DOI: 10.5061/dryad.sf7m0cg3c
title: Data from: Colony size affects breeding density, but not spatial distribution type, in a seabird

description: The spatial distribution of individuals within populations can result in
 fine-scale density-dependence and affect the social environment that is
 encountered. As such, it is important to quantify within-population
 spatial structuring and understand the factors that shape it. In this
 study, we make use of point process statistics to test whether colony size
 affects the statistical type of spatial nest distribution produced by
 common terns (Sterna hirundo) breeding at identical man-made rectangular
 and homogeneous islands of fixed physical size. Comparing sub-colonies of
 variable density both within and across years, we find that inter-nest
 distances are smaller at higher local and overall breeding density, but
 that the spatial distribution type does not vary across the observed
 densities. This suggests that the birds’ main settlement rules do not
 depend on density. In our case, analyses of fine-scale density-dependence
 or potential social effects therefore do not need to account for
 between-individual heterogeneity in settlement decision rules or
 acceptance of these rules. We urge, however, other studies to similarly
 test for density-dependence of the spatial distribution of individuals
 before undertaking such ‘down-stream’ analyses.
description: The common tern is a
 Holarctic colonially breeding and migratory seabird (Becker and Ludwigs
 2004). The data we present here come from a long-term study population
 located in the Banter See at Wilhelmshaven on the German North Sea coast
 (53°36’N, 08°06’E). In 1992, 101 adult birds of this population were
 caught and marked with transponders (TROVAN ID 100; TROVAN, Köln,
 Germany), and since 1992 all locally hatched birds have similarly been
 marked with a transponder shortly prior to fledging.
 The colony site consists of a line of six concrete islands
 (denoted A to F, land to lakeward; Becker 2015), each of which measures
 10.7 x 4.6 m, is homogeneously covered with gravel, and is surrounded by a
 0.6 m wall. Despite the distance between adjacent islands only being 0.9
 m, they can be considered functional sub-colonies (Dittmann et al. 2007,
 Becker 2015). Three-times-weekly checks of the six sub-colonies are used
 to mark each nest, to assess laying date and to record reproductive
 parameters. During incubation, which is shared between partners, antennae
 are placed around each nest for 1–2 days to identify breeding individuals,
 such that we can distinguish between first, replacement, as well as second
 clutches. Since 1993, all nest sites have been mapped at the end of each
 breeding season by measuring the distance from the wall of one long and
 one narrow side of each island, leading to <i>x</i>- and
 <i>y</i>-coordinates of nest midpoints. For
 this study, we include all first clutches for 2 out of 6 sub-colonies (C
 and F) and for 6 out of 26 available years (1996, 2000, 2005, 2010, 2015
 and 2018; table 1). For each sub-colony and year, and for the moment when
 the last of the clutches was established, we interpret the midpoints of
 these clutches as a point pattern (figure 1) and use point process
 statistics (Illian et al. 2008, Wiegand and Moloney, 2014, Baddeley et al.
 2016) to analyse this pattern.