BRANTA — Roger Jovani
Scaling in bird coloniality: Individual behaviour, spatial patterns and population dynamics
Institution: Universidad de Sevilla, Spain
Supervisors: José L. Tella, Daniel Oro
Details: PhD 2006 (Completed)
Subject Keywords: Ecology, self-organization, social organization, group living, coloniality, seabirds, storks, raptors, power law, scale, fractals, colony size
Species Keywords: Ciconia ciconia, Falco naumanni, +20 seabirds, White Stork, Lesser Kestrel
Thesis available online here
In this dissertation we explore a new framework and a new quantitative approach to the breeding ecology of colonial (many of them migratory) birds. New patterns of nest distribution and colony sizes are described, and it is shown the power of individual behavioural decisions to create these patterns. On doing so, we fusion previous knowledge about bird coloniality (individual behaviour, population dynamics) with new concepts coming from psychology (bounded rationality) and complex systems research (self-organization in decentralized systems) and with tools such as fractal geometry, statistical physics of long-tailed frequency distributions and individual-based simulation models. We show the potential of this new approach to answer old unsolved questions of bird coloniality research doing a simulation model and analysing empirical data from several colonial birds from Spain, Britain and Ireland, Alaska, Greenland and Canada. The results can be grouped in two interconnected blocks:
PATTERNS OF NEST DISTRIBUTION AND OF COLONY SIZES
We collected data and analysed the spatial distribution of nests of White storks (Ciconia ciconia) breeding in Do├▒ana National Park and surrounding areas (>1,000 nests; 6,400 Km2) during four years. We show that in this population nests do not distribute in any of the known ways. Not random, not uniform (as expected for a colonial species), nor simply clumped! They distributed nests in a fractal way, that is, nests occurred in clusters within clusters at different scales without a preferred scale of aggregation. In other words, there was not a colony scale in the system. That may be a natural explanation of why is so difficult to define what is a colony in our study population but also in many other species and populations. Moreover, independently of how we defined colonies in our population (e.g. a colony is a clump of nests interconnected by less than 50m between neighbouring nests) the same shape on the distribution of colony sizes emerged. That gives some hope that irrespective of how colonies are described in the field, it may be worthy to compare colony size frequency distributions for different species and for same species in different parts of the world – this is was our next step.
Before this thesis the idea about natural patterns of bird colony size variation was that all species show long-tailed frequency distributions (histograms). That is, that all species show many small colonies and some of large ones. The only difference between species was thought to be what means 'small! and 'large! for the different species (how large was the tail of their distribution). We first studied the colony sizes of 20 seabird species breeding in Britain and Ireland (>4 million nests, >18,000 colonies), with data coming from the Seabird2000 project (http://www.jncc.gov.uk/page-1548). We found that different species not only differed in how large where their larger colonies, but also in different quantitative characteristics of their distribution. Particularly interesting was that there were species showing log-normal and others power law distributions of colony sizes. Moreover, analysing together the different characteristics of each species we found that the different species can be distributed in a two-dimensional space and that they show a gradual transition between them. Then, we enlarged this analysis to study populations of the same seabird species occurring in very distant parts of the world (Britain and Ireland, Alaska, Greenland and Canada), thus with potentially contrasting environmental and historical peculiarities. We found that species showed very similar patterns of colony size variation when occurring in distant areas (and again that in a given area species differed in their colony sizes). Thus, we have identified, and shown how to describe, natural patterns of colony size variation, and that these patterns are species-specific more than environment-specific. Thus, for the first time, we have interesting patterns to try to explain, and not only verbal (and weak) ideas of colony size variation. Our next step was to try to understand show these patterns are created.
FROM INDIVIDUAL BEHAVIOUR TO POPULATION PATTERNS
We did two approaches to the scaling-up from individual behaviour to population patterns. First, we linked the properties of colony size frequency distributions with individual behaviour. We did so analysing the size of all the colonies (from 1 to 40 pairs each one) of the endangered migratory falcon Lesser kestrels (Falco naumanni) breeding in the Ebro valley (10,000 Km2) from 1993 to 2000. During this time, the population grew in numbers and expanded along the Ebro valley. In previous works in this same population it was shown that juveniles are very often displaced when trying to settle in large colonies, finally joining to smaller colonies or creating new ones. Thus, we predicted that during population geographic expansion (mainly mediated by juveniles) the distribution of colony sizes would reflect this despotic behaviour of adults towards first breeders. We found that while the initial subpopulation showed a power law distribution of colony sizes, the new subpopulations created during the study period showed a truncated power law distribution. Moreover, that the breakpoint of this power law was around colonies of 10 breeding pairs, that is, at the colony sizes where the despotic behaviour of adults becomes to be relevant. This is because these falcons bred in the roof of small (50m2) farm houses, and a colony of 10 nests means 20 individuals (mainly adults) defending the surroundings of their nests, thus collectively defending the whole colony towards newcomers (mainly juveniles). Thus, we have shown how a very local behaviour (individuals defending the few centimetres-meter around their nests) produce a shift on the size of colonies at large scales (10,000Km2).
Second, we explored through an individual-based model how these (fractal) nest distributions and (power law) colony size distributions are created. We did a very simple model where individuals searching for a nest site were attracted to already nesting individuals. This behaviour is known to operate in colonial (and even also territorial species): conspecific attraction. Individuals searched places at random and then decided to nest there according to an increasing probability shaped by an increasing number of conspecifics. Only by doing so, we created fractal patterns of nest distribution and power law distributions of colony sizes in our simulation. Thus, we suggest that complex spatial nest distributions found in nature (such as in our White stork population) and long-tailed frequency distributions of colony sizes (such as the ones found in the studied seabirds, White storks and Lesser kestrels) may emerge by simple behaviours of individuals adaptively trying to nest near conspecifics, and thus reducing the uncertainty of breeding in previously empty areas, while creating a positive feedback that creates colonies.
Pueyo, S. y Jovani, R. (2006) Comment on 'A keystone mutualism drives pattern in a power function!. Science 313: 1739c
Jovani, R. y Tella, J.L. (2007) Fractal bird nest distribution produces power law colony sizes. Proceedings of ther Royal Society of London B 274: 2465-2469.
Jovani, R., Serrano, D., Urs├║a, E. y Tella, J.L. (2008) Truncated power laws reveal a link between low-level behavioral processes and grouping patterns in a colonial bird. PlosONE 3(4): e1992. doi:10.1371/journal.pone.0001992
Jovani, R., & Mavor, R. (2011) Group size versus individual group size frequency distributions: a nontrivial distinction. Animal Behaviour 82: 1027-1036. pdf doi:10.1016/j.anbehav.2011.07.037
Jovani, R., Mavor, R. & Oro, D. (2008) Hidden patterns of colony size variation in seabirds: a logarithmic point of view. Oikos 117: 1774-1781. doi: 10.1111/j.1600-0706.2008.17065.x
Jovani, R., Schielzeth, H., Mavor, R. & Oro, D. (2012). Specificity of grouping behavior: comparing colony sizes for the same seabird species in distant populations. Journal of Avian Biology 43: 397-402. pdf doi: 10.1111/j.1600-048X.2012.05643.x