No description available.

Cavity-nesting communities are structured by the creation of and competition for cavities as nest-sites. Viewing these communities as interconnected webs can help identify species interactions that influence community structure. This study examines cavity-nesting bird community interactions within the fire-maintained longleaf pine (Pinus palustris) ecosystem at Eglin Air Force Base, Florida. In chapter 1, I provide a background review of the ecology of my study system. In chapter 2, I use nest webs to depict the flow of cavity-creation and use at Eglin. I identified 2 webs into which most species could be placed. One web contained 6 species associated with pines. The second web contained 5 species associated with hardwoods. Red-cockaded woodpeckers (Picoides borealis) and northern flickers (Colaptes auratus) created most cavities used by other species within this community. In chapter 3, I describe snag densities and nest-site selection of the cavity-nesting bird community at Eglin. Large, mature pine snags were abundant, exceeding other reported densities for southern pine forests. Pine snags were heavily-used, despite the abundance of available red-cockaded woodpecker cavities in living pine. Hardwood snags accounted for 10% of nests found, and were used by 12 of 14 species. Diameters of nest-trees and available snags were below the range of optimal nest-snag diameters reported in other studies, indicating the need for site-specific snag management guidelines. In chapter 4, I combine a study of basic ecological principles with endangered species management to examine interactions within the cavity-nesting bird community at Eglin. I used a nest web to identify a potential indirect interaction between the red-cockaded woodpecker and large secondary cavity-nesters, mediated by the northern flicker. I used structural equation modeling to test a path model of this interaction. By experimentally manipulating cavity availability, I blocked links described in the model, confirming cavity creation and enlargement as mechanisms that influence this indirect relationship. I demonstrated that a red-cockaded woodpecker cavity-management technique could disrupt this indirect relationship by affecting northern flicker behavior, and provided an empirical example of how, in interactive ecological communities, single-species management can have indirect effects on non-target species. / Ph. D.

I conducted demographic and behavioral studies of Western Sandpipers (Calidris mauri) breeding on the Yukon-Kuskokwim River Delta, Alaska (1998-2005). In chapter one, I estimated apparent annual survival (product of true survival and site fidelity) while correcting for the probability of encounter for 237 males and 296 females. Overall return rates (individual returned to the site in a subsequent season) were lower for females (40%) than males (65%), as was apparent annual survival (± SE, females = 0.65 ± 0.05, males = 0.78 ± 0.03), and encounter rate (females = 0.51 ± 0.07, males = 0.74 ± 0.04). In chapter two, I examined the effects of mate and site fidelity on nesting success (N = 430 nests). Annual divorce rate ranged between 37-83%, with 17-63% of pairs reuniting annually. Reuniting pairs initiated clutches earlier than newly formed pairs, and clutches that were initiated early in the season had higher nest success rates compared to late-season nests. When I controlled for clutch-initiation date, nests tended by individuals with prior breeding-site experience had higher daily survival rates compared to birds breeding at the site for the first time. The effect of site experience was greater for males than females. In chapter 3, I reported that Western Sandpipers exhibited aggregated breeding behavior on a 36 ha plot. Breeding aggregations occurred when dominant and/or older individuals excluded younger, subordinate individuals from preferred habitat. The pattern of habitat occupancy conformed to an ideal despotic distribution with aggregated nesting birds in less preferred habitat experiencing lower reproductive success. In chapter 4, I described and demonstrated the form and function of parent-chick communication in the Western Sandpiper. Through experimental playback of adult vocalizations to chicks in the field, I demonstrated: (1) chicks respond to the alarm call by vocalizing relatively less often and moving away from the signal source, (2) chicks respond to the gather call by vocalizing relatively more often and moving toward the signal source, (3) and chicks respond to the freeze call by vocalizing relatively less often and crouching motionless on the substrate for extended periods of time. I also describe two distinct chick vocalizations (chick-contact and chick-alarm calls). / Ph. D.

Positive biotic interactions such as mutualism, commensalism and facilitation are ubiquitous in nature, but historically have received considerably less research attention than negative interactions such as competition, predation and parasitism. The paucity of research on positive interactions is particularly evident in stream ecosystems and in vertebrate communities. Stream fishes clearly provide an ideal system for advancing research on positive interactions. Many minnows (Cyprinidae) of eastern North America engage in a potentially mutualistic reproductive interaction known as nest association, in which individuals of one species (nest associates) spawn in nests constructed by host species. In nest association, hosts provide unsilted gravel substrate for spawning nest associates, and increased parental care to associate broods. High associate: host egg ratios can create a dilution effect, reducing the probability that host eggs will be preyed upon by egg predators. Nest associative interactions are common, but are relatively understudied compared to other interactions among stream fishes. The goals of this study were to apply general ecological models to this novel system to (a) gain new insight into the mechanisms structuring nest associative stream fish communities, and (b) to use inference from stream fish communities to potentially expand and improve the general ecological models. These goals required completion of three objectives, including (1) examining the influence of abiotic and biotic contexts on reproductive behavior and fitness outcomes between a cyprinid host and associate, using the biological markets model to generate predictions; (2) examining the utility of the nest web framework (previously only used for cavity nesting vertebrate communities) and the stress gradient hypothesis (previously applied almost exclusively to plant communities) for predicting which associate species spawn on nests built by various nest building species, and the consequences of these choices, respectively; and (3) using two-species occupancy modeling to determine the relative influence of biotic interactions and habitat covariates on the co-occurrence of a host and two nest associates. To accomplish these goals, I conducted a large-scale experiment to manipulate presence of mutualists (Nocomis leptocephalus, host; Chrosomus oreas, associate), egg predators (biotic context) and habitat quality (abiotic context). I conducted behavioral nest observations and conducted repeated stream fish stream fish community surveys to collect demographic data. I constructed a nest web from observational data, and implemented structural equation modeling through an information-theoretic framework to identify nest web plausibility across a large spatial extent. I tested some predictions of the stress gradient hypothesis by regressing juveniles-per-nest and a metric of cyprinid community structure on a composite measure of physical stress (scaled gradients of catchment-scale agricultural land use and catchment area). I used two-species occupancy modeling to model co-occurrence of N. leptocephalus hosts and two associates, C. oreas and Clinostomus funduloides, and used an information-theoretic framework to compare hypotheses representing the importance of biotic interactions, habitat covariates or both at determining species co-occurrence. Results corroborated some (but not all) model predictions, and identified room for improvement in each of the general models. Nest associative spawning by C. oreas was not context dependent; C. oreas did not spawn in the absence of a reproductively active male N. leptocephalus at any treatment level. However, the net fitness outcome of host and associate species was mutualistic, and the interaction outcome switched from commensalistic to mutualistic with abiotic context. N. leptocephalus reproductive success was improved by C. oreas presence in less-silted habitats, but not in heavily-silted habitats. This is most likely because broods were subject to predation in both habitat types, but were also negatively affected by siltation in silted habitats. Accordingly, egg dilution by associates was not sufficient to support a mutualistic relationship in less favorable habitats. Results suggest that the biological markets model may be a useful tool for predicting fitness outcomes of nest associative mutualism, but may not be as useful for predicting the behavioral outcomes of obligate mutualisms. Future applications of the biological markets model should carefully consider species traits, specifically the degree to which trading behavior is obligate for participants. Future work with this model will yield more insight by considering highly facultative associates. Nest webs constructed from nest observational data suggested an interaction topology in which strong (nearly-obligate) associates relied most frequently on N. leptocephalus nests, and less frequently on nests constructed by Campostoma anomalum. Weak (facultative) associates were seldom associated with nests constructed by either species, and probably spawned before hosts began nesting activity. Structural equation models corroborated this topology throughout the New River basin, although some less-supported model evidence specified some nest association by weak associates. Juveniles-per-nest of strong associates responded positively to physical stress, while this metric for other cyprinid reproductive groups showed no relationship. Proportional representation of Nocomis and strong associates also increased predictably with physical stress. This study suggests that the nest web framework can be informative to systems outside the ones for which it was developed; future studies may be able to use this framework to better understand the role of habitat-modifying species in communities other than cavity nesting terrestrial vertebrates and nest associative stream fishes. This work extended the nest web framework by (a) modeling the outcomes of interactions instead of the interactions themselves, and (b) by using structural equation modeling to test nest web predictions with an information-theoretic framework. This study also suggests that the stress gradient hypothesis can be useful for understanding interaction dynamics in vertebrate communities; this represents the first direct evidence that this model can be used in vertebrate communities. Further, I demonstrate that the stress gradient hypothesis may be extended to predict community structure. However, more research in a diversity of systems will be needed to determine the extent to which this can be applied. This study provides some of the first evidence of large-scale positive co-occurrence patterns in vertebrates. However, the precise roles of habitat covariates and biotic interactions were species-specific. Occupancy results suggest that co-occurrence between N. leptocephalus and nest associate C. funduloides is driven only by reproductive behavioral interactions. Alternatively, evidence suggests that co-occurrence between N. leptocephalus and C. oreas is driven by both nest association and habitat covariates. That two-species occupancy modeling can be a useful tool for comparing difficult-to-test hypotheses involving biotic interactions at large spatial scales. This study represents the first quantitative, multi-scale treatment of positive interactions in stream ecosystems. This study demonstrates that applying general ecological models to stream fish communities can yield new insights about both the study system and the models themselves. While models of negative interactions, food webs and dispersal have been applied to stream fishes, we stand to gain much ground by also considering positive biotic interactions. In doing so, stream fish ecologists will also be able to contribute to the advancement of general ecology, and thus raise awareness for these understudied ecosystems and taxa. / Ph. D.

The Wilson’s Plover (Charadrius wilsonia) is a species of concern in most southeastern U.S. coastal states, where it breeds and winters. The U.S. Shorebird Conservation Plan listed this species as a Species of High Concern (Prioritization Category 4), and the U.S. Fish and Wildlife Service has designated it as a Bird of Conservation Concern (BCC). Despite its conservation status, Wilson’s Plover population trends are poorly understood and little research has been conducted examining habitat factors affecting this species’ breeding and foraging ecology. I collected Wilson’s Plover demographic data and explored which habitat characteristics influenced breeding success and foraging site selection among three coastal habitat types (i.e. fiddler crab (Uca spp.) mud flats, beach front, and interdune sand flats) at Onslow Beach, Marine Corps Base Camp Lejeune, North Carolina, 2008-2009. I observed little difference between years in nest success (≥ 1 egg hatched), failure, and overall nest survival. The majority of nest failures were caused by mammalian predators. For those nests that hatched successfully, greater proportions were located in clumped vegetation than on bare ground or sparsely vegetated areas. In-season chick survival for both years was higher for nests that hatched earlier in the season, and for nests farthest from the broods’ final foraging territory. Productivity estimates (chicks fledged per breeding pair) were not significantly different between years (0.88 ± 0.26 fledged/pair in 2008, 1.00 ± 0.25 fledged/pair in 2009) despite a shift in foraging behavior, possibly related to habitat alterations and availability in 2009. My findings indicate that Wilson’s Plover adults and broods were flexible in establishing final foraging territories; in 2008 all final brood foraging territories were on fiddler flats while in 2009, final foraging territories were sometimes split between fiddler flats, beach front, and interdune sand flats. For those Wilson’s Plovers establishing territories on fiddler flats, area of the flat was the most important feature explaining use versus non-use of a particular flat; area ≥ 1250 m² was preferred. Close proximity to water and vegetative cover were also important habitat features in foraging site selection on fiddler crab mud flats, and in all habitat types combined. My findings will directly contribute to population and habitat research goals outlined in the U.S. Shorebird Plan and will supplement limited data about foraging and habitat use related to Wilson’s Plover breeding ecology. / Master of Science

No description available.

Résumé : En Amérique du Nord, la superficie des monocultures utilisatrices d’intrants tels que des fertilisants et des pesticides couvre aujourd’hui 85 % des surfaces agricoles et autant de pâturages ont disparus en 50 ans afin de répondre aux besoins de l’Homme. Ces changements d’usage des terres ont profondément transformé le paysage et altéré la biodiversité des agro-écosystèmes. Parmi les espèces d’oiseaux champêtres, les insectivores aériens tels l’Hirondelle bicolore, Tachycineta bicolor, ici étudiée, présentent un fort taux de déclin des effectifs, reflétant probablement une dégradation des agro-écosystèmes. Les mécanismes biologiques à l’origine de ce déclin sont encore méconnus ainsi que les patrons de dispersion chez les passereaux migrateurs. Le présent travail vise à étudier les effets environnementaux sur les traits individuels (survie et succès reproducteur) et la dispersion de reproduction (probabilité de disperser et taux d’occupation des sites de reproduction) chez l’Hirondelle bicolore. Pour ce faire, 2200 reproducteurs et 8000 oisillons ont été bagués entre 2004 et 2013 et suivis en reproduction pendant 10 ans sur une aire d’étude au Sud du Québec, laquelle est composée de 40 sites et couvre une mosaïque de paysages agricoles hétérogènes. Le développement d’un nouveau modèle de capture-marquage-recapture, flexible d’utilisation, a permis de réduire les biais d’estimation des probabilités de survie et de dispersion de l’espèce. Cette approche a permis de tester l’effet de plusieurs variables en lien avec la qualité de l’habitat, l’information publique et les caractéristiques individuelles sur la variabilité des paramètres de dispersion, de survie et de succès reproducteur au niveau individuel. Les milieux cultivés intensivement associés à la présence du Moineau domestique, Passer domesticus, un compétiteur pour les cavités de reproduction, diminuaient au maximum de 19 % la survie des mâles. Les femelles étaient quant à elles affectées par le coût de la reproduction, qui était d’autant plus grand en milieu intensif qu’il y avait une présence de moineaux et une disponibilité moindre en ressources alimentaires. Pour autant, la décision de disperser n’était pas affectée par les conditions environnementales que ce soit par la présence de compétiteurs ou la qualité du milieu, et ne dépassait pas les 4 % chez les mâles. Les femelles dispersant jusqu’à 14 fois plus s’appuyaient sur leur expérience personnelle pour décider de disperser. Un échec de reproduction augmentait jusqu’à 7 fois la probabilité de disperser et pour la première fois chez une espèce à courte durée de vie, nous avons montré que la probabilité de disperser était augmentée l’année suivant une première dispersion. Ces patrons de dispersion étaient stables dans le temps. La dispersion semblait donc être un processus coûteux, comparé à la fidélité au site, qui apparaissait comme un phénotype minoritaire dans la population. Elle répondait à des conditions défavorables de reproduction. Enfin, ce travail montre l’utilisation de l’information publique dans la décision de s’établir sur un site généralement fortement dense et productif en jeunes l’année précédente et ce, une fois que la décision de disperser est amorcée. La présence de moineaux et la forte proportion de cultures intensives dans un rayon proche participaient également à ll‘évitement des sites lors de l’établissement. D’après ces résultats, le milieu intensif contribuerait au déclin de l’Hirondelle bicolore. / Abstract : In North America, monoculture areas using high inputs of fertilizers and pesticides cover 85 % of agricultural lands, and as many pastures disappeared in the past 20 years to satisfy human food needs. These land-use changes have deeply transformed landscapes and altered the biodiversity of agro-ecosystems. Among farmland birds, aerial insectivores such as Tree Swallow, Tachycineta bicolor, studied here show a high rate of decline in their abundance, partly reflecting the degradation of agro-ecosystems where they breed. Biological mechanisms leading to the decline and dispersal patterns in migratory passerines are still poorly known. The present work quantifies the environmental effects on Tree Swallow individual traits (survival, reproductive success) and breeding dispersal (probability to disperse and occupation rates of breeding sites). Between 2004 and 2013, 2200 breeders and 8000 chicks were ringed and monitored during 10 breeding seasons on a study area composed of 40 sites and covering a mosaic of heterogeneous agricultural landscapes in southern Québec. The development of a new, flexible capture-mark-recapture model has reduced estimate bias of survival and dispersal probabilities of Tree Swallows. This approach allowed to assess the effect of several variables linked to habitat quality, public information and individual characteristics on dispersal, survival and reproductive success parameters of individuals. Within intensively cultivated landscapes associated to the presence of a nest site competitor, the House sparrow, Passer domesticus, male survival is up to 19 % lower. Females were affected by the cost of reproduction, especially in intensive landscapes where House sparrows and found and where food resources and nest site appear limited. Dispersal probability was not affected by environmental conditions either through the presence of House sparrows or habitat quality, and was restricted to 4 % in males. Females, which dispersed up to 14 times more than males, based their decision on their personal experience. Breeding failure increased up to 7 times the probability to disperse and for the first time in a short-lived species, I show that dispersal likelihood increased if the individual had dispersed in the previous year. These dispersal patterns were stable in time. Dispersal appeared as a costly process compared to site fidelity and was a minority phenotype in the population. It appeared an answer to unfavorable condition for reproduction. Finally, this work shows the use of public information (partners density, fledglings productivity the previous year) on the decision to settle on a site and this, after the dispersal decision had been initiated. The presence of House sparrows and the proportion of intensive areas near nest boxes were used as information to select a breeding site. Based on these findings, agricultural intensification likely plays a role in the decline of Tree swallow populations.

Em espécies que exibem cuidado paternal exclusivo, a qualidade esperada do comportamento paternal pode influenciar as decisões de acasalamento das fêmeas e determinar o sucesso de acasalamento dos machos. Nesta tese, investigamos a inter-relação entre o cuidado paternal e a seleção sexual utilizando o opilião Quindina limbata como organismo modelo. Os machos nesta espécie constroem ninhos na forma de taça que são visitados pelas fêmeas na busca de um sítio de oviposição. No primeiro capítulo, nós experimentalmente avaliamos a eficiência do atendimento aos ovos provido pelos machos e testamos se os ninhos desatendidos são adotamos por fêmeas e/ou por machos não relacionados com os ovos. As conclusões mais importantes deste capítulo são: (1) a proteção do macho é crucial para a sobrevivência dos ovos porque ninhos desatendidos são prontamente atacados por predadores; (2) a compensação do cuidado parental por fêmeas é rara, provavelmente porque elas estão associadas aos machos e não aos ninhos; (3) os machos adotam ovos não relacionados com eles e protegem eles tão eficientemente quanto os machos originais, provavelmente porque o cuidado dos ovos é um comportamento selecionada sexualmente. No segundo capítulo, nós testamos a existência de duas táticas alternativas de acasalamento nas fêmeas: residentes, na qual as fêmeas permanecem perto de um ninho, repelem fêmeas conespecíficas e copulam preferencialmente com um único macho dono de ninho, e visitantes, na qual as fêmeas não permanecem espacialmente associadas a ninhos, não repelem conespecíficas e copulam com vários machos donos de ninhos. Também investigamos se a monopolização de ninhos por fêmeas afeta o sucesso reprodutivo dos machos. As conclusões mais importantes deste capítulo são: (1) o comportamento das fêmeas parece ser uma tática reversível, na qual as fêmeas podem mudar de residentes a visitantes durante sua vida, provavelmente em resposta à condição corporal, e (2) a monogamia social imposta pelas fêmeas residentes pode afetar negativamente o grau da promiscuidade dos machos, mas não sua taça de acasalamento. Em conclusão, nós provemos evidência de que a preferência das fêmeas por machos que provem cuidado pode ter favorecido a manutenção do cuidado paternal. Além disso, demonstramos que a monopolização dos melhores machos ou ninhos pode ter favorecido a evolução das táticas alternativas de acasalamento nas fêmeas / In species exhibiting exclusive male care, the expected quality of paternal behavior can influence female mating decisions and thus determine male mating success. In this thesis, we investigated the interplay between paternal care and sexual selection using the harvestman Quindina limbata as model organism. Males of this species build cup-like mud nests that are visited by females in search of an oviposition site. In the first chapter, we experimentally evaluated the efficiency of egg-attendance provided by males and tested if unattended nests are adopted by females and/or unrelated males. The most important conclusions of this chapter are: (1) male protection is crucial for egg survival because unattended nests are promptly attacked by predators; (2) flexible compensation of parental care by females is rare, probably because they are associated to males and not to the nests; (3) males adopt unrelated eggs and protect them as efficiently as original owner males, probably because egg-attendance is a sexually-selected behavior. In the second chapter, we tested the existence of two reproductive tactics in females: resident, in which females remain close to a nest, repel conspecific females, and mate preferentially with a single nest-owner male, and wanderer, in which females are not spatially associated with nests, do not repel conspecific females, and mate with different nest-owner males. We also investigated whether nest monopolization by females may affect males\' reproductive success. The most important conclusions of this chapter are: (1) females\' behavior seems to be a reversible tactic, in which females can switch from resident to wanderer during their lifetime, probably in response to body condition, and (2) the social monogamy imposed by resident females may negatively affect the degree of male promiscuity, but not male\'s mating rate. In conclusion, we provide evidence that female preference for parental individuals may have favored the evolution and maintenance of paternal care. Moreover, we show that the monopolization of the best males or the best nests may have favored the evolution of alternative reproductive tactics in females

Au cours de leur vie, les individus doivent constamment prendre des décisions qui peuvent fortement affecter leur valeur sélective. Pour optimiser leur prise de décisions, ces individus peuvent utiliser des informations soit issues de leurs propres interactions avec l’environnement (informations personnelles), soit issues de l’observation d’autres individus (informations sociales). La propension à utiliser des informations sociales et leur signification dépend certainement de paramètres individuels et environnementaux. Pour comprendre le potentiel évolutif de ce comportement à priori adaptatif, il est nécessaire de comprendre les causes de ces variations inter- et intra-individuelles. Le but de cette thèse était donc de déterminer les sources de variations individuelles dans l’utilisation d’information sociales hétérospécifiques pour le choix d’habitat de reproduction, chez le gobe-mouche à collier (Ficedula albicollis). A partir de données à long terme et d’expérimentations en nature dans la population de Gotland (Suède), j’ai montré que l’utilisation d’informations sociales n’est pas héritable dans cette population, mais dépend de l’âge et de l’agressivité des mâles, ainsi que de la taille de ponte des compétiteurs au moment où les gobe-mouches font leur choix. A partir d’une expérience de repasse, j’ai également montré que les femelles peuvent ajuster, en fonction de leur propre niveau d’agressivité, leur choix de site de nidification en fonction de caractéristiques de chants supposément liés à la qualité des mésanges charbonnières (Parus major). Cette thèse souligne l’importance de la personnalité dans l’utilisation d’informations sociales hétérospecifiques pour la sélection d’habitat de reproduction dans cette population, et montre que des caractéristiques fines de signaux à l’intention de congénères peuvent aussi être utilisées par d’autres espèces. Cela nous aide ainsi à mieux comprendre les mécanismes évolutifs de ce comportement / All their life, individuals have to make decisions that may strongly affect their fitness. To optimize their decisions, they can use personally acquired information but also information obtained from observing other individuals (“social information”). The propensity to gather and use social information and the information meaning might depend on both individual and environmental factors. Studying what drives within- and between-individual differences in social information use should help us understand the evolutionary potential of this supposedly adaptive behaviour. The aim of my PhD was to empirically investigate sources of variability in heterospecific social information use for breeding habitat selection. I worked on a natural population of collared flycatchers (Ficedula albicollis, Gotland Island, Sweden), a passerine species shown to cue on the presence, density, reproductive investment and nest site preference of dominant titmice for settlement decisions. Using both long term and experimental data, I showed that the use of heterospecific social information, measured as the probability to copy tit nest preference, is not heritable but depends on male age and aggressiveness and on tit apparent breeding investment at the time of flycatcher settlement. Using a playback experiment, I also showed that female flycatchers can fine-tune nest site choice according to (i) song features supposedly reflecting great tit (Parus major) quality and (ii) their own aggressiveness level. This thesis highlights the importance of personality in the use of heterospecific social information for breeding site selection in this population, and broadens the traditionally known sources of heterospecific information to fine song characteristics reflecting heterospecifics’ quality. To fully understand the evolutionary mechanisms and consequences of heterospecific social information use, genetically based plasticity and fitness consequences remains to be explored

La construction du nid chez les fourmis génère des structures assumant de nombreuses fonctions, qui sont en grande partie dépendantes de l’architecture produite. L’omniprésence de ces fonctionnalités contraste avec la forte diversité intra et inter-spécifique de la morphologie du nid. Ce travail a pour objectifs de déterminer d’une part la morphogenèse du nid durant son excavation par les fourmis (Chapitre 3), et d’autre part de quantifier l’impact de la taille du groupe (Chapitre 3) et de la qualité de l’environnement (qualité du matériau, gravité; Chapitres 4 & 5) sur cette séquence. Pour répondre à ces questions, nous avons utilisé un dispositif de creusement en deux dimensions (2D) afin de suivre la dynamique d’excavation et l’évolution de la morphologie du nid au cours du temps. Nous avons tout d’abord mis en évidence (Chapitre 3) un changement brutal de la morphologie du nid au cours de sa croissance. Durant une première période d’excavation homogène, le nid est constitué d’une seule cavité de forme circulaire et au contour régulier. Par la suite, le pourtour de la cavité devient plus irrégulier, l’apparition de ‘‘bourgeons’’ lui donnant une apparence plus plissée. Enfin, la cavité centrale cesse de croître lorsque des ramifications se déploient depuis certains des ‘‘bourgeons’’. Nous avons qualifié de transitions morphologiques ces brusques changements de forme, dont la fréquence d’apparition augmente avec la taille de la population. Notre analyse et le recours à la simulation semblent indiquer que ce phénomène soit basé non pas sur un changement de comportement des ouvrières excavatrices mais sur la densité d’activité au front de creusement. Lorsque celle-ci est importante, des phénomènes d’encombrement se manifestent et le nid est excavé de manière homogène. À l’inverse, lorsque l’encombrement diminue les ouvrières peuvent focaliser leur travail et excavent des galeries. Un modèle analytique (Chapitre 6) a permis d’étudier les conditions d’apparition de cette transition morphologique. Nos autres résultats indiquent que le matériau de construction influence très fortement la morphogenèse du nid (Chapitre 4) : les nids excavés dans un milieu cohésif sont plus fréquemment ramifiés et la transition morphologique apparaît à des surfaces de nid plus petites que dans un milieu granulaire. Il semblerait qu’à nouveau, il n’y ait aucune variation comportementale impliquée, mais que l’environnement joue un rôle de ‘‘médiateur’’ des interactions entre les fourmis: le milieu change le temps d’extraction et donc la probabilité individuelle d’excavation, cette variation est exacerbée par les mécanismes d’amplification à l’œuvre, modifiant alors sensiblement la dynamique collective de creusement et la structure du nid. Enfin, une série d’expériences (Chapitre 5) nous a permis de déterminer l’influence de la gravité sur la morphogenèse. Il apparait que la gravité sert de gabarit à la construction, puisque les nids excavés dans un dispositif à l’orientation verticale sont tous dirigés vers le bas. Aussi, si le nid prend directement une forme de long puit vertical sans jamais présenter de chambre, le mécanisme de transition morphologique reste visible, sous forme de bifurcation des galeries à leur extrémités. Ce phénomène de tip-splitting semble aussi apparaître sous la contrainte de la densité d’ouvrières excavant au fond des galeries. Notre travail a donc mis en évidence un phénomène de transition morphologique capable de produire les modules de base de tout nid - chambres et galeries -, et qui semble découler non pas de modifications comportementales, mais de la seule interaction de l’amplification de l’activité et de l’encombrement au front d’excavation. Ce phénomène ubiquiste propose une explication à la grande variété de structures observées sur le terrain, en lien notamment avec la diversité et l’hétérogénéité des milieux, et la dynamique des colonies. Enfin, les similitudes de la transition morphologique avec les instabilités de croissance observées dans de nombreux systèmes biologiques et physico-chimique invitent à considérer ces derniers phénomènes à la lumière de nos résultats.