S. James Reynolds

Winter feeding of birds increases productivity in the subsequent breeding season.

Supplementary food given to birds can have contemporary effects by reducing the risk of starvation, increasing survival and altering movements and reproductive performance. There is, however, a widely held perception that birds benefit from extra food over winter, but that it is better that they ‘look after themselves’ during breeding. Here we describe a landscape-scale experiment showing for the first time that the effects of increasing food availability only during the winter can be carried over to the subsequent breeding season. Even though food supplementation stopped six weeks prior to breeding, birds living on sites provisioned over winter had advanced laying dates and increased fledging success compared with birds living on unprovisioned sites. Thus, supplemental feeding of wild birds during winter, in a manner mimicking householders provisioning in gardens and backyards, has the potential to alter bird population dynamics by altering future reproductive performance. With levels of bird feeding by the public continuing to increase, the impacts of this additional food supply on wild bird populations may be considerable.

Food supplementation and possible mechanisms underlying early breeding in the Florida Scrub-Jay (Aphelocoma coerulescens).

Food supplementation studies demonstrate the importance of resources in the timing of reproduction. Studies of Florida Scrub-Jays (Aphelocoma coerulescens) found that supplemented jays bred earlier than unsupplemented jays and that protein may play a critical role. In this study, free-living scrub-jays were provided with supplemental diets high in fat and protein (HFHP) or high in fat and low in protein (HFLP). Jays in both treatments bred earlier than unsupplemented controls (CNT), but HFHP-supplemented jays bred earlier than HFLP jays. To assess possible mechanisms, we measured testosterone (T) in males, estradiol (E2) in females, and corticosterone (CORT) in both. HFHP males had higher T than HFLP and CNT males, but treatment did not affect E2 levels of females. Pilot studies of scrub-jays in suburban environments suggest that the spatial and temporal predictability of food may influence corticosterone (CORT) levels. Suburban jays have year-round access to human-provided foods and breed earlier than wildland jays; thus, we compared CORT in all treatments in the natural site (wildlands) with those of suburban jays. CORT levels of suburban jays were lower than HFLP, HFHP, and CNT jays. HFHP-supplemented jays had lower CORT levels than those of HFLP and CNT jays. The observed differences in the timing of breeding, both between suburban and wildland populations and between experimental groups in the wildlands, may result from differences in the spatial and temporal predictability of food, and the nutritional differences in diets. Because CORT can negatively affect the reproductive axis, we postulate that nutrient availability, the predictability of food, CORT levels, and initiation of reproduction are inextricably linked.

Nutritional quality of prebreeding diet influences breeding performance of the Florida scrub-jay

Food supplementation studies of breeding birds have traditionally concentrated on energetic constraints on breeding performance. It is only recently that the nutritional quality of the prebreeding diet has also been considered influential. We examined the importance of specific nutrients in the prebreeding diet of the Florida scrub-jay (Aphelocoma coerulescens). Birds were provided with one of two supplements (rich in protein and fat or rich in fat only) prior to breeding in 2000 and 2001 and their breeding performance, in relation to unsupplemented (control) birds, was examined. Birds receiving both supplements significantly advanced laying in both years, and increased clutch size in 2000 but not in 2001. Laying date explained variation in clutch size in birds on dietary supplements. Egg mass and volume declined with laying order, irrespective of dietary treatment, but birds on the high fat, high protein diet laid heavier third eggs than controls and this was independent of laying date. Laboratory analysis of 14 abandoned and unhatched eggs revealed that as egg mass increased so did the absolute amount of protein and water while fat content remained relatively fixed. Using these relationships between the masses of egg components and fresh egg mass, we calculated that heavier third eggs laid by birds on high fat and high protein, compared with those laid by controls, contained more water that may be fundamental to chick growth and survival. This is the first demonstration for an avian species that nutritional quality of prebreeding diet can simultaneously influence laying date, clutch size, and egg size and composition.

Does food supplementation really enhance productivity of breeding birds

Food availability influences multiple stages of the breeding cycle of birds, and supplementary feeding has helped in its understanding. Most supplementation studies have reported advancements of laying, whilst others, albeit less numerous, have also demonstrated fitness benefits such as larger clutches, shorter incubation periods, and greater hatching success. Relatively few studies, however, have investigated the effects of supplementary feeding for protracted periods across multiple stages of the breeding cycle. These effects are important to understand since long-term food supplementation of birds is recommended in urban habitats and is used as a tool to increase reproductive output in endangered species. Here, we compare the breeding phenology and productivity of blue tits Cyanistes caeruleus and great tits Parus major breeding in food-supplemented and non-supplemented blocks in a broadleaf woodland in central England over three seasons (2006–2008). Supplementation was provided continuously from several weeks pre-laying until hatching, and had multiple significant effects. Most notably, supplementation reduced brood size significantly in both species, by half a chick or more at hatching (after controlling for year and hatching date). Reduced brood sizes in supplemented pairs were driven by significantly smaller clutches in both species and, in blue tits, significantly lower hatching success. These are novel and concerning findings of food supplementation. As expected, supplementary feeding advanced laying and shortened incubation periods significantly in both species. We discuss the striking parallels between our findings and patterns in blue and great tit reproduction in urban habitats, and conclude that supplementary feeding may not always enhance the breeding productivity of birds.

The Effects of Electromagnetic Fields From Power Lines on Avian Reproductive Biology and Physiology: A Review

Electrical power lines are ubiquitous in the developed world and in urban areas of the developing world. All electrical currents, including those running through power lines, generate electric and magnetic fields (EMFs). Electrical power lines, towers, and distribution poles are used by birds for perching, hunting, and nesting. Therefore, many bird species, like humans, are exposed to EMFs throughout their lives. EMFs have been implicated in adversely affecting multiple facets of human health, including increasing the risks of life-threatening illnesses such as leukemia, brain cancer, amyotrophic lateral sclerosis, clinical depression, suicide, and Alzheimer’s disease. A great deal of research and controversy exists as to whether or not exposure to EMFs affects the cellular, endocrine, immune, and reproductive systems of vertebrates. Laboratory work has used mice, rats, and chickens as models for this EMF research in an effort to understand better the possible implications of EMF exposure for humans. However, EMF exposure of wild birds may also provide insight into the impacts of EMFs on human health. This review focuses on research examining the effects of EMFs on birds; most studies indicate that EMF exposure of birds generally changes, but not always consistently in effect or in direction, their behavior, reproductive success, growth and development, physiology and endocrinology, and oxidative stress under EMF conditions. Some of this work has involved birds under aviary conditions, while other research has focused on free-ranging birds exposed to EMFs. Finally, a number of future research directions are discussed that may help to provide a better understanding of EMF effects on vertebrate health and conservation. We dedicate this paper to Professor Ross Adey whose exceptional talents as a researcher and mentor will be greatly missed. We thank Ross Adey and Bob McGivern for advice about the physiological effects of EMFs and with respect to avian fertility. K. J. Fernie thanks Hydro Québec, Elliot Block and Dave Bird for logistical support and/or advice during her EMF research, as well as the Canadian Wildlife Service during her current research. We also thank Wolfgang Wiltschko and Roland Prinzinger for access to literature. Financial support for EMF work by K. J. Fernie has been provided by McGill University, Hydro-Québec, the Department of Natural Resource Sciences, the John K. Cooper Foundation, the Wilson Ornithological Society, the Province of Québec Society for the Protection of Birds (P.Q.S.P.B.), the International Osprey Foundation, and the Orville Erickson Memorial Scholarship. Financial support for S. J. Reynolds has been provided by the Natural Environment Research Council, the National Science Foundation, the University of Memphis, and the University of Birmingham.

Dietary Calcium Availability and Reproduction in Birds

Reproduction is energetically costly. Egg formation in birds requires 37–55% of basal metabolism for small passerines and 160–216% for ducks and the Southern Brown Kiwi (Apteryx australis) (Walsberg 1983). The production of eggs requires an adequate supply of water, macronutrients (carbohydrate, protein, and fat), and micronutrients (essential fatty acids, amino acids, vitamins, and ions). Of this latter group of nutrients, calcium is probably the most limiting micronutrient required by the laying bird (Burley and Vadehra 1989); 98% of the dry mass of the eggshell consists of calcite, a crystalline form of calcium carbonate (CaCO3) (Romanoff and Romanoff 1949). The eggshell encases all of the nutrients required for embryonic development and survival during the incubation period, preventing the developing chick from being crushed by the incubating adult. The shell also protects the embryo from dehydration, prevents the entry of pathogens from the external environment, and allows gaseous exchange. It also acts as a source of calcium for skeletal development of the embryo. The withdrawal from the skeleton results in “natural thinning” of the eggshell from ∼6% (e.g., Balkan et al. 2006) to 21% (e.g., Booth and Seymour 1987) as the embryo absorbs calcium from the shell during incubation. After egg production, the calcium requirements of the adult in many species must remain elevated during chick-rearing, when birds feed calcium-rich foods to their young (see Table 3 in Graveland 1996a) for the continued mineralization of their skeletons (Starck 1998).

Local temperature and not latitude determines the design of Blue Tit and Great Tit nests

Recent studies are documenting the extent to which the mass and construction of bird nests varies between individuals and locations. In the Blue Tit (Cyanistes caeruleus) and Great Tit (Parus major), temperatures experienced by females during nest construction are inversely related to nest mass. Moreover, Mainwaring et al. (Journal of Biogeography, 2012) showed that nests constructed at high latitude are heavier and better insulated than nests built by conspecifics in the south. Although mean spring temperature was used as a proxy for latitude in the Mainwaring et al. study, it remains untested whether individual birds build nests in response to a narrower range of temperatures experienced at the start of the breeding season. Our study showed that irrespective of latitude nest mass, and in particular nest cup mass, of Blue Tits and Great Tits was significantly affected by the temperature experienced by the birds for the seven days preceding clutch initiation. Similar results were seen with the insulatory properties of nests. The potential impact of variation in nest construction and insulation on subsequent incubation and chick-rearing behaviour is discussed.

Calcium availability affects bone growth in nestlings of free‐living great tits (Parus major), as detected by plasma alkaline phosphatase

Recent studies have revealed that calcium limitation of avian reproduction may be a widespread phenomenon, affecting both egg properties and chick development. The effect of calcium shortage on the final body size of fledglings is usually rather weak, possibly owing to compensatory growth. Achieving full skeletal size is not, however, a reliable indication of complete ossification and it is possible that chicks with similar tarsus lengths are in different stages of skeletal development. We hypothesized that measuring plasma activity of the bone alkaline phosphatase (ALP), a highly specific marker for bone calcification, may reveal subtle developmental differences in full-grown fledglings, having experienced different levels of calcium availability during growth. In two seasons, a number of pairs of great tit Parus major were provided with calcium-rich material during the nestling period, while others were not supplemented (controls). While no significant differences in size of fledglings were detected between groups, bone-ALP activity at the pre-fledging stage was lower in the calcium-provided nestlings than in the control nestlings. This may indicate that supplemented chicks had completed the rapid phase of bone formation, but this process was delayed in controls. Measuring ALP as a marker of skeletal development expands our knowledge of how delayed skeletal development of chicks can result in protraction of the nestling period, thereby reducing the breeding success of adult birds.

Diet quality during pre-laying and nestling periods influences growth and survival of Florida scrub-jay ( Aphelocoma coerulescens ) chicks

Food availability is one of the most important determinants of successful reproduction in birds. In a previous study, it was shown that Florida scrub-jays Aphelocoma coerulescens maintained a third egg mass when supplemented with a high fat and high protein diet during pre-breeding, as opposed to birds that were not supplemented that laid third eggs markedly lighter in mass than first- and second-laid eggs. Increases in egg quality promote chick growth and survival in many avian species. In this study, scrub-jays were provided with supplements throughout the pre-laying and nestling periods of 2001 and 2002 to investigate whether diet quality influenced growth and survival of chicks. Fledging success and survival to independence of unsupplemented chicks were higher for 2002 compared with 2001 indicating that breeding conditions (e.g. natural food availability) for jays were probably improved in the former year. Consistent growth patterns were found during the first 11 days post-hatch between chicks on supplemented and those on unsupplemented territories. Of the three growth measures, only seventh primary feather growth tended to be greater for third-hatched chicks on supplemented territories compared with third-hatched chicks on control territories. Food supplementation influenced survival of chicks. Compared with chicks hatching on unsupplemented territories, fledging success of chicks hatching from second-laid eggs was significantly greater in 2001, but not in 2002. Chicks from third-laid eggs on supplemented territories also tended to survive better in 2001 compared with chicks from unsupplemented territories, but, again, no such effect was found in 2002. We conclude that food supplementation in the pre-laying and nestling periods results in little measurable improvement in chick growth. However, improvements were found in survival of chicks that may be mediated through improvements in egg quality, but also may be the result of changes in foraging strategies of provisioning birds on supplemented territories. Chicks on supplemented territories might be in better body condition than those on unsupplemented ones. Methods other than morphometrics may be required to detect elevations in body condition of chicks raised on supplemented territories.

Chick Development in Free‐Living Great Tits Parus major in Relation to Calcium Availability and Egg Composition

Evidence from a number of avian studies suggests that limitation of exogenous calcium (Ca) may reduce egg quality and retard nestling growth. However, it is poorly understood whether reduction in chick growth in Ca‐poor areas is due to insufficient intake of dietary Ca or caused by maternal Ca limitation mediated through subtle changes in composition of eggs. In this study, we provide new evidence that Ca availability during egg formation may indeed affect egg composition and influence chick development of the great tit Parus major at early developmental stages. Ca‐supplemented birds breeding in base‐poor pine forests produced eggs with elevated yolk Ca concentration compared with controls, while no such effect of supplementation was detected in case of eggshell thickness. Nestling tarsus length in the first half of the nestling period was positively influenced by both yolk dry mass and yolk Ca concentration. The effect of supplementary Ca did not persist throughout the nestling period; initial effects of egg components disappeared as nestlings aged. We conclude that apparently normal eggs may harbor poor nutrient and mineral conditions for chick growth. Such subtle changes in composition of eggs can depress chick growth, especially in harsh years.