Reoccupation Of Local Breeding Areas

In spring, individual birds are assumed to be under pressure to return to breeding areas early, in order to gain precedence in competition for territories or nest-sites, and to gain time, increasing their chance of raising young (Chapter 27). But there are limits to earliness because, unless they have substantial body reserves, birds cannot survive in breeding areas until food becomes sufficiently plentiful there. Typically, among the early-arriving species, individuals first concentrate in particular places where food is available, moving only later to their nesting places as conditions permit. For example, hirundines often first appear in spring over wetlands, where midges first become abundant, and only later spread to their nesting sites, which may be scattered through the surrounding landscape. Montane species often appear first in valleys, moving up as the higher ground becomes snow-free and habitable. These 'pre-breeding areas' could be important in enabling birds to build up body condition in preparation for breeding. Later arriving species seem to settle directly on their territories, especially those returning to their territories of the previous year. Nocturnal migrants, absent in the evening, are often

Day from first arrival

Figure 14.8 The pattern of arrival by Prairie Warblers Dendroica discolor in a breeding area at Bloomington, Indiana. The diagram is based on the combined data from 1958 to 1965, but corrected for annual variation in arrival periods by counting the first day of arrival each year as Day 1. In different years, the first arrival date varied between 11 and 22 April for males, and between 21 and 28 April for females. The mean interval separating the first male and female in each year was 5.1 days (extremes 1 and 9 days). As may be seen, males in general arrived before females. From Nolan (1978).

Day from first arrival

Figure 14.8 The pattern of arrival by Prairie Warblers Dendroica discolor in a breeding area at Bloomington, Indiana. The diagram is based on the combined data from 1958 to 1965, but corrected for annual variation in arrival periods by counting the first day of arrival each year as Day 1. In different years, the first arrival date varied between 11 and 22 April for males, and between 21 and 28 April for females. The mean interval separating the first male and female in each year was 5.1 days (extremes 1 and 9 days). As may be seen, males in general arrived before females. From Nolan (1978).

found on their territories at dawn, when they start singing and chasing intruders (e.g. Nolan 1978). They can switch instantly from migration to reproductive mode.

In many species, re-occupation of local breeding areas each spring follows the pattern depicted in Figure 14.8. In the population as a whole, arrival may be spread over three or more weeks, but most individuals arrive around the middle of the arrival period, within a few days of one another, unless disrupted by poor weather. In most species, males tend to arrive before females, and older individuals in better body condition are commonly seen to arrive and pair up before younger or poorer condition ones (Francis & Cooke 1986, Hill 1988, Lundberg & Alatalo 1992, M0ller 1994). This may be because older, better condition birds depart earlier from the wintering areas (as noted, for example, in the Great Reed Warbler Acrocephalus arundinaceus, Nisbet & Medway 1972, and American Redstart Setophaga ruticilla, Marra et al. 1998). Alternatively, it may be because they spend the winter nearer to their breeding areas (as in many species, Ketterson & Nolan 1983, Chapter 4), or migrate faster (as in many species, Hilden & Saurola 1982, Ueta & Higushi 2002, Chapter 8). Whatever the mechanism, birds in good condition reach the breeding areas before others, and can presumably also better survive the costs associated with early arrival, including a poorer food supply than prevails later in spring (M0ller 1994, Kokko 1999).

Where birds pair up soon after arrival in their breeding areas, early-arriving females would be expected to pair with early-arriving males, and vice versa. This pattern seems to hold not just in birds pairing for the first time, but also in established pairs re-uniting on their nesting areas after spending the winter apart. In several species studied in detail, members of a pair arrived closer in time than expected by chance, even though (as shown by colour-ringing or radio-tracking) partners may have migrated independently of one another, and wintered hundreds of kilometres apart (for Black-tailed Godwit Limosa limosa see Gunnarsson et al. 2004, for White Stork Ciconia ciconia see Tryjanowski 2005). In many species that have been studied, males are surplus to females, and it is the latest arriving males that often end up without a breeding partner.

How might competition for territories and mates influence the arrival patterns of migrants? In mathematical models of this situation, increasing the number of competitors for territories can generate cascading pressure for early arrival, which advances arrival dates even further ahead of optimal breeding dates. If the habitat is saturated, so that latecomers risk not obtaining any territory, or if the worst territories are of much lower quality than the rest, competition may lead to most breeders arriving within a short interval, followed by a much later non-breeding contingent (as seen in some birds of prey and others). The penalties for later arrival are not necessarily greatest for the earliest birds, but for those that have the most to lose if they drop a few places in the arrival sequence (Kokko 1999).

The fact that some populations seem to arrive on breeding areas up to several weeks before they start nesting has been attributed to competition for territories, which provides strong selection pressure for early arrival but, as emphasised above, birds can only respond to that selection if the breeding areas offer sufficient food at that time. An extreme example is provided by the Snow Bunting Plectrophenax nivalis in arctic Greenland, where males arrive in early April, 6-8 weeks before nesting and 2-4 weeks ahead of females (Salomonsen 1967). During this lengthy pre-nesting period, males commonly experience severe storms and temperatures down to —30°C, and considerable mortality can occur. Yet year after year, the males continue to arrive at this early date, apparently in order to compete for territories in the limited high-quality nesting habitat.

The importance of an early return is also evident in some colonial cliff-nesting seabirds, in which pairs compete for limited space on cliff ledges. Apparently in order to secure their sites, birds return weeks or months before egg-laying, and in some species return dates became progressively earlier as populations grew and competition intensified. In some species, such as the Northern Fulmar Fulmarus glacialis, birds are now present on their nest-sites almost year-round in Britain, mates taking turns to guard the site, foraging between times. Eggs are laid in May. Similarly, over most of their breeding range, Common Guillemots (Murres) Uria aalge return to their breeding colonies in late winter or early spring, some two months before the first eggs are laid. Return dates to cliff colonies on the Shetland Isles, off Scotland, became earlier by 25 weeks from March to October during a 10-12 year period. This change coincided with a period of continued population growth, and was attributed to intensified competition for nest-sites (Harris et al. 2006). Autumn returns persisted for about 10 years, after which return dates gradually reverted to late winter, as the population declined. Over the whole period, the correlation between mean annual return date and population size was highly significant (r = —0.695, P < 0.001, n = 29 years). This link between arrival dates and potential competition levels did not rule out an influence of food supply, which, through a period of change, could itself facilitate earli er arrival or overwintering, and at the same time promote population growth.

Not all species follow this pattern, probably because, in the conditions prevailing, birds cannot survive in breeding areas until shortly before nesting can begin. In such species, the gap between arrival and nesting is much shorter, and late-arriving individuals can start breeding activities within days. There are occasional extreme examples, as illustrated by a male Wood Warbler Phylloscopus sibilatrix that was caught on spring migration and ringed on the Isle of Man on 8 May. The following morning, less than 24 hours after ringing, the same bird had established a territory 205 km further north, in Scotland, and by that evening it had attracted a mate and started nest-building (Morton 1986).

The basic problem inherent in the timing of spring migration is that individuals arriving early in breeding areas stand to benefit from greater production of young that year, but they may die if conditions at the time of arrival are bad. Birds that nest at high latitudes often risk facing an untimely cold snap or snowfall that cuts off the food supply, and the earlier in spring they arrive, the more likely this is to happen, killing a large proportion of the early-arriving individuals (Chapters 20 and 28). On the other hand, within the limits of the possible season, the earlier birds begin to nest, the greater the number of young they are likely to produce. Not only do first arrivals tend to acquire better territories, nest-sites or mates than later arrivals, they also lay earlier in the season, and often achieve better nest success. Examples of species showing relationships between arrival date, territory quality, laying date and reproductive success are given in Table 14.2, and include both passerines and non-passerines (see also Sergio et al. 2007). In some of these species, the latest birds to arrive failed to get either a territory or a mate.

The Barn Swallow Hirundo rustica illustrates the conflicting pressures on migration timing, as it benefits greatly from an early start to breeding, but the costs of early breeding are represented in cold seasons by mortality among early-arriving males (M0ller 1994, 2001). In these inclement seasons, birds suffer from snowstorms, when their increased energy demand is coupled with an absence of insect prey. Catastrophic losses early in the season can exert a measurable selection pressure on migration dates if losses hit early-arriving individuals more severely than others, as evident among migrant Cliff Swallows Hirundo pyrrhonata in Nebraska (Chapter 12; Brown & Brown 1998, 2000). Arrival date has a hereditary component in both these swallow species, leading in cold springs to selection for later arrival. If early arrival confers the competitive advantage of prior occupancy, but increases the risk of mortality, arrival date can be viewed as a trade-off between opposing pressures, but sometimes the 'best' males may be able to survive when others could not (Drent et al. 2003). Other features of the individual, besides inheritance, also influence arrival dates. In particular, many individuals may be unable to get sufficient food on migration to travel at the optimal rate and keep up with improving conditions. As a result they arrive later than they otherwise would, and suffer the reproductive consequences. Other individuals, having arrived in breeding areas, may be unable to acquire enough food for egg production at the optimal date. In some species, such as Styan's Grasshopper Warbler Locustella pleskei and Bar-tailed Godwit Limosa lapponica, individuals that arrived in the first part of the season were most successful, but not the very first birds, which did less well (Takaki et al. 2001, Drent et al. 2003). Whether the poor performance of the very earliest arrivals is general in these species, or a feature of the particular study years, remains to be seen.

Territory settlement

In any particular area, it is commonly found after detailed study that territories vary in quality: that is, in the fitness benefits they confer on their occupants. The quality of nesting habitat has often been assessed from measurements of cover,

Table 14.2 Relationship in various bird species between spring arrival date in breeding areas and subsequent breeding performance. In general, an early arrival relative to other individuals resulted in better performance

Species Mate Territory Laying Clutch Young Source acquisition quality date size per nest

Table 14.2 Relationship in various bird species between spring arrival date in breeding areas and subsequent breeding performance. In general, an early arrival relative to other individuals resulted in better performance

Northern Wheatear Oenanthe oenanthe + +

+ + + Currie et al. (2000)

Black Redstart Phoenicurus ochruros + +

Andersson (1995),

0 0

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