Jennings, Gemma (2012) The ecology of an urban colony of common terns Sterna hirundo in Leith Docks, Scotland. PhD thesis. http://theses.gla.ac.uk/3910/ Copyright and moral rights for this thesis are retained by the author A copy can be downloaded for personal non-commercial research or study, without prior permission or charge This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the Author The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the Author When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given Glasgow Theses Service http://theses.gla.ac.uk/ [email protected] The ecology of an urban colony of common terns Sterna hirundo in Leith Docks, Scotland Gemma Jennings Submitted in fulfillment of the requirements for the Degree of Doctor of Philosophy Institute of Biodiversity, Animal Health and Comparative Medicine College of Medical, Veterinary and Life Sciences University of Glasgow June 2012 2 Abstract The Imperial Dock Lock Special Protection Area (SPA) in Leith Docks on the Firth of Forth currently supports the largest common tern (Sterna hirundo) colony in Scotland. The nest site, a former lock wall in an operational port, was designated as an SPA for the species in 2004 but very little is known about the ecology of common terns in this man- made environment. This thesis examined their ecology using a combination of long-term data for the Firth of Forth region and field research at the colony. The dynamics of the Firth of Forth breeding population of common terns was linked both to local influences of predators and the regional status of their main food source, the Firth of Forth sprat stock. Colonisation of Leith Docks resulted from relocation of birds from natural islands in the Firth of Forth which were abandoned due to unsustainable levels of predation by gulls. Herring gulls (Larus argentatus) and lesser black-backed gulls (L. fuscus) are active predators in Leith Docks but at relatively low levels. Predation attempts by mink present a serious threat and could be highly detrimental to the colony. Foraging studies revealed that terns are feeding primarily in the Forth of Forth rather than within the docks, and that their diet consists mostly of sprat, but also sandeels and gadoids. The importance of sprat in the diet is discussed in relation to the potential reopening of the sprat fishery. Surveys of birds commuting between the colony and the feeding grounds showed that a range of flight lines are used but to different extents, and found no evidence of collisions with buildings or other man-made structures. Terns were well-habituated to regular human activity but were sensitive to unusual or high-level human disturbance factors. Gulls and crows, rather than humans, were the greatest disturbance factors for nesting birds overall. Currently the Imperial Dock Lock SPA is the only site in the region that could support common terns breeding in considerable numbers, and so the future of the Firth of Forth population of common terns is now dependent on this one site. There are a number of management options available, and the future persistence of the population relies on the continued monitoring of breeding numbers of terns, of predation levels and further assessment of the sprat stock. 3 Table of Contents 1 General Introduction ................................................................................................. 12 2 A review of the use of urban and industrial environments by breeding terns........... 14 2.1 Introduction ....................................................................................................... 14 2.2 Seabirds in urban environments ........................................................................ 14 2.3 Nest site selection in terns: a shift towards nesting on man-made structures? . 15 2.4 Challenges at man-made sites ........................................................................... 21 2.5 Productivity at man-made sites ......................................................................... 22 2.6 Management considerations for urban and industrial colonies ......................... 27 2.7 Conclusions ....................................................................................................... 28 3 Common terns in the Firth of Forth: Population trends and distribution patterns .... 29 3.1 Introduction ....................................................................................................... 29 3.2 Methods ............................................................................................................. 31 3.2.1 Numbers and distribution .......................................................................... 31 Common tern records pre-1969 ................................................................................ 31 3.2.2 Productivity ............................................................................................... 32 3.2.3 Relationship with gull numbers ................................................................ 33 3.2.4 Relationship with sprat abundance............................................................ 34 3.3 Results ............................................................................................................... 36 3.3.1 Numbers and distribution .......................................................................... 36 3.3.2 Productivity ............................................................................................... 41 3.3.3 Relationship with gull numbers ................................................................ 41 3.3.4 Use of tern breeding numbers as indicators .............................................. 43 3.3.5 Relationship between common tern breeding numbers and the status of the sprat fishery ............................................................................................................... 44 3.4 Discussion ......................................................................................................... 46 3.4.1 Numbers and distribution .......................................................................... 46 3.4.2 Productivity ............................................................................................... 47 3.4.3 Relationship with gull numbers ................................................................ 47 3.4.4 Use of tern breeding numbers as indicators .............................................. 48 3.4.5 Relationship between common tern breeding numbers and the status of the sprat fishery ............................................................................................................... 48 3.4.6 Conclusions ............................................................................................... 49 4 Foraging ecology of common terns breeding at the Imperial Dock Lock Special Protection Area in Leith Docks ......................................................................................... 52 4.1 Introduction ....................................................................................................... 52 4.2 Methods ............................................................................................................. 54 4.2.1 Diet studies ................................................................................................ 54 4.2.2 Foraging areas ........................................................................................... 55 4.3 Results ............................................................................................................... 56 4.3.1 Diet composition ....................................................................................... 56 4.3.2 Prey size .................................................................................................... 59 4.3.3 Chick provisioning rate ............................................................................. 61 4.3.4 Foraging locations ..................................................................................... 62 4.4 Discussion ......................................................................................................... 63 4.4.1 Diet ............................................................................................................ 63 4.4.2 Chick provisioning rate ............................................................................. 64 4.4.3 Foraging locations ..................................................................................... 65 5 Flight lines of common terns at the Imperial Dock Lock SPA ................................. 66 5.1 Introduction ....................................................................................................... 66 5.2 Methods ............................................................................................................. 68 5.2.1 Data analysis ............................................................................................. 70 4 5.3 Results ............................................................................................................... 72 5.4 Discussion ......................................................................................................... 80 5.4.1 Temporal patterns of foraging activity ...................................................... 80 5.4.2 Environmental conditions ........................................................................