water meet. These areas often have strong temperature or salinity gradients and are where large aggregations of plankton and fish can be found. They are often visible from the air and are known to be important foraging areas for many different marine predators from whales to seabirds.

In contrast, immature birds ranged much more widely than the adults and didn’t have the strong association with oceanic fronts which indicates that gannets use their comparatively long immature stage (up to 5 years) to learn where the best feeding grounds are.

Each colour represents trips made by a single individual.

The results provide crucial information on how seabirds efficiently locate and exploit patchy food resources vital to their survival and long-term fitness. The time taken for individuals to learn how to recognise good foraging sites and where they’re likely to occur probably goes a long way to explaining why seabirds and other long-lived predators don’t start breeding until they’re several years old.

The differences in the foraging ranges of adults and immature birds may also mean they face different levels of risk at sea, for instance from collision with offshore wind turbines, which is something we’re now investigating further.

The paper “Understanding the ontogeny of foraging behaviour: insights from combining marine predator bio-logging with satellite-derived oceanography in hidden Markov models” has been published in the Journal of the Royal Society Interface and is free for anyone to download so if you’d like to read more about what we did and found out, grab yourself a nice cup of tea, put your feet up and download it here.

Making tracks

One of the most exciting aspects of the field season is getting to download the data from the GPS tags and seeing where the gannets have been.  As in previous years, we left the tags on for 7 to 10 days allowing us to record multiple trips from each bird.

A popular foraging location for many of the gannets this year has been the Fladen Ground, a large fishing area in the middle of the North Sea between Scotland and Norway.

This gannet made 5 trips to the Fladen Ground in 10 days, covering an average of 670 km on each journey (each trip is a different colour). To put this in perspective, this is the equivalent to you making a round trip from Leeds to Bristol for your food shop – every 2 days!


In complete contrast, other gannets have been staying much closer to the colony. This male only went a maximum of 35 km from Bass Rock when it went up to St Andrews, however it still managed to cover between 90 and 200 km during each of these four trips.


This female gannet also chose to forage close to shore but instead headed south to the coast of Northumberland. During the yellow trip she covered a distance of 375 km and spent time foraging close to Holy Island. During the pink trip she spent time foraging close to the Farne Islands in areas that shags forage for sandeels.



It’s always fascinating to see the variation in foraging strategies used by gannets who are neighbours at the colony. Establishing why they have developed these individual strategies is something I and other researchers are hoping to try and understand a little better.



The newest member of Team Gannet

Sadly this will be the last summer I get to spend working with the Bass Rock gannets. But fear not, the blogs will be continuing as Ruth Jeavons, and from October Chris Pollock, will be continuing the research in pursuit of their PhD’s. So for this post I’m handing over to Ruth to introduce herself and her research.


I am currently spending a hot, but windy, afternoon looking out to sea at the white crests preventing us from landing on Bass Rock today. What better excuse to sit down with an excellent cup of coffee, piece of carrot cake, and introduce myself?!

I’m Ruth, a 1st year NERC DTP PhD student at the University of Leeds, studying the foraging effort and behaviour of breeding gannets. Despite officially being half way through the field season, it’s only just beginning for me in terms of getting my hands on some actual data…

Very excited to be retrieving a GPS logger from one of the birds tagged in April (Photo – Emily Thornton)

Like Jude, I am attaching data-logging devices to the birds to see what’s going on whilst they’re out at sea. In addition to GPS loggers to see where they go, I’m using accelerometers. These measure movement, in the form of acceleration, in three directions (see below!) From this it’s possible to work out body postures and behaviours that we simply wouldn’t be able to see otherwise. For example, the angle of a dive, or individual wingbeats during flight and underwater. As movement is associated with using energy, accelerometry data can also be used as an indication of how energetically costly some activities are to gannets. These are some of the things I hope to look at during my PhD… But for now, it’s all about getting that data!

I have always been a big fan of fieldwork and have been lucky enough to work in some amazing places, but Bass Rock fieldwork is like nothing else I’ve ever experienced! As everyone says, the senses are utterly assaulted one way or another by the 150,000 birds. From getting to know the ideal distance to stay away from a path side nest to avoid a jab to the calf, to the ever constant ‘is it raining? Oh, nope, that’s not rain’, it’s a one of a kind experience!

One of the more vocal and feisty path side birds – one to avoid getting too close to! (Photo – Ruth Jeavons)

I am attaching accelerometers to the backs of the gannets rather than the tail, which involves the fiddly task of manoeuvring small feathers, positioning tape under them at the base and then laying the device on top before wrapping it up nice and securely… Sounds simple? I didn’t so! To be on the safe side I made some dummy devices out of fimo (basically playdough that hardens in the oven for anyone not familiar!) and attached them to some pioneering participants. Two out of three birds were seen 6 days later with their dummy devices still attached, so I went ahead and deployed 4 of the real things on our next trip, with a clear conscience!

Accelerometer attached to the back and GPS logger to the tail (Photo – Ruth Jeavons)

As the newbie in the team, I’ve had a lot of firsts this field season; First trip to Bass in April, first bird caught (B262 you trooper), first dummy device attached and re-sighted (B066 you star), first real device attached (B295 you legend)… The list (and excitement!!) is endless, but I’m still waiting on the biggest first… Which leads back to hoping for a calm day to be able to retrieve some devices and download the first of my data!

Season so far

We are now four weeks in to our eight week gannet tracking season, so how have we been getting on?

A quagmire

It’s been an interesting summer so far, after such a promising May, June, as you may have seen in the last blog, brought wind, rain and stormy seas. The wild weather has been a bit of a disaster for many seabirds, this blog from Isle of May describes the conditions their seabirds have been experiencing.

The storm over the 29/30th June certainly caused gannet nests to fail on the Bass. Many of the nests on the flatter areas of the rock succumbed to the quagmire of mud and guano which is created during heavy rain, however those on the steeper ledges, the prime locations, have fared much better.

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Tough year for gannets and researchers

So the gannets have been having a tough year and as a result so are we! You would think on a colony of 150,000 gannets it would be easy to catch and tag a few of them, but unfortunately for us it isn’t quite the case. The first limitation we face is that it is only possible to access a very small area within the breeding population (many of the gannets below the chapel are non-breeders), secondly we can then only catch birds we can reach with a 6 m pole, thirdly we need to know the sex of the birds we catch [see this blog on why sexing gannets is a tricky business] and fourthly they need to be feeding chicks. Because we work on one of the flatter areas of the colony, many of the nests have failed this year, making it much harder than usual to meet all the criteria.

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Interesting tracks

It hasn’t all been stormy seas and rain though. The chicks that survived are now a good size with many now standing tall next to their parents tentatively flapping their newly discovered wings. The birds we have managed to track so far have given us some interesting foraging tracks, with lots of activity close in to the coastline, so we’re just keeping our fingers crossed that we get some more calm seas and light winds over the next few weeks to allow us to track a few more.

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Summer storms

For the seabirds in the Firth of Forth, June 2017 has been a shocking month. Reported as being the wettest June on record in parts of Fife, the impact of all that rain was very evident when we made our first trip over to the Bass on the 19th June. The part of the colony within which we work appeared to have many failed nesting attempts with adult gannets standing around on empty territories instead of sitting tight incubating eggs or brooding young chicks.

New year, new team

The University of Leeds research team this year comprises myself, Ruth Jeavons who is embarking on her first summer of PhD field work and two MSc students Emily Thornton and Solange Ponce. Our first two visits, as last year, were made with the objective of spotting as many of the previously colour-ringed birds as possible and to establish their breeding stage – were they incubating eggs, brooding chicks or not breeding/failed.

Weather watching

Since those first visits the east coast has been battered by strong winds making trips out to the Bass almost impossible. A small window in the weather on Monday 26th allowed us to get on for a morning and attach loggers to the first birds this summer but since then the wind has howled and today has seen the coast inundated with heavy rain to accompany the winds.

This clip was shot on my phone from the Seabird Centre … can you make out the Bass in the distance?

A small positive

How the rest of the summer will turn out is anyone’s guess. What is certain though is that after today’s weather there will have been yet more chicks and eggs lost.

It will be interesting to see how the adult gannets respond to this bad weather when out foraging. As a far-ranging seabird their ability to adapt to environmental change has allowed them to be one of the few species of UK seabirds not to be suffering from long-term breeding failures. Being able to track their foraging behaviour during this unseasonable weather may give us some insights into how they adapt and how they might fare in our changing climate.

Battles on Bass Rock

By mid-April almost all the gannets have returned to Bass Rock and tensions are high. Fights are common with birds on the edge of the colony trying to establish themselves on a territory.  In between catching and GPS tagging we would watch these fights unfold in awe of the brutal blows they inflict one another. The dogged determination they show in trying to win is unbelievable. Just when you think one bird is down and out, bam, it strikes back with even more purpose than before.

The photos below document one such fight. The bird with the colour ring is B116, a young bird we ringed in 2015 when it was in it’s 4th year. These two must have been fighting for at least 20 minutes, locked beak to beak, jabbing at each other, grabbing the backs of each others necks only pausing for a second or two before resuming their intense power struggle. If you look closely at the pictures you’ll see how B116 causes its opponents bill to splinter. At one point the neighbouring bird got involved but soon turned away and left them to it!

Eventually B116 appeared to come out as the winner but not before blood was split.

During these fights the birds spread their wings widely which allows them to balance and prevent themselves being pushed backwards.

The ornithologist Brian Nelson describes perfectly in his book The Gannet, how the winner of such fights is “often not content with winning; he prevents his rival from disengaging, pursues him and renews the fight even when it could have ended.” It’s therefore not surprising that we see a fair number of gannets with battle wounds. However they are incredibly resilient, I  am just in awe of their robustness in the wake of such ferocious fights and what look to be quite severe injuries.



Early season success

Our return visits to the Bass certainly didn’t disappoint!

If you read the previous blog, you’ll know we put out some GPS loggers on gannets in April this year in a first attempt at finding out how they behave early in the breeding season before they are constrained by incubating eggs or feeding chicks.

The weather and tides dictated the days we were able to go back to retrieve the loggers and fortunately they gave us a two day window, a week after the deployment, to try and get them all back.

With huge relief, four of the birds were waiting for us on the first morning so we were quickly into the process of catching and removing tags. Unfortunately though, two of the four were about to leave on foraging trips at the same time and so frustratingly one of them managed to get away before we could retrieve it’s tag.


Hoping that wasn’t to be the last time we’d see B154, we then had 6.5 hours to spot colour-ringed birds and indulge ourselves with observing and photographing the day-to-day life on a gannet colony. At this time of year is even more frenetic than in the summer with birds trying to establish and defend territories, copulate, meet and greet and steal nesting material; it really was intense out there with some truly ferocious fighting witnessed. I’ll blog another time about the ‘battles on the Bass’. Then, just in the nick of time before we had to head down to get on the boat, our old friend B099 (we have tagged him in the summers of 2015 and 2016) returned with his GPS!

The trip back across the Forth was slightly more exhilarating than usual with Jack Dale of Forth Wild demonstrating some superior boat handling skills to get us back into Seacliff harbour on a high tide. We than had an hour and a half to hunker down out of the wind and watch the male gannets come into Seacliff bay to collect seaweed before the tide was low enough to safely walk back to the beach.

April high tide at Seacliff harbour (Photo: Jude Lane)

The high tide preventing us from getting back to the beach (Photo: Jude Lane)

Day two was a more slow and steady day with nerves creeping in at times as we pondered about the prospect of not getting any more tags back. Thankfully though we did, the gannets just appeared more sporadically throughout the day, keeping us on our toes and looking through our binoculars for the entire 10 hours we were there. We were well rewarded for our efforts though as by the end of the (cold and windy) day we had successfully retrieved all but one of the GPS devices we’d deployed including, with much relief, B154, the ‘one that got away’ the day before!

So it was with big smiles that we headed back to Seacliff, not only had we shown that catching at this time of the year on the Bass can be done but we also now have some great data to crunch to see whether the Bass gannets are behaving differently during the pre-laying period to when they are feeding chicks!

A Gannet tracking first?!

My first field work blog of the year is a little earlier than usual. This is because, excitingly, we are attempting to gather some slightly different data this year.

As I explained in Gannet Grabbing, the GPS tagging of adult gannets has traditionally taken place during the period of the breeding season when the birds are feeding chicks. At this stage the gannets are almost guaranteed to return to the nest, allowing them to be re-caught to retrieve the loggers. This has given us a really fabulous insight into where the gannets are going during the summer months when they are feeding chicks but we know very little about their behaviour during the remaining 10 months of the year.

This year we are going to attempt to change that and see if we can get GPS data from adult gannets making foraging trips in April before they lay their eggs and begin incubation.

So why are we interested in doing this? If successful, it will be very exciting as no-one has managed to gather data on gannet foraging trips during this stage of the breeding season before. During the chick-rearing period we know adults are repeatable in their foraging behaviour with individuals consistently returning to the same areas presumably because they know they will find prey there – see last summer’s post about B037. Their time away from the colony is constrained by the need to return with food for the chick so their foraging trips need to be as efficient as possible in terms of time and energy expenditure. When there is no chick to feed we don’t know whether they are repeatable in their foraging trip destinations or whether they are able to be more flexible in the amount of time they spend away from the colony.


I use the words ‘attempt’ and ‘if successful’ as we are as yet unsure whether it will be possible to catch the gannets in April. When gannets have chicks they will remain with the chick and avoid straying into the territories of neighbours who will give them a hard time. With no chick to protect and potentially less aggressive neighbours (as they also have no chicks) the gannets may simply run or fly off! If we do catch them and attach the loggers the second question mark lies over whether we’ll be able to re-catch them again for much the same reasons plus the fact they will be wiser to being caught when they see the pole approach for a second time!


So keep your fingers crossed for us and hopefully my next post will be able to bring news of a successful April tagging trip!


Hot rock

“UK weather: Britain is hotter than Saudi Arabia as country sizzles in 35C heat” was the Mirror’s headline on 19th July this year. It probably wasn’t quite 35C in the Firth of Forth but it was still pretty hot!

The early morning trip across from Seacliff was magical … the sea was as calm as I’ve ever seen it, the sun shone and the gannets soared.

Beckie and I didn’t get much tagging or colour ringing done that day, it was too hot for both us and the birds, but I did take some footage and made it into a short movie you might enjoy.