Tuesday, 26 February 2019

All your gulls coming home to roost

DBBP’s very own resident rock star, Ricky Whelan, has been bringing his sell out speaking tour, 'The Secret Life of Crows', to packed audiences across the nation. The intelligence of these birds has captivated crowds and, much like a revival, has led to conversions to the ‘cult of the crow’, or in less dramatic terms, people left with a renewed appreciation for the cleverness of corvids.

An avian family which has been compared to corvids, both in terms of their cerebral qualities and their sometimes-controversial behaviour, is the gull family. While perhaps not as innovative as the New Caledonian Crow which uses sticks to transport multiple items (Jacobs et al 2016), gulls have shown a remarkable ability to ‘think outside the box’.

Herring Gull with chick
Photo credit: Brian Burke

If you have ever noticed Herring or Common Gulls, ‘dancing’ in your local park, what you are witnessing is them simulating rain drops by drumming their feet on the ground to encourage earthworms to the surface (Tinbergen 1953). For a video of a Herring Gull performing a ‘rain dance’ click hereTool-use has also been recorded, with a report of a Herring Gull using bread as bait to lure goldfish (Henry & Aznar 2006). Take a walk along the coast and chances are you will see gulls repeatedly dropping shellfish from a height until the shell breaks open (Ingolfsson & Estrella 1978). This ability to learn, innovate and adapt behaviour accordingly is what defines intelligence (van Schaik & Burkart 2011).

Common Gull
Photo credit: Brian Burke

Last week, as darkness was falling, the DBBP Team got in to position all around Dublin Bay - from Dun Laoghaire to Sutton - and counted over 29,000 gulls coming in to roost. But more on numbers shortly.

In all, Dublin Bay regularly supports five species of gull. These are, in descending order of relative abundance, the Black – headed Gull; Herring Gull; Common Gull; Lesser Black – backed Gull and the Great Black – backed Gull. Small numbers of Mediterranean Gulls have also been consistently recorded during the autumn and winter months on Dublin’s south side for the last number of years, a relatively new arrival to our shores. 

Mediterranean Gull
Photo credit: Brian Burke

We count gulls in Dublin Bay in our monthly surveys during which all waterbird species, as well as their behaviour and location, are recorded. Gulls, however, are tricky. They are often feeding inland or out at sea during the day so can be missed during the regular surveys. To get a more accurate picture we schedule additional, dedicated counts at dusk to get an estimate of the numbers using the bay to roost at night. What we have found is pretty fascinating.

Mixed gull flock
Photo credit: Brian Burke

At high tide at dusk - the conditions we usually select for these counts - the highest number of gulls are recorded on the south side of Dublin Bay, when overall numbers there can go northwards of 20,000. However, at low tide, it appears that the north side of the bay, specifically the Tolka Estuary, is the place to be for those of the gull persuasion. More than 26,000 gulls were recorded there at low tide in February 2012 (Cummins & Crowe 2012), and more than 12,000 were recorded over the same tidal stage in February 2014 (Tierney et al 2014). 

Great Black-backed Gull
Photo credit: Brian Burke

Black–headed gulls certainly rule the roost in terms of sheer numbers, often with 16,000+ recorded across the whole bay, and most of these usually at Sandymount and Booterstown. The second most numerous species during roost surveys is the Herring Gull, with more than 7,000 birds counted during a dusk survey. The difference in numbers of gulls recorded during core counts of all waterbirds present compared with dedicated dusk counts is particularly striking for the Black-headed Gull. The peak count for this species during a standard low tide core count in 2018 was over 5,000 in February compared to the 17,000+ recorded in the same month but during a dusk gull roost count.

Black-headed Gull in breeding plumage
Photo credit: Brian Burke

But why, aside from scientific curiosity, is this information important? Given the number of sandwiches and chips pilfered from unsuspecting humans, surely the population of Herring Gulls, if not all gulls, is very healthy? Published data, however, states that Herring Gull and Black–headed Gull are both red-listed as species of conservation concern in Ireland (Colhoun & Cummins 2013). This is due to a 90% and 70% decline respectively in the breeding populations of these birds between 1968 and 2002.

Common Gull, Lesser Black-backed Gull and Great Black-backed Gull are all amber listed as species of conservation concern. More recent monitoring of breeding populations in Ireland will hopefully show something more positive. And by gathering data on long-term trends and distribution in Dublin Bay, we can highlight any changes at what appears to be an important winter roosting site for these beautiful and intelligent birds, and hopefully the information can help inform policy to protect them.

Black-headed Gull in non-breeding plumage
Photo credit: Brian Burke

Finally, a note on our relationship with gulls. One aspect which we tend to forget is that gulls have lived in coastal areas and inland with us for centuries. Their presence now appears to be greater in coastal towns and cities for a number of reasons. One is our continued overfishing of the seas. As a result of overfishing, these intelligent birds were forced to find alternative methods of foraging and landed on, (both figuratively and literally), landfills. This led to wide outbreaks of avian botulism, a disease which causes paralysis, and in turn resulted in the Herring Gull population crash which led to the 90% decline cited above. When landfills closed, these birds were once more forced to find alternative food supplies, and as proficient scavengers, have been able to take advantage of the waste found on our streets and overflowing bins. Further to this, by feeding them scraps from our sandwiches and what have you, we have trained some of these birds to see us as a source of food, which has led to lunches being pinched by a few rogue gulls.

This change in the food web, from sea, to landfill, to scavenging from bins and streets, has also resulted in some species, the Herring and Lesser Black–backed Gull, to begin to nest on rooftops during the summer months, where they can more easily scavenge for their young broods. It is during this period that most of the human – Gull conflict arises. Gulls are fantastic parents who are, much like most human parents, wonderfully protective. This means that, particularly during the fledging stage during late June/early July - when Gull offspring are at their most vulnerable - these parents will protect their young by emitting high pitched warning calls or even putting their own safety on the line by swooping at people or pets who come too close to their young. This stage lasts for a couple of weeks after which Gulls go back to largely ignoring us.

During the summer months we need to remember that it is our actions which have driven them to nest amongst us. It is a visible, (and often audible), warning that our fishery policies and practices are causing knock-on effects across the ecosystem and that major reforms are needed to correct this. For an excellent piece on fisheries policies in Ireland see this recording by BirdWatch Ireland’s Fintan Kelly.

Lesser Black-backed Gulls
Photo credit: Brian Burke

Colhoun, K. & Cummins, S. (2013) Birds of Conservation Concern in Ireland 2014–2019. Irish Birds 9: 523-544.

Cummins, S. & Crowe, O. (2012). Collection of baseline waterbird data for Irish coastal Special Protection Areas 2011/ 2012, s.l. Report commissioned by the National parks and Wildlife Service and prepared by BirdWatch Ireland.

Henry, P.Y. & Aznar, J.C. (2018). Tool-use in Charadrii: active bait – fishing by a Herring Gull, Waterbirds, 29(2), pp. 233-234.

Ingolfsson, A. & Estrella, B.T. (1978). The development of shell – cracking behaviour in Herring Gulls, Auk, 95, pp. 577-579.

Jacobs, I.F., von Bayern, A., & Osvath, M. (2016). A novel tool-use mode in animals: New Caledonian crows insert tools to transport objects, [online], Animal Cognition, 19(6), pp. 1249-1252.

Tierney, N., Whelan, R., Boland, H., Valentín, A., & Crowe, O. 2014. Dublin Bay Birds Project Technical Report. BirdWatch Ireland report to Dublin Port Company.

Tinbergen, N. (1953). The herring gull’s world: a study of the social behaviour of birds. London, England: Collins.

van Schaik, C.P. & Burkart, J.M. (2011). Social learning and evolution: the cultural intelligence hypothesis, Philosophical Transactions of the Royal Society, 366.