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Overwintering in shallow habitats presents a serious obstacle for aquatic invertebrates. Here we investigated the little-known ability of water boatmen (Hemiptera: Corixidae), an aquatic insect, to survive the winter encased in air pockets within the ice of shallow wetlands. We extracted and experimentally thawed large blocks of ice from prairie wetlands in Saskatchewan, Canada, from which we examined the species composition and revival of corixids. While multiple corixid species were present in wetlands prior to freeze-up, a single species, Cymatia americana Hussey 1920, comprised the vast majority of corixids that were found within the ice later in winter. Only 4%-9% of corixids, all Cymatia americana, revived after ice thawing over both study years. Being encased within an air pocket appeared to be necessary for the survival of corixids in the ice, with up to 300 individuals grouped together. Other invertebrate taxa also revived after thawing, including Haliplidae and Dytiscidae (Coleoptera) encased within air pockets both alongside corixids and on their own, as well as Coenagrionidae (Odonata), Phryganeidae and Leptoceridae (Trichoptera), Chironomidae (Diptera), and Physidae and Planorbidae (Basommatophora), which appeared to be encased in solid ice. The ability to overwinter inside ice represents a little understood survival mechanism of aquatic invertebrates in shallow wetlands, which could confer energetic and reproductive advantages to those that endure until spring. Key words: overwintering, winter ecology, aquatic insect, wetland, ice, Corixidae, Introduction Winter is a challenging time for animals in regions that experience sub-zero temperatures. Aquatic invertebrates face a number of chemical and physical stressors resulting from seasonal changes in temperature [...]

Within species, bird parents defend their nests with different levels of intensity. Previous studies have examined several hypotheses for this variation by looking at factors such as morphological characteristics of the parents and the characteristics of the brood, but few have controlled for multiple factors simultaneously. Here we examined a broad range of factors that may influence the nest defense behaviour of Mountain Bluebirds (Sialia currucoides (Bechstein, 1798)), including the adult's morphology (sex, age, plumage colour, body size, and body condition) and characteristics of their brood (size, hatch date, and stage of nestling development). Parental aggression was triggered using a taxidermy mount of an American red squirrel (Tamiasciurus hudsonicus (Erxleben, 1777)). We found no relationship between nest defense and any morphological characteristic of adults, but the intensity of nest defense was positively correlated with the stage of ofspring development, clutch size, and hatch date consistent with parental investment theory. Males also defended nests more strongly than females, consistent with the premise of the renesting hypothesis that males have fewer future opportunities to breed and, therefore, are more invested in the current brood. Our results provide strong support for parental investment theory and renesting as the primary drivers of nest defense intensity in breeding birds. Key words: breeding behaviour, plumage colour, nest defense, predation, parental care, passerine, Mountain Bluebird, Sialia currucoides Chez les oiseaux, les parents font preuve de differents niveaux d'intensite dans la defense de leur nid. Si des etudes passees ont examine plusieurs hypotheses pour expliquer cette variation en considerant differents facteurs comme des caracteristiques morphologiques des parents et les caracteristiques de la nichee, peu de ces etudes ont tenu compte de l'efet simultane de differents facteurs. Nous examinons un grand eventail de facteurs qui pourraient influencer le comportement de defense du nid de merlebleus azures (Sialia currucoides (Bechstein, 1798)), dont la morphologie des adultes (sexe, age, couleur du plumage, taille du corps et embonpoint) et des caracteristiques de leur nichee (taille, date d'eclosion, etape du developpement des oisillons). Un ecureuil roux (Tamiasciurus hudsonicus (Erxleben, 1777)) empaille est utilise pour declencher le comportement d'agression parentale. Aucune relation n'est relevee entre la defense du nid et des caracteristiques morphologiques des adultes, mais l'intensite de la defense du nid est positivement correlee a l'etape du developpement des oisillons, la taille de la nichee et la date d'eclosion, ce qui concorde avec la theorie de l'investissement parental. Les males defendent en outre leur nid avec plus d'intensite que les femelles, ce qui concorde avec la premisse de l'hypothese de la renidification voulant que les males aient moins d'occasions futures de reproduction et, donc, qu'ils s'investissent davantage dans la nichee actuelle. Nos resultats appuient fortement l'interpretation voulant que la theorie de l'investissement parental et la renidification soient les principaux facteurs expliquant l'intensite de la defense du nid chez les oiseaux reproducteurs. [Traduit par la Redaction] Mots-cles : comportement de reproduction, couleur du plumage, defense du nid, predation, soins parentaux, passereau, merlebleu azure, Sialia currucoides, Introduction Nest predation is among the leading causes of reproductive failure for most passerine species (Ricklefs 1969), sometimes causing the loss of half of the nests in a population (Martin [...]

Author(s): Philip L. Munday [sup.1] , Danielle L. Dixson [sup.2] , Megan J. Welch [sup.1] , Douglas P. Chivers [sup.3] , Paolo Domenici [sup.4] , Martin Grosell [sup.5] , Rachael [...]

Lake food web structure dictates the flow of energy and contaminants to top predators, and addition of invasive species can shift these flows. We examined trophic position (TP), proportional reliance on the littoral zone ([Pro.sub.plittoral]), and mercury (Hg) concentrations across the life-span of two predatory fishes, walleye (Sander vitreus) and northern pike (Esox lucius), in lakes with and without invasive virile crayfish (Faxonius virilis). The littoral zone was the dominant foraging zone for both species regardless of size, accounting for 59% and 80% of the diet of walleye and pike, respectively. Both species increased in TP and Hg with body size, as did crayfish. Walleye in crayfish-present lakes had lower [Pro.sub.plittoral], TP, and Hg concentrations compared with non-present lakes, but trophic magnification of Hg through the food web was consistent across all six lakes. These findings underscore a strong role for the littoral zone in channeling energy and contaminants to higher trophic levels and how invasive species can occupy new habitats at low abundance while altering food web structure and contaminant bioaccumulation. La structure des reseaux trophiques de lacs dicte les flux d'energie et de contaminants vers les predateurs de niveau trophique superieur, et l'ajout d'especes envahissantes peut modifier ces flux. Nous examinons la position trophique (PT), le recours proportionnel a la zone littorale ([Pro.sub.plittoral]) et les concentrations de mercure (Hg) durant la vie de deux poissons predateurs, le dore jaune (Sander vitreus) et le grand brochet (Esox lucius), dans des lacs abritant ou non des ecrevisses a pinces bleues (Faxonius virilis), une espece envahissante. Le littoral est la zone d'approvisionnement predominante pour les deux especes, peu importe la taille, representant 59 % et 80 % des regimes alimentaires des dores jaunes et des grands brochets, respectivement. La PT et les concentrations de Hg augmentent avec la taille du corps chez les deux especes, tout comme chez l'ecrevisse. Les dores jaunes dans les lacs abritant des ecrevisses presentent des [Pro.sub.plittoral], PT et concentrations de Hg plus faibles que les dores jaunes dans les lacs sans ecrevisses, mais il y a uniformement amplification trophique du Hg le long de la chaine alimentaire dans les six lacs etudies. Ces constatations soulignent l'important role de la zone littorale dans le transfert d'energie et de contaminants vers des niveaux trophiques superieurs et le fait que des especes envahissantes peuvent occuper de nouveaux habitats en faible abondance, mais modifier tout de meme la structure du reseau trophique et la bioaccumulation de contaminants. [Traduit par la Redaction], Introduction The structure of lake food webs has important implications for energy flow and contaminant bioaccumulation (Vander Zanden and Rasmussen 1996). Both horizontal and vertical food web structure play roles [...]