Most cavity-nesting birds are considered limited by the availability of nesting sites, either in terms of obtaining suitable cavities for nesting or the time or energy to excavate new cavities. We investigated the rate of nest cavity reuse in a cold, multispecies forest for cavity-nesting birds in southwestern Iran. We measured 80 Syrian Woodpecker (Dendrocopos syriacus) nest trees for 9 years and their cavity reuse by 6 cavity-nesting bird and mammal species. Analysis examined the relationship between nest cavity reuse and the characteristics of the cavities and nest trees. In all cases, cavities that were abandoned after a year of re-occupancy remained unoccupied for the subsequent years thereafter. Once a species had adopted a cavity then it was used by the same species thereafter, with only seven (2.5% of 282) exceptions - in one nest, the occupying species changes in three successive years. Over the entire study period, for all the cavities and occupancies, the average number of years a cavity was reused was highest for the Eastern rock nuthatches and lowest for the Syrian woodpecker, but this was unaffected by species. The Mount Atlas Mastic was used by Syrian woodpeckers most frequently followed by Narrow-laved Ash trees and then Wild Pear trees (n = 54, 22, and 4, respectively). Almost all of the physical characteristics of the tree or cavity did not correlate with the proportion of nest re-occupied, except for circumference at breast height and diameter at breast height, which exhibited significant positive relationships. Vertical depth of the nest cavity was significantly affected by species reusing the cavity, largely because the Eastern rock nuthatches (Sitta tephronota) reused swallower nest cavities. As agriculture increases, logging and the removal of older trees decrease the availability of suitable nesting cavities for birds.

Although more than 60 hypotheses predict associations between characteristics of nest architecture and at least one measure of breeding success, reproductive consequences of nest architecture remain quantitatively understudied. In addition, most detailed studies of animal constructions in natural conditions were conducted on species built inside human-made nest-boxes that facilitate monitoring. Here, we present the first review of methodologies and research findings in studies that examined the reproductive consequences of nest characteristics in wildlife species that build nests outside cavities, mainly focusing on nest-size components, animal-derived nest material, anthropogenic nest material, or fresh greenery (i.e., fresh vegetative materials placed in the nest). We found 84 published field studies that were conducted on 51 non-cavity nesting bird species from 10 different avian orders, mainly members of the Passeriformes (69%). We discuss why most investigations reported weak relationships between aspects of nest design and breeding success. We propose suggestions for future research on bird nests, and also highlight the importance of poorly studied research problems.

Psittaciformes have a broad distribution across the southern hemisphere, and this wide ecological range is coupled with high levels of morphological diversity, particularly in the skull and beak structure, which has been previously linked to diet and body size. This paper studies how the beak and cranial shape vary in relation to predominant diet across a broad taxonomic range of parrots, using 2D geometric morphometric techniques and regression analyses. The data suggest that whilst there are some levels of significance between diet and beak shape, body mass was a much stronger co-variate. This indicated that skull morphology is more likely explained by parrot body mass and that whilst diet may partially explain beak shape in parrots when compared to other avian groups, it is not determining beak shape within the clade. Further study into other factors related to beak morphology, such as bite force, may prove key in explaining the evolutionary forces shaping the skull in parrots and other birds.

Avian flight is possible because of functional adaptations of the skeleton, musculature, and the integument in the form of feathers. Bird species with short wings have a greater stroke amplitude angle than bigger birds with longer wings. The m. pectoralis generates the downstroke and the m. supracoracoideus facilitates the upstroke. Striated muscle has a limited capacity for contraction so greater stroke amplitude angles could reflect a longer muscle. The length of both flight muscles must reflect the size of the underlying skeleton, i.e. the sternum and coracoid. It was hypothesised that, to increase the distance between the sternum and the shoulder, short wings would be associated with longer coracoids, and vice versa. It was predicted that, in an inter-species comparison, coracoid dimensions would exhibit negative allometry with total forelimb skeletal length but isometry with body mass. Values for coracoid length and width, total forelimb length, and body mass were collated from the literature and measured from photographs for 146 species of bird. Phylogenetically controlled analysis showed that coracoid length scaled isometrically with body mass, but coracoid width showed positive allometry. As predicted, both coracoid length and width exhibited negative allometry with total forelimb length. Order of bird was important in determining variation in coracoid dimensions. Although coracoid length will determine muscle length, variation in sternum dimensions will also affect length of breast muscles. Further studies aiming to improve our understanding of the mechanics of flight in birds need to consider the pectoral-sternum anatomy in conjunction with the associated musculature.

Avian incubation is characterised by the contact between the eggs and the bird's skin to transfer heat to increase egg temperature above ambient conditions. Birds can be attentive to the clutch all of the time or, particularly if only one parent incubates, attentiveness may be quite low. Attentiveness is related to egg size with large eggs having high attentiveness, whereas small eggs (<10 g) can have attentiveness ranging from 50% to 100%. Previous studies have suggested that incubation duration is a function of attentiveness albeit for small birds. This study tested the hypothesis that, after controlling for egg size and phylogeny, incubation duration would be a function of attentiveness. Data for 444 bird species representing 24 orders were analysed. Whilst egg mass had a significant relationship with incubation duration, there was no relationship with attentiveness for all of the species or a subset of the passerines. Despite egg temperature drops during an incubation recess, average day-time and night-time temperatures are similar in a range of species. Re-examination of previously reported temperature profiles recorded by dummy eggs over a 24-h period shows that after an incubation recess there seems to be an additional heat flux that raises egg temperature above that seen during night-time periods of constant incubation. The reasons why eggs under intermittent incubation are not considerably cooler than eggs during constant incubation are discussed.

Whilst many birds glide briefly with wings held horizontally, some species maintain this posture for extended periods during soaring. This is considered possible because of the contraction of the m. pectoralis that holds the wing in place, although albatrosses seem to have a physical shoulder lock that helps with this action. However, studies of this flight style have not considered the cranially orientated long-axis rotation of the humerus induced by the contraction of the main flight muscles that would depress the ulna and change the angle of the aerofoil downwards. This study explored whether the m. deltoideus major helps counteract this rotation. Muscle masses were collated from the literature and from dissections of birds to allow exploration of the allometry of muscle masses versus body mass. All muscles exhibited isometry with body mass, but relative to the size of the m. pectoralis, the m. deltoideus major was large but only in a few species that regularly soar or glide for long periods. By contrast, other elevator muscles were less variable among species. The presence of relatively large deltoideus major muscles in soaring species was suggestive that this muscle, since it originates on the scapula extending caudally and inserting on the dorsal humerus, may counteract humeral long-axis rotation around its longitudinal axis during contraction of the breast muscles. The results of this study are suggestive of previously unconsidered substantial roles for other muscles of the pectoral girdle and forelimb during different flight styles in birds.

Numerous hypotheses predict associations between phenotype-associated breeder traits and reproductive success. In secondary cavity-nesting passerine birds, which have been investigated most, up to more than 90 parental characteristics have been linked to at least one field measure of breeding performance. However, within study populations, different parental traits, such as clutch size, first-egg date, egg size, nest architecture, plumage colour, adult morphometry, or song performance, are often weakly correlated across female and/or male breeders. Furthermore, many of these studies have been conducted outside the theoretical framework of the ‘Individual Optimisation Hypothesis’ of clutch size (IOH). IOH predicts that: (i) females with larger clutches will have larger broods at hatching and fledging compared to those with smaller clutches; and (ii) clutch size, which is adaptively adjusted to the parents' ability to rear nestlings, should always maximize the percentage of eggs producing fledglings so that variation in clutch size becomes disconnected from variation in breeding success. In this paper, we present the first detailed review of implications of IOH for parental characters other than clutch size. Our review covered 188 non-experimental studies and 1074 statistical results that examined how parental traits influence breeding success in Western Palearctic Great Tits (Parus major), Blue Tits (Cyanistes spp.), and flycatchers (Ficedula spp.). Clutch size explained one third of the variation in brood size at hatching and fledging within study populations. However, most parental characteristics associated weakly with the number of hatchlings or fledglings, likely because they did not correlate with clutch size. Overall, parental traits were poorly correlated with the proportion of hatchlings and fledglings per egg. We discuss why intraspecific variation in phenotype-associated breeder traits is often disconnected from interindividual differences in breeding success, and highlight the importance of underexplored research problems in avian breeding biology.

Powered flight in birds is reliant on feathers forming an aerodynamic surface that resists air pressures. Many basic aspects of feather functionality are unknown, which hampers our understanding of wing function in birds. This study measured the dimensions of primary and secondary flight feathers of 19 species of parrots. The maximum force the feathers could withstand from below was also measured to mimic the pressures experienced during a downstroke. The analysis tested whether: (1) feather dimensions differed along the wing and among secondary and primary remiges; (2) the force that feathers could withstand varied among the remiges; and (3) there would be isometric relationships with body mass for feather characteristics. The results show that body mass significantly affected vane width, rachis thickness, maximum force, and ultimate bending moment, but the relationship for feather length only approached significance. Many of the proximal secondary feathers showed significantly lower values relative to the first primary, whereas for distal primaries the values were greater. There were isometric relationships for force measurements of primary and secondary feathers with body mass, but there was positive allometry for feather lengths and vane widths. The forces feathers can withstand vary along the wing may be a proxy for the aerodynamic properties of the feathers in situ. Broader taxonomic studies that explore these topics are required for other species representing a range of different orders. A better understanding of the functionality of feathers will improve our understanding of how avian flight works particularly considering the variety in flight style and wing shape in birds.

Thermistors embedded on and within the nest wall were used to measure the changes in temperature between different parts of the architecture of the wall of Song Thrush nests as heat from a light bulb was conducted across the wall. The wood pulp forming the nest cup provided the greatest resistance to heat loss across the nest wall. Despite differences in nest mass and architecture, thermal conductance for Song Thrush nest walls were not statistically different from values previously reported for Common Blackbird Turdus merula nests. Song Thrush nests constructed over many years in the same geographical location exhibited an increase in the thickness of the wood pulp cup and a reduction in thermal conductance, i.e. the most recent nests were better insulated. However, nest mass or thermal insulation did not affect reproductive success between years.

Nest construction is a feature of a range of taxa, yet the functional properties of nests are poorly understood. Avian nests offer thermal insulation, structural support and protection from rain, but to date there are few studies that have explored these functions for mammal nests. Here nests constructed by harvest mice (Micromys minutus) were studied ex situ to determine the thermal insulation provided by the nest wall and the degree to which simulated rainfall was absorbed. Nests were collected from across Great Britain and analysis explored whether nest size and geographical location affected insulation or rainproofing. Nests were constructed from grass leaves woven into an outer wall that surrounded smaller pieces of grass that filled the interior of the nest. Nest mass was positively related to thermal insulation but unrelated to geographical location. By contrast, nest mass was positively related to the amount of water nest absorbed after simulated rain but volume and longitude were inversely related to the time it took the nest to dry out. In many ways, harvest mice nests had similar functional properties to those of small songbirds, i.e., to provide thermal insulation and rainproofing. This study was the first to explore the environmental protection potentially offered to harvest mice by their nests. The study has highlighted our poor understanding of the factors that determine the function of mammal nests. There is scope for more research into the functional properties, e.g., thermal insulation or rainproofing, of a wide variety of mammal nests.

Jaw morphology and function determine the range of dietary items that an organism can consume. Bite force is a function of the force exerted by the jaw musculature and applied via the skeleton. Bite force has been studied in a wide range of taxa using various methods, including direct measurement, or calculation from skulls or jaw musculature. Data for parrots (Psittaciformes), considered to have strong bites, are rare. This study calculated bite force for a range of parrot species of differing sizes using a novel method that relied on forces calculated using the area of jaw muscles measured in situ and their masses. The values for bite force were also recorded in vivo using force transducers, allowing for a validation of the dissection-based models. The analysis investigated allometric relationships between measures of body size and calculated bite force. Additionally, the study examined whether a measure of a muscle scar could be a useful proxy to estimate bite force in parrots. Bite force was positively allometric relative to body and skull mass, with macaws having the strongest bite recorded to date for a bird. Calculated values for bite force were not statistically different from measured values. Muscle scars from the adductor muscle attachment on the mandible can be used to accurately predict bite force in parrots. These results have implications for how parrots process hard food items and how bite forces are estimated in other taxa using morphological characteristics of the jaw musculature. Bite force in parrots has been explored using data from muscle masses. The values were validated by comparison with data for bite force collected from live parrots. The parrot's reputation for a strong bite force is well founded.image

The shape of birds' eggs has fascinated scientists for many years. It is now possible mathematically to describe shape accurately, allowing exploration of the physical and ecological factors driving the evolution of egg shape. However, there has been relatively little consideration of how egg shape is established in the oviduct or, given that even without an external calcitic layer eggs retain their shape, how shape is fixed in the isthmus. This paper proposes a hypothesis that attempts to explain how egg shape is established and fixed in the oviduct. The hypothesis suggests that as the egg mass (i.e. yolk and albumen) moves from the magnum into the isthmus, it is squeezed by the physical restriction imposed by the isthmus lumen and cannot easily move into the isthmus. As the leading edge of the egg mass enters the isthmus, the egg mass in the distal magnum is forced to bulge outwards, resulting in an asymmetrical shape. The various egg shapes observed in birds are, hence, produced by the interaction between the size of the egg mass relative to female body mass, and the degree of the restriction of the isthmus. Thus, a large egg mass, i.e. relative to female body mass, entering a narrow isthmus will produce a pointed egg shape. If the egg mass is relatively small, and the isthmus lumen wide, more of the egg mass could enter the isthmus and the degree of asymmetry would be reduced. It is further proposed that egg shape is fixed during the formation of the shell membranes in the isthmus because the constituent protein fibres permanently stick together as they are deposited. For the first time this hypothesis helps explain the pattern of deposition and characteristics of the calcitic egg in relation to the diversity of egg shapes in birds and reptiles.

There is increasing interest from evolutionary biologists in the evolution of avian bill shape, how the bill is used during feeding and, in particular, the bite forces the bill can deliver. Bite force exhibits isometry with the total mass of the jaw musculature, but there is variation in the functional categories of the jaw muscles in different avian taxa. Qualitative descriptions of the jaw musculature do not allow analysis of the relative contributions that adductor or retractor muscles play in generating a bite force. This study is a meta-analysis of published data for body mass and the mass of the jaw musculature in 66 bird species from 10 orders. The masses of the different muscles contributing to adduction and retraction in closing the jaw, and to depression and protraction in opening the jaw, were summed and allometric relationships explored before investigating the effects of taxonomic order on these relationships. The categories of muscles, and the masses of each category of jaw musculature varied among avian orders. Some species, such as the flightless ratites, had relatively small jaw muscle mass but parrots had an additional adductor muscle. Phylogenetically controlled relationships between body mass and the mass of each muscle category irrespective of taxonomic order were isometric. However, analysis of covariance revealed significant interactions between body mass and taxonomic order. Most orders had low values for body-mass-specific muscle masses in the jaw with the notable exceptions of the Passeriformes (songbirds) and Psittaciformes (parrots). The values of these orders were 3-4 times greater, although the relative amounts of muscles contributing to adduction and retraction were similar in Psittaciformes, but adduction was markedly higher in Passeriformes. The results of these analyses highlight the lack of species-specific data for most birds, which is adversely impacting our understanding of the anatomical features that are determining the functional properties of the bill during feeding.

Bird wings vary in size and morphology in terms of both size and number of feathers and the underlying skeletal anatomy. The number of primary remiges does not seem to vary much between bird species but, by contrast, the number of secondary remiges is reported to range between 6 and 40 depending on bird size. Given that the primaries are attached to the manus, and the secondaries are attached to the ulna, it was predicted that as bone lengths increased with increasing size of the bird, then feather count would increase. Data were collected for 268 species from 25 different orders, and phylogenetically controlled analysis explored the allometry between feather count and bone size. The number of primaries was typically 10 or 11 and did not vary with manus size. By contrast, the number of secondaries increased with ulna length, but only in some orders. For example, in Gruiformes, the number of secondary feathers increased concomitantly with ulna length but despite a two orders of magnitude range in body mass, almost all species in the Passeriformes had nine secondary remiges. It is unclear why, for instance, species with an ulna length of 70 mm can have between 9 and 24 secondaries depending on their order. This variation in secondary feather number can be added to variation in relative wing bone lengths, flight feather lengths, flight feather mechanical properties, and flight feather vane densities as another potential mechanism of adaptation to flight requirements. The apparent constraint of wingspan is scaling as approximately body mass1/3. Further research is needed to explore whether changes in secondary feather number relative to ulna length are accompanied by changes in feather vane width or the overlap of adjacent feathers and how this relates to wing aerodynamics. Bird wings vary in size and morphology in terms of both size and number of feathers and the underlying skeletal anatomy. This study determined the numbers of primary and secondary flight feathers and related these values to the length of the manus and ulna, respectively. The number of primaries was typically 10 or 11 and did not vary with manus size. By contrast, the number of secondaries increased with ulna length, but only in some orders. Songbirds and parrots had 9 or 10 secondaries, respectively, irrespective of ulna length but larger birds in other orders had a positive relationship between secondary count and ulna length.image

More than 40 hypotheses predict associations between features of nest architecture and at least one measure of reproductive success. However, quantitative studies of reproductive consequences of nest characteristics remain scarce. In addition, most studies were conducted on model species of which nests can be easily monitored with artificial nest-boxes. Here, we review the replicability of research protocols and findings in model species, with many repeat studies focusing mainly on nest-size components, animal-derived nest material, or fresh greenery in model species of secondary hole-nesting birds: Blue Tits (Cyanistes caeruleus), Great Tits (Parus major), Tree Swallows (Tachycineta bicolor), and Starlings (Sturnus spp.). The studies looked for correlations between nest traits and aspects of breeding performance that can be easily quantified in the field, such as clutch size, brood size at hatching or fledging, the percentage of eggs that hatch or fledge, or nestling characteristics assumed to reflect qualities associated with survival probabilities (e.g., morphometry, body condition, blood profiles). We discuss the consequences of poor replicability of research methodologies and provide explanations for why many of these studies reported poor associations between nest design and breeding success at different spatiotemporal scales. We also make suggestions for future research.

Animals often leap from substrates that give way under them, such as leaves, soft ground or flexible branches. This provides an added complexity for latch-mediated spring-actuated (LaMSA) jumping animals because the spring-loaded system often works so quickly that neural feedback cannot adjust for errors caused by a yielding substrate. We studied a LaMSA jumper, the grasshopper, to determine how the mechanical properties of a substrate giving way under them would affect the kinematics of the jump. We measured this by allowing grasshoppers to leap from two diving boards, a long one that could generate a whole range of relative stiffnesses, and a shorter, much lighter, but stiffer board. Substrate stiffness was manipulated by then placing the grasshopper on different locations on that diving board, presenting from 30% of the grasshopper's leg stiffness to 200 times the grasshoppers leg stiffness. For platform stiffnesses that were less than that of the grasshopper, take-off velocity and kinetic energy were reduced, but jump elevation (the jump trajectory) was unaffected. For stiffnesses that were greater than that of the grasshopper, there was no effect on take-off velocity and kinetic energy. When jumping from an extremely light and stiff substrate, recoil of the surface allowed the grasshopper to recover some of the lost energy. Consequently, when jumping from substrates that are less stiff than they are (such as floppy leaves), grasshoppers must contend with lower take-off velocities, but jump direction is unaffected.

Nest architecture in birds is highly variable ranging from simple scrapes through to elaborate woven constructions. In species that nest in open situations the nest can have a cup open to the elements or the nest has a dome positioned over the cup. The functional properties of domed nests have yet to be explored and this study compared the thermal insulation and rainproofing properties of nests built by four species of European songbird, two of which build domed nests whereas the others build open cup nests. Insulatory values were recorded using temperature loggers. Nests were exposed to a simulated rain event and the amount of water absorbed and the minutes for the nest to dry were determined. The nests were then deconstructed into their component parts. Materials used to build the nests differed among the species. Differences in thermal insulation were associated with the species and the size of the nest. The amount of water absorbed by a nest during a simulated rainfall event was negatively associated with the size of the nest. Feathers and moss had significant positive effects on thermal insulation and rainproofing, respectively. There was no significant difference between domed and open nests in terms of thermal insulation or rainproofing, except for the time taken for a nest to dry, which showed a significant interaction between nest mass and type of nest. Insulatory values and degree of rainproofing were like data from previous reports for songbird nests of comparable size. That no differences observed between domed and open cup nests in the species studied may reflect similarity among species, although it may be due to a paucity of data from a wider range of species building domed nests.

Abstract: More than 40 hypotheses predict associations between features of nest architecture and at least one measure of reproductive success. However, quantitative studies of reproductive consequences of nest characteristics remain scarce. In addition, most studies were conducted on model species of which nests can be easily monitored with artificial nest-boxes. Here we review the replicability of research protocols and findings in model species with many repeat studies focusing mainly on nest-size components, animal-derived nest material, or fresh greenery in blue tits (Cyanistes caeruleus), great tits (Parus major), tree swallows (Tachycineta bicolor), and starlings (Sturnus spp.). The studies looked for correlations between nest traits and aspects of breeding performance that can be easily quantified in the field, such clutch size, brood size at hatching or fledging, the % of eggs that hatch or fledge, or nestling characteristics assumed to reflect qualities associated with survival probabilities (e.g., morphometry, body condition, blood profiles). We discuss the consequences of poor replicability of research methodologies and provide explanations for why many of these studies reported poor associations between nest design and breeding success at different spatiotemporal scales. We also make suggestions for future research. Keywords: Nest design, Breeding success, Nest-box, Cavity-nesting birds, Replicability