The effects of habitat gaps on breeding success and parental daily energy expenditure (DEE) were investigated in great tits (Parus major) and blue tits (Cyanistes caeruleus) in urban parkland (Cardiff, UK) compared with birds in deciduous woodland (eastern England, UK). Tree canopy height, the percentage of gap in the canopy and the percentage of oak (in the wood only) within a 30 m radius of nest boxes were obtained from airborne remote-sensed data. Breeding success was monitored and parental DEE (great tits: both habitats; blue tits: park only) was measured using doubly labelled water in birds feeding young. In the park, mean (± SD) tree height (7.5 ± 4.7 m) was less than in the wood (10.6 ± 4.5 m), but the incidence of gaps (32.7 ± 22.6%) was greater (9.2 ± 14.7%). Great tits and blue tits both reared fewer young in the park and chick body mass was also reduced in park-reared great tits. Park great tits had a higher DEE (86.3 ± 12.3 kJ day -1 ) than those in the wood (78.0 ± 11.7 kJ day -1 ) and, because of smaller brood sizes, worked about 64% harder for each chick reared. Tits in the park with more than about 35% gap around their boxes had higher DEEs than the average for the habitat. In the wood, great tits with less oak around their boxes worked harder than average. Thus structural gaps, and functional gaps generated by variation in the quality of foraging habitat, increased the costs of rearing young.Keywords airborne LiDAR · ATM multi-spectral · blue tit · energy expenditure · great tit · habitat quality · habitat structure · parkland · reproductive success · urban birds

Mixed-species flocks result from co-evolved relationships between participants: interspecific gregariousness that varies in strength in different species pairs or groups. Such inter-specific associations of birds are a characteristic of most avian communities, observed in diverse habitats from forests to grasslands, estuaries to the open ocean, worldwide in both high latitudes and in the tropics. We explore the consequences of mixed-species flocks in shaping the selective environment and discuss whether the participants in flocks should be identified as niche constructors. As a result of the decision to associate with other species, a bird alters its relationship with competitors; potentially gains access to resources; becomes safer from predators; and may change microhabitat use. The recurrent patterns in the behaviour of disparate unrelated species active in mixed-species flocks have led investigators to conclude that similar selective pressures have shaped their behaviour. However mixed-species flocks are variable in their characteristics, the birds active in them are diverse in form and behaviour, and the selective pressures that have shaped their propensity to join mixed-species flocks must be various. In forming mixed-species flocks, species with specialised roles at the centre of flocks are creating a complex social environment that represents an important biotic selective force shaping their own ecology and behaviour, and that of other species within the avian community. In this review we consider how the spectrum of inter-specific relationships in mixed-species flocks make them an interesting focus for further studies of niche construction by relocation.

We studied hydrographic structure, zooplankton distributions, and foraging by planktivorous seabirds in the Anadyr Strait, northern Bering Sea, during 4 summer cruises (1984)(1985)(1986) 1993). The western portion of the strait was occupied by cold, dense Anadyr water that was mixed from top to bottom. This mixed water was separated from the stratified Bering Shelf water on the eastern side of the strait by a sharp surface front (the 'Anadyr Front'). Net sampling indicated that calanoid copepods were the numerically dominant component of the zooplankton, and that densities of several species were elevated in the frontal zone, apparently due to mechanical accumulation resulting from surface convergence. Hydroacoustic surveys showed that overall zooplankton biomass was concentrated along the thermocline and at the front. Although the location of the Anadyr Front was highly variable over time scales as short as 1 d, large numbers of least auklets Aethia pusilla often flew 25 to 50 km from their breeding colonies to feed at the front. Diet samples indicated that the copepod Neocalanusplumchrus was the principal prey taken by least auklets both at the front and away from it, indicating that heavy use of the distant frontal habitat was due to the higher densities of their preferred prey (i.e. rather than absence of suitable prey species closer to shore). Whenever aggregations of least auklets were found away from the front, there was evidence that they were exploiting near-surface hgh-density patches of zooplankton, though the exact mechanisms responsible for the formation of such patches are unclear. In contrast to least auklets, crested auklets Aethia cristatella were usually found away from the front. In several cases, compact aggregations of crested auklets were located over acoustically observed epibenthic layers of zooplankton. Hydrographic data suggested that intense subsurface jets and/or upwelling along the eastern side of the strait might have increased the availability of the crested auklets' preferred euphausiid prey. Thus, spatial segregation of the 2 principal planktivores in Anadyr Strait likely arises because different physical mechanisms cause concentrations of preferred prey originating at different depths.

There is increasing recognition of the importance of "positive interactions" among species in structuring communities. For seabirds, an important kind of positive interaction is the use of birds of the same species, birds of other species, and other marine predators such as cetaceans, seals and fishes as cues to the presence of prey. The process by which a single bird uses, say, a feeding flock of birds as a cue to the presence of prey is called "local enhancement" or "facilitation." There are subtly different uses of each of these terms, but the issue we address here is the ubiquity of positive interactions between seabirds and other marine predators when foraging at sea, and whether as a result of their associations the feeding success, and therefore presumably the fitness, of individual seabirds is increased. If this contention is true, then it implies that conservation of any one species of seabird must take into consideration the status and possible conservation of those species that the focal species uses as a cue while foraging. For example, conservation of great shearwaters (Ardenna gravis), which often feed over tuna (e.g., Thunnus) schools, should take in to consideration conservation of tuna. Ecosystem management depends on understanding the importance of such processes; the loss of biodiversity, and the consequent threat to foraging success, may be a substantial threat to the stability of marine ecosystems.

Abstract-Scyphozoan jellyfish are preyed on by 11 species of birds in the Bering Sea: Fulmarus glacialis, Pufinus griseus, Pujinus tenuirostris, Oceanodroma furcata, Laws hyperboreus, Rissa tridactyla, Rissa brevirostris, Uria aalge, Uria lomvia, Cyclorrhynchus psittacula, and Aethia cristatella. Parasitic amphipods on scyphomedusae also contribute to avian diets.Predation by birds on gelatinous zooplankton has been documented anecdotally (Anthony 1895;Foxton 1966;Madin 1974;Murie 1979), but has not been suggested as a significant trophic pathway. Gelatinous organisms are abundant in the pelagic environment and are important in creating spatial heterogeneity, acting as "floating substrates" for a suite of other organisms (Hamner et al. 1975). Medusae, salps, and siphonophores, with their parasitic or symbiotic amphipods (Lava1 1980), may constitute easily captured patches of prey for marine birds. I here report 11 species of arctic birds feeding on scyphozoan jellyfish and their associated amphipods. For only one of the eleven, the northern fulmar (Fulmarus glacialis), has feeding on medusae been previously described (Anthony 1895;Murie 1979).The stomachs of 17 species of birds were examined for evidence of predation on jellyfish: F. glacialis; Pufinus griseus (sooty shearwater); Pufinus tenuirostris (shorttailed shear-water); Oceanodroma furcata (fork-tailed storm-petrel); Larus hyperboreus (glaucous gull); Larus glaucescens (glaucous-winged gull); Rissa tridactyla (black-legged kittiwake); Rissa brevirostris (red-legged kittiwake); Uria aalge (common murre); Uria lomvia (thick-billed murre); Cepphus columba (pigeon guillemot); Ptychoramphus aleuticus (Cassin's auklet); Cyclorrhynchus psittacula (parakeet auklet); Aethia pusilla (least auklet); Aethia cristatella (crested auklet); Fratercula cirrhata (tufted puffin); and Fratercula corniculata (horned puffin). Nine species of birds (2 15 individuals)were collected in the southeastern Bering Sea in August 1982 by D. Schneider and N. Harrison. Additional collections were made in October 1982 near St. Matthew Island (7 species, 25 individuals) and in July 1983 in the northern Bering Sea, St. Matthew Island, and the Pribilof Islands (9 species, 86 individuals) by G. Hunt, Z. Eppley, and N. Harrison. All birds were killed with a 12-gauge shotgun. The digestive tracts of the birds were removed immediately after collection, opened, and preserved in 80% ethanol. Reference samples of scyphomedusae were collected and identified by J. Morin and W. Hamner. These samples were preserved in both Formalin and 80% ethanol, the latter for comparison with preserved avian stomach contents. The study was completed as part of the multidisciplinary investigation of the southeastern Bering Sea (PROBES).Jellyfish have probably been overlooked in the analysis of avian stomach contents in earlier studies because of the rapid breakdown of their tissue. I observed the deterioration of the medusae preserved in ethanol through time and learned how to identify the shreds of dehydrated tissues...