DOI: 10.5061/dryad.2jm63xsxc
title: Age-specificity in territory quality and spatial structure in a wild bird population

description: Age influences behaviour, survival, and reproduction; hence variation in
 population age structure can affect population-level processes. The extent
 of spatial age structure may be important in driving spatially-variable
 demography, particularly when space-use is linked to reproduction, yet it
 is not well understood. We use long-term data from a wild bird population
 to quantify covariance between territory quality and age and examine
 spatial age structure. We find associations between age and aspects of
 territory quality, but little evidence for spatial age structure compared
 to the spatial structure of territory quality and reproductive output. We
 also report little between-year repeatability of spatial age structure
 compared to structure in reproductive output. We suggest that high
 breeding site fidelity among individuals that survive between years, yet
 frequent territory turnover driven by high mortality and immigration
 rates, limits the association between age and territory quality and
 weakens overall spatial age structure. Greater spatial structure and
 repeatability in reproductive output compared to age suggests that habitat
 quality may be more important in driving spatially-variable demography
 than age in this system. We suggest that the framework developed here can
 be used in other taxa to assess spatial age structure, particularly in
 longer-lived species where we predict from our findings there may be
 greater structure.
description: <strong>(i) Study system and data
 collection</strong> The great tit <em>Parus
 major</em> is a passerine bird found in woodlands across Europe,
 with breeding ages ranging 1–9, averaging 1.8 years [1–3]. Although there
 are some continuous changes with age [1], the main age effects on
 individual-level traits are captured by two age-classes: first-years
 (hereafter yearlings) and older (hereafter adults [2–6]). The species is
 socially monogamous, with pairs defending territories during annual
 breeding seasons [7]. Data used here are from a long-term study in Wytham
 Woods, Oxford (51°46’N, 1°20’W), a 385ha mixed deciduous woodland
 surrounded by farmland [8]. The tit population has been monitored since
 1947, where breeding adults and their chicks have been marked with unique
 BTO (British Trust for Ornithology) rings since the 1960s; and standard
 reproductive metrics are collected [9]. Individuals breed almost
 exclusively in the 1026 nest-boxes which are in fixed positions with known
 GPS coordinates [10,11]. All chicks are ringed at 14-days of age, while
 adults are trapped at nest-boxes and identified by ring number, or marked
 with a new ring if they are immigrants. Age is based on year of hatching
 for local birds, or plumage characteristics for immigrants [12]. Although
 immigration rates are high (46%), most are first caught as yearlings (78%)
 and can therefore be aged accurately.
 <strong>(ii) Data selection</strong>
 We constructed a dataset that assigns the year of hatching to all
 individuals between 1950–2022, across which exact age was calculated for
 88.8% of 46062 identified breeding individuals. In this study, we included
 birds in analyses that attempted to breed between 1978–2022, for which
 data were more complete compared to earlier dates. Individuals that were
 first caught post-fledging are assumed to be immigrants, as
 locally-hatched tits are marked as nestlings in nest-boxes and the
 proportion of birds hatched in natural cavities is very low [13].
 Immigrants that entered the population with adult plumage were assigned a
 minimum age of 2, and subsequent age estimates were based on this (6.7%
 and 10.0% of breeding females and males). Age was therefore determined for
 68.7% of breeding individuals where at least one egg was laid (due to a
 combination of nests failing prior to adult trapping and unsuccessful
 trapping attempts, there are cases where the identity of parents is
 unknown). <strong>(iii) Statistical
 analyses</strong> <strong>Determining
 breeding territories</strong> We defined annual
 breeding territories through a Dirichlet tessellation technique that forms
 Thiessen polygons [14,15] around each occupied nest-box. The polygon
 includes all space within the habitat that is closer to the focal box than
 any other (with a boundary also imposed by the woodland edge). This metric
 of territory has been shown to be biologically meaningful in terms of
 territory size and territorial neighbours in tit species and is strongly
 related to other methods of calculating territories [10,16–19]. However, a
 limitation is that unrealistically large polygons are formed in areas
 where nest-boxes are placed at great distances from each other. We
 therefore capped territories at 2ha, which is a more realistic maximum
 spatial scale at which individuals use territories, as supported in
 previous analytical and field studies [10,11,20,21].
 <strong>Age and territory quality</strong>
 We first assessed covariance between the age of individuals and
 their territories’ quality. We measured territory quality through four
 measures: the number of oak trees <em>Quercus</em> spp. within
 75m of the nest-box; average territory density; the edge distance index;
 and the long-term nest-box popularity index. Each of these is justified
 below. Great tits predominantly provision offspring
 with caterpillars collected close to their nests [3], thus variation in
 caterpillar availability is directly linked to reproductive success
 [22–24]. Caterpillars are found most abundantly on oak trees [3,22],
 therefore oak proximity, health and abundance is important for breeding
 success [10,18,25,26]. A radius of 75m was chosen as the abundance of oaks
 within this distance has been shown to be particularly important for
 breeding [27,28]. The density of conspecifics breeding
 in proximity may influence resource availability if foraging ranges
 overlap, and therefore territory density may also represent an aspect of
 territory quality. Additionally, territory density may affect site quality
 through social mechanisms, such as increased competition and emergent need
 for territory defence leading to reduced foraging [29], or conversely
 mutual benefits between familiar neighbours [18,30,31]. We calculated
 average territory density directly from the Thiessen polygon area produced
 from tessellation by taking the reciprocal of the mean polygon
 area. Territories at woodland edges are associated with
 lower reproductive success in great tits [21]. Following Wilkin et al.
 (2007) [21], we defined the edge distance index (EDI) for each nest-box by
 multiplying the distance to forest edge by the proportion of woodland
 habitat within a 75m radius of the box. Thus, boxes within 75m of the edge
 have an EDI value in proportion to the amount of woodland habitat within
 this radius, therefore considering not only the distance to edge, but also
 the number and geometric arrangement of edges relative to
 nest-box. Finally, the frequency a territory is
 occupied in the long-term may provide a measure of quality as individuals
 may choose sites that confer reproductive benefits more often, as
 evidenced in other species [32–36]. There is evidence of this in Wytham,
 where the number of times a nest-box has been occupied positively
 correlates with the average number of offspring that fledge per breeding
 attempt. We therefore calculated the frequency of occupancy of each
 nest-box independent of individual breeding site fidelity by calculating
 the number of times a nest-box has been occupied since 1965 by a new
 breeding individual (i.e. attempts where either the female or male had
 previously used the same box were removed). However, nest-box occupation
 frequency is related to nest-box density, because in areas of high density
 there are multiple unoccupied boxes which would likely be associated with
 the same territorial range if they were occupied (thus, in regions of high
 box density, birds may re-occupy the same territory over multiple years,
 but not necessarily the exact same box). To correct for this, we ran a
 linear model between the number of boxes within 30m of a focal nest-box
 and the number of times said box has been occupied by a new breeding
 individual, and took the residuals as the long-term nest-box popularity
 index. We constructed a generalised linear
 mixed-effects model assuming a binomial error distribution to analyse the
 association between these four measures of territory quality and the age
 of the breeding individual. We modelled age (juvenile/adult) as the
 response variable, with the territory quality measures as explanatory
 variables, which were z-transformed to compare their relative effects in
 predicting age. Individual ID, nest-box ID, and breeding year were
 included as random effects. We ran three sets of these models: one with
 all individuals; one with only females; and one with only males, allowing
 us to assess potential sex-specific differences in the association between
 aspects of territory quality and age. All models were conducted in R
 statistical software [37] using the <em>lme4</em>
 package [38]. <strong>Spatial age
 structure</strong> For each year’s breeding
 population, we constructed an individual-by-individual matrix denoting
 breeding neighbours i.e. a network of breeding territories, where nodes
 represent individuals and edges represent the spatial connectivity of
 territories. Specifically, edges connected individuals if their
 territories share a boundary from the tessellation technique and were
 weighted relative to the distance between nest-boxes of neighbouring
 territories. We created three networks per year: one
 with all individuals (but removing edges within breeding pairs that occupy
 the same territory); one with only females; and one with only males. Edges
 connecting individuals of unknown identity were removed. Across these
 networks, we calculated the assortativity coefficient of age
 (juvenile/adult), which measures the correlation between individuals’ age
 and that of their territorial neighbours accounting for edge weight
 (proximity of neighbouring nest-boxes) and the relative proportion of the
 two age classes across the network. Through this technique, the
 age-composition of neighbourhoods of birds that are likely to interact
 during territorial and foraging behaviour contribute to the emergent
 quantitative signal of spatial age structure to a greater degree than they
 would if spatial autocorrelation were calculated through pairwise distance
 of all individuals across the system. Thus, this method allows us to
 assess evidence for spatial age structure at a biologically relevant
 scale. We also ran analyses to compare spatial age structure with spatial
 structuring of territory quality and reproductive output. To calculate
 territory quality structure, we assigned the value from the previously
 described four measures of quality to each node (associated with an
 occupied nest-box) and calculated the territory quality assortativity
 coefficient across the network. For spatial reproductive output structure,
 we ran parallel analyses, but calculated the assortativity in clutch size,
 chick number, fledgling number, and binary success (where 0 is no
 fledglings and 1 is at least one fledgling) associated with each nest-box.
 These analyses were run using the <em>assortnet</em>
 package[39]. <strong>Temporal repeatability of
 spatial age structure</strong> Finally, we tested
 whether spatial regions show temporal repeatability in age-composition,
 territory quality, and reproductive output. To assess this, within each
 year, we defined a radius around each occupied nest-box that corresponded
 to an area of 25, 50, 100, 150 and 200ha, representing neighbourhoods of
 breeding individuals of variable population sizes. Within each radius we
 calculated: the proportion of individuals that were adults; the mean
 number of oaks within 75m of the focal boxes; mean territory density; mean
 edge distance index; mean nest-box popularity index; mean clutch size;
 mean chick number; mean fledgling number; and proportion of boxes with
 binary success. We then calculated the same metrics for all nest-boxes
 outside of the focal radius. From this, we calculated a ratio index of
 within versus outside radius for each calculated measure, where a value of
 one represents the same annual average measure within and outside the
 radius. We then tested the repeatability of the ratio index associated
 with each spatial scale across the 45 years of data by calculating the
 intraclass correlation coefficient (ICC) derived from a linear
 mixed-effects model, where the response variable is the ratio index, and
 year and radius area are fitted as categorical grouping
 variables.
 <strong>References</strong> 1.         Bouwhuis S, Sheldon BC, Verhulst S, Charmantier A. 2009 Great tits growing old: Selective disappearance and the partitioning of senescence to stages within the breeding cycle. Proceedings of the Royal Society B: Biological Sciences 276, 2769–2777. (doi:10.1098/rspb.2009.0457) 2.         Perrins CM. 1979 British tits. 1st edn. London: Collins.  3.         Gosler Andrew. 1993 The great tit . London: Hamlyn.  4.         Farine DR et al. 2015 The role of social and ecological processes in structuring animal populations: A case study from automated tracking of wild birds. R Soc Open Sci 2. (doi:10.1098/rsos.150057) 5.         Greenwood PJ, Harvey PH, Perrins CM. 1979 The Role of Dispersal in the Great Tit (Parus major): The Causes , Consequences and Heritability of Natal Dispersal. Journal of Animal Ecology 48, 123–142. 6.         Sandell M, Smith HG. 1991 Dominance, prior occupancy, and winter residency in the great tit (Parus major). 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description: # Age-specificity in territory quality and spatial structure in a wild
 bird population This README.txt file was updated on 09/06/2025    A. Paper
 associated with this archive: Age-specificity in territory quality and
 spatial structure in a wild bird population Citation: Woodman et al.,
 under review. Brief abstract: Age influences behaviour, survival, and
 reproduction; hence variation in population age structure can affect
 population-level processes. The extent of spatial age structure may be
 important in driving spatially-variable demography, particularly when
 space-use is linked to reproduction, yet it is not well understood. We use
 long-term data from a wild bird population to quantify covariance between
 territory quality and age and examine spatial age structure. We find
 associations between age and aspects of territory quality, but little
 evidence for spatial age structure compared to the spatial structure of
 territory quality and reproductive output. We also report little
 between-year repeatability of spatial age structure compared to structure
 in reproductive output. We suggest that high breeding site fidelity among
 individuals that survive between years, yet frequent territory turnover
 driven by high mortality and immigration rates, limits the association
 between age and territory quality and weakens overall spatial age
 structure. Greater spatial structure and repeatability in reproductive
 output compared to age suggests that habitat quality may be more important
 in driving spatially-variable demography than age in this system. We
 suggest that the framework developed here can be used in other taxa to
 assess spatial age structure, particularly in longer-lived species where
 we predict from our findings there may be greater structure. B.
 Originators: Woodman J. P., The Edward Grey Institute of Field
 Ornithology, University of Oxford. Firth, J. A., School of Biology,
 University of Leeds. The Edward Grey Institute of Field Ornithology,
 University of Oxford. Cole, E. F. The Edward Grey Institute of Field
 Ornithology, University of Oxford. Sheldon, B. C. The Edward Grey
 Institute of Field Ornithology, University of Oxford. J.P.W., J.A.F.,
 E.F.C. and B.C.S. conceived the study and contributed to data collection.
 J.P.W. analysed the data. J.P.W. wrote the first draft of the manuscript,
 with substantial input from J.A.F., E.F.C. and B.C.S. C. Contact
 information: Name - Dr Joe Woodman email -
 [jwoodman999@hotmail.co.uk](mailto:jwoodman999@hotmail.co.uk) D. Dates of
 data collection: 1978–2022 E. Geographic Location of data collection:
 Wytham Woods, Oxford (51°46’N, 1°20’W) F. Funding Sources:The Edward Grey
 Institute of Field Ornithology ACCESS INFORMATION 1. Licenses/restrictions
 placed on the data or code: CC0 1.0 Universal 2. Data derived from other
 sources: NA 3. Recommended citation for this data/code archive: Woodman et
 al., 2025. DATA & CODE FILE OVERVIEW This data repository consist of 4
 data files, 1 code script (uploaded at Zenodo), and this README document,
 with the following data and code filenames and variables: Data files and
 variables 1. breeding.inds.base.csv. Individual breeding data 1978-2022
 with columns: "Box" - nest-box id "Section" - section
 within study site "Year" - year of breeding "Ring" -
 individual ring number. NA refers to non-available data, where the
 breeding individual was never identified at the nest-box. "Sex"
 - female (F) or male (M) "Known.dob" - year of hatching of focal
 individual. NA refers to non-available data, where the breeding individual
 was never identified at the nest-box and therefore their year of hatching
 is not known. "Latest.dob" - latest year of hatching of focal
 individual when exact age is not known. NA refers to non-available data,
 where the breeding individual was never identified at the nest-box and
 therefore their year of hatching is not known. "Known.age" -
 exact age of focal individual in years since hatching. Cell is blank when
 only the minimum age can be calculated. NA refers to non-available data,
 where the breeding individual was never identified at the nest-box and
 therefore their age cannot be estimated. "Min.age" - minimum age
 of focal individual when exact age cannot be calculated in years since
 hatching. Cell is blank if the exact age can be calculated. NA refers to
 non-available data, where the breeding individual was never identified at
 the nest-box and therefore their age cannot be estimated.
 "Bin.age" - age; either a yearling (Juv) or older (Ad). NA
 refers to non-available data, where the breeding individual was never
 identified at the nest-box and therefore their age cannot be estimated.
 "Disc.age" - age; 1, 2, 3, 4, or 5+ (in years since hatching).
 NA refers to non-available data, where the breeding individual was never
 identified at the nest-box and therefore their age cannot be estimated.
 "Cont.age" - age; 1-9 (in years since hatching). NA refers to
 non-available data, where the breeding individual was never identified at
 the nest-box and therefore their age cannot be estimated.
 "Imm.status" - whether individual hatched within (Resident) or
 outside (Immigrant) the study site. NA refers to non-available data, where
 the breeding individual was never identified at the nest-box and therefore
 their residency status cannot be estimated. "Settled.status" -
 whether the individual immigrated into the study site within the last year
 (New) or not (Settled). NA refers to non-available data, where the
 breeding individual was never identified at the nest-box and therefore
 their residency status cannot be estimated. "April.lay.date" -
 day that the first egg is laid starting from April 1st (i.e. 1 = April
 1st). NA refers to non-available data, where the date at which the first
 egg was laid cannot be calculated due to fieldworker error.
 "Clutch.size" - number of eggs laid\ "Num.chicks" -
 number of chicks hatched "Num.fledglings" - number of chicks
 fledged "Binary.succ" - binary measure of if any chicks fledged
 (1) or not (0) 2. nest.box.attributes.csv. Nest-box information with
 columns: "Box" - nest-box id "X" - X coordinate of
 nest-box "Y" - Y coordinate of nest-box "Num.oaks" -
 number of oaks within 75m of nest-box "Num.occupied" - number of
 times the nest-box has been occupied 1965-2022 "Num.occ.dstnct"
 - number of times the nest-box has been occupied 1965-2022, excluding
 occupation where either the male or female has previously occupied the box
 "Boxes.30" - number of nest-boxes within 30m of the focal
 nest-box "Pop.index" - the long-term popularity index of a box,
 derived from the residuals of a linear model where the number of times
 occupied (excluding cases where either the male or female has previously
 occupied the box) ~ number of boxes within 30m 3. wood.outline.rds.
 Polygon of study site border 4. wood.outline.linestring.rds. Linestring of
 study site border Code scripts ANALYSIS - Age-specificity in territory
 quality and spatial structure in a wild bird population.R. SOFTWARE
 VERSIONS R version 4.4.1 (2024-06-14) Platform: x86_64-apple-darwin20
 Running under: macOS 15.3.2 Packages: dplyr_1.1.4 stringr_1.5.1
 ggplot2_3.5.1 GGally_2.2.1 Matrix_1.7-0 lme4_1.1-35.5 assortnet_0.20
 sf_1.0-16 tidyr_1.3.1 geodist_0.1.0 spaa_0.2.2 spData_2.3.1 spdep_1.3-5
 progress_1.2.3 sp_2.1-4 broom.mixed_0.2.9.5 gt_0.11.1 purrr_1.0.2