Eggs vary substantially in shape, size and weight. Within any species egg shape tends to be relatively constant and similar to that in related species. Some generalizations as to shape include rather spherical eggs in owls and kingfishers, long eliptical eggs in the more aerial species (swallows, hummingbirds), and hyriform (top-shaped) eggs in many species with four-egg clutches (for compactness, e.g. waders) or nesting on bare ground (for stability against rolling, e.g. auks). Egg weights vary across orders of magnitude from a hummingbird's 0.3g to an Ostrich's 1,600g. Egg weight is closely correlated with the weight of the female, though there can be considerable differences in egg weight between different species groups. For example, the 4.5g egg of a 100g Cuckoo to the 21g egg of a 100g shearwater; in part this results from different modes of development.
Is given as the average length and width (both in mm).
Is approximate fresh weight (from a calculation based on egg size). Shell weights are broadly correlated with egg weight but some species laying on bare rock e.g. Guillemot - produce eggs with unusually thick shells over some or all of their area. Several species near the top of their food chains (raptors, herons, etc.) have been shown to lay unusually thin-shelled eggs when contaminated with persistent agricultural organochlorines (such as DDT).
The data presented here are from Schönwetter & Miese (1960) and are generally for the nominate race of each species.
Nest Record Cards
Number of Nest Records Each Year: is the average number of nest records submitted each year to give an indication of how much information is available in the archives as part of the BTO's Nest Record Scheme. To reliably monitor productivity, we need at least 150 cards a year. For many common species we get fewer than this, so we are especially looking for extra cards of these species, particularly where they are Birds of Conservation Concern. We have noted this on the relevant species pages, but a complete list is available here, together with details on how to begin nest recording.
Within most species, the number of eggs laid by a female tends to be relatively constant, but it varies enormously across species. Long-lived birds, such as many seabirds, may lay only a single egg, while some gamebirds and rails can lay clutches of twenty or more. Instances of egg-laying by multiple females in the same nest can lead to apparently larger clutch sizes in some groups, e.g. waterfowl and ostrich.
For all species we give an indication of the most usual, or typical, clutch size that is likely to be encountered. This information is taken from Snow & Perrins (1998) and Harrison (1975). Where data from the Nest Record Scheme or a particular study are available, we also give the mean clutch size ± 1 standard deviation, together with the minimum and maximum sizes recorded (in parentheses) and the sample size. A reference for the study is also given.
In most species, eggs are usually laid at intervals of one or two days, though larger species such as some of the larger birds of prey, may have longer laying intervals. Incubation can start as soon as the first egg is laid, in which case eggs will hatch asynchronously (one or two days apart); when the clutch is completed, in which case eggs hatch synchronously; or at some point in between. Amongst British species, the herons, gulls, birds of prey, owls and near-passerines tend to have asynchronous hatching and the passerines synchronous hatching. There is some evidence that weather conditions, for example particularly cold spells, can extend the normal incubation period by a day or two, particularly amongst passerines. Incubation may be by either, or both, sexes depending on the species, and the most usual pattern is indicated.
For all species we give an indication of the most usual, or typical, incubation period (in days) that is likely to be encountered. This information is taken from Snow & Perrins (1998) and Harrison (1975). Where data from the Nest Record Scheme or a particular study are available, we also give the mean incubation period ± 1 standard deviation, together with the minimum and maximum periods recorded (in parentheses) and the sample size. A reference for the study is also given (after fledging period).
When nestlings hatch they may be either Precocial i.e. down-covered and with their eyes open, in which case they are capable of leaving nest in a day or two of hatching, or Altricial i.e. with little or no down feathers, closed eyes, and incapable of leaving the nest. Amongst British birds, gulls and terns are somewhat intermediate between these two strategies, being capable of walking, but rarely moving far from the nest, these are termed Semi-Precocial. For altricial species, the fledging period is typically the time spent in the nest, though birds may be dependent on their parents for a much longer period out of the nest (in some cases several weeks). For precocial species, the fledging time is harder to measure, as birds may wander far from the nest, but is typically the time taken to grow a set of flight feathers.
For all species we give an indication of the most usual, or typical, fledging period (in days) that is likely to be encountered. This information is taken from Snow & Perrins (1998) and Harrison (1975). Where data from the Nest Record Scheme or a particular study are available, we also give the mean incubation period ± 1 standard deviation, together with the minimum and maximum periods recorded (in parentheses) and the sample size. A reference for the study is also given.
The breeding cycle varies in timing from year to year, mainly in response to weather conditions and food availability. In the most northerly (Arctic) species, no breeding may take place in cold years. Commencement may also vary from one part of the species' range to another, usually commencing earlier in the south and west than in the north and east, there is also good evidence that the breeding season of most species are getting earlier (more details). Information on the termination of the breeding cycle is harder to come by. In principle the time required to complete an appropriate number of broods may be calculated, however, it is not straight-forward to extrapolate the total length of the breeding season - failed breeding attempts may be followed by replacement clutches and multiple broods may be entirely separate events or may overlap to a point where the male feeds the young of one brood while the female is incubating a further brood and so on.
First Egg Date
For species that breed in Britain an indication of the median date of first egg-laying is given, where possible this is given for England only, except for species where the bulk of the breeding population occurs elsewhere, together with an indication of the range of laying dates (in parentheses), excluding extremes. This may be taken as the range of dates on which the first egg of the first clutch may be laid; for multiple brooded species egg-laying (of second or later clutches) may be recorded for a much longer period.
Number of Broods
An indication of the typical number of broods laid by one pair in a season (excluding relacements for failed broods) is given; occassionally a larger numbers of broods (given in parentheses) may be attempted, for example in particularly good years. Information taken from Snow & Perrins (1998) and Harrison (1975).
Number Ringed Each Year: Is the average number of birds ringed (including nestlings) each year to give an indication of how much information is available in the BTO archives. For more details on the British and Irish ringing scheme, click here. Information on movements from birds ringed in Britain and Ireland has recently been summarised in the monumental Migration Atlas.
Survival (or, conversely, mortality) is a key parameter in a bird's life-history and influences many other things, for example long-lived birds are often larger, develop more slowly and lay fewer eggs, both in a clutch and over their lifetime. Annual survival rates and, and consequent life-expectancy, vary enormously between bird species, for example, the life-expectancy of some small rails may only be a few months, while the larger albatrosses can expect to live many decades.
Estimation of survival rates require studies of marked birds, normally through ringing individuals and tallying either the number resighted (particularly when they are fitted with individually identifiable colour marks) or subsequently found dead. Reports of dead birds often yield better estimates of survival rates, since all birds have the potential to be found. Studies of colour marked birds, particularly those restricted to particular area, often measure 'return-rates', which combine an individuals chnace of surviving with the probability that it returns to the area where re-sighting occurs. This will be lower than the 'true' survival.
Mortality is not constant through a bird's life, being highest shortly after hatching. For example, two percent or more of Song Thrushes may die die each day in the weeks immediately following leaving the nest, as they come to terms with their environment (Robinson et al. 2004). By the time they reach adulthood, the mortality rate will have decreased to about one bird in every hundred dying each week. Thus, most Song Thrushes (and other birds) that hatch will live for only a few months; only about 20% will survive to breed. This example will be broadly typical for most passerine birds, but less so for longer-lived species, such as seabirds and waders.
The average proportion of adult birds (i.e. of at least breeding age) which survive each year. Figures are only included if they are considered to be reliable estimates of true survival rate, i.e. studies which estimate return rates to a particular area are excluded. Where possible, the estimates refer to British birds.
Average proportion of birds in the first year of life which survive each year.
This is the total age (in whole years) that an individual can be expected to live, given that it reaches breeding age (Ba). It is calculated from the adult survival rate (phi) as: ex = Ba + (-1/ln(phi)). Comparable figures for people living in the U.K. are 77 and 81 years for men and women respectively. It should be noted that most indviduals that hatch will not make it to breeding age, mortality rates early in life are high.
Age At First Breeding
The age at which a typical individual starts breeding. This will be correlated with overall life expectancy, the longer a bird lives, typically the more time it takes to develop and mature and consequently start breeding. There will also be some variation in this, with individuals of particularly good, or poor, quality, attempting to breed earlier, or later, than the average.
Maximum Recorded Lifespan
The longest time an individual has been recorded carrying a ring. Where a bird is marked in the nest, this will be the true age, but for most birds it will be less than their actual age, as they will have been ringed as free-flying birds. Tables of longest lifespans recorded are available for species in Britain, Europe and North America.
Harrison, C. 1975. Nests, eggs and nestlings of British and European Birds. Collins, London.
Robinson, R.A., Green, R.E., Baillie, S.R., Peach, W.J. & Thomson, D.L. 2004. Demographic mechanisms of the population decline of the song thrush Turdus philomelos in Britain. Journal of Animal Ecology 73:670-682.
Schönwetter, M. & Miese, W. (1960-1992) Handbuch der Oologie. Academie Verlag, Berlin.
Snow, D.W. & Perrins, C.M. (1998) The birds of the western Palearctic, Concise ediditon. Oxford University Press.