May June

Figure 14.9 Settling and egg-laying dates of Pied Flycatchers Ficedula hypoleuca in broad-leaved and coniferous forests in central Sweden. In the preferred broad-leaved areas, food supply increased earlier in spring, the birds settled earlier and at greater density, and annual breeding success (in terms of young per female) was higher than in coniferous areas. Redrawn from Lundberg et al. (1981).

May June

Figure 14.9 Settling and egg-laying dates of Pied Flycatchers Ficedula hypoleuca in broad-leaved and coniferous forests in central Sweden. In the preferred broad-leaved areas, food supply increased earlier in spring, the birds settled earlier and at greater density, and annual breeding success (in terms of young per female) was higher than in coniferous areas. Redrawn from Lundberg et al. (1981).

disturbance, local food supplies or proximity to good feeding areas (whichever was most relevant in the population concerned), or more directly from the feeding rates, survival or reproductive rates of previous occupants. On the basis of such assessments, places classed as best are generally occupied first each spring. As these places become occupied, later arrivals are relegated to poorer places. For example, among migrant Willow Ptarmigan Lagopus lagopus in Alaska, the first birds to arrive settled near a stream, while later settlers were forced by territorial competition onto adjacent hillsides, where food was shown to be poorer (Moss 1972). In a previous year, when numbers were lower, only the stream area was occupied. Similarly, Pied Flycatchers Ficedula hypoleuca arriving in central Sweden in spring settled in deciduous areas in preference to coniferous (Figure 14.9). In deciduous areas, food supply increased earlier in spring, the birds settled at greater density, laid earlier and raised larger broods than in the conifers. Also, the males (but not the females) that occupied deciduous areas were larger than those in coniferous areas, a difference attributed to the effect on settling patterns of fights and other interactions (Lundberg et al. 1981).

Patterns of spring settlement, with the best areas occupied first, have been described in many other bird species, including Painted Bunting Passerina ciris (Lanyon & Thompson 1986), Great Reed Warbler Acrocephalus arundinaceus (Bensch & Hasselquist 1991), Collared Flycatcher Ficedula albicollis (Wiggins et al. 1994), Savi's Warbler Locustella luscinioides (Aebischer et al. 1996) and Northern Wheatear Oenanthe oenanthe (Currie et al. 2000). In several such species, the same sequence of territory settlement held year after year in the same area, even though the occupants changed, and even though some early settlers were displaced by former owners which returned later. In some species, however, the sequential pattern was disrupted by site-fidelity, as some returning birds settled in poor areas where they had previously bred, despite the presence of vacancies in better habitat (Lanyon & Thompson 1986). Site-fidelity sometimes led individuals to re-occupy for several further years habitats that had deteriorated (Hilden 1965, Wiens & Rotenbury 1986). The use of such areas declined over time, however, as existing occupants died or left, and were not replaced.

In not all species do individuals establish a nesting territory as soon as they arrive in breeding areas. Instead they feed in flocks for some days or weeks before they eventually take up a nest-site (e.g. Pied Avocets Recurvirostra avosetta, Hotker 2002). Acquisition of the best nesting places in such species cannot therefore be due directly to early arrival, but birds that arrive earliest may also be the ones most able to compete for the best nesting places.

Manipulation of arrival date

The better breeding of early-arriving individuals, recorded in many species, could have been because early birds were of higher quality and would have bred well at any time in the season, or because they acquired the best territories, or because date itself was important, perhaps in relation to food supplies for the young. In an attempt to test whether individual quality or date was most important, Cristol (1995) delayed the effective arrival date of female Red-winged Blackbirds Agelaius phoeniceus. The females arrived when their Indiana breeding marshes were still ice-covered, and almost two months before nests were built. Newly arrived females were caught, and some were released immediately (the controls), while others were removed to an aviary and released later, but before the control females had started nest-building. The subsequent breeding performance of both groups was compared. In this polygynous breeder, the late-released birds were subdominant to the control females mated to the same male, suffered from less male help later in the cycle, and raised fewer young. The 20 delayed females nested, on average, one week later than the 30 controls, a date difference expected to reduce breeding output. In this species, therefore, early arrival conferred a reproductive advantage via an enhancement of social dominance over late-arriving birds, and when the early arrivals were removed from social contact, they lost this prerogative.

These findings from a polygynous species may not apply to monogamous ones. Another experimental study involved manipulation of clutch sizes and hatching dates of Collared Flycatchers Ficedula albicollis on Gotland Island, Sweden (Wiggins et al. 1994). Findings suggested that both bird/territory quality and environmental changes during the season contributed to the seasonal decline in reproductive success.

The role of territory establishment in the regulation of breeding density

The first arrivals in breeding areas can settle in the preferred areas partly because they encounter little or no resistance from other individuals. Typically, they establish large territories which they might later contract to some extent under pressure from later settlers; but as more birds arrive, newcomers find it increasingly difficult to find a place, and eventually local density reaches a plateau (Figure 14.10). The area is then occupied to capacity and no further birds can settle that

Total numbers

Figure 14.10 Model showing how the numbers of territorial and non-territorial birds might change according to changes in the total numbers of potential settlers. C indicates the range of total numbers over which territories could be compressed under pressure from further contenders, and R indicates the range of total numbers over which replacements of dead (or removed) territorial birds could be expected. From Newton (1998b).

At low numbers, all individuals can establish a territory, and territorial behaviour serves merely to space out birds within the habitat. At higher numbers (zone C) an increasing number of birds can establish territories, but an increasing proportion is excluded from doing so, providing a mechanism for the regulation of local density. Territorial behaviour can thus limit density from the start of zone C, even though higher densities can be reached (to the end of zone C) under pressure from further rise in the number of contenders. Beyond C, no further increase in territorial numbers occurs, despite further rise in the number of contenders. At this level, for every additional territorial bird that settles, one must die or leave, perhaps joining a surplus of non-territorial, non-breeders.

Over short periods of years, any one population would normally be expected to fluctuate within only part of the density range shown on the horizontal axis.

year unless they displace others already there. Both adult numbers and subsequent overall breeding output are thereby limited. This pattern of spring settlement has been observed in a wide range of bird species (for reviews, see Klomp 1972, Davies 1978, Newton 1998b). It provides a clear behavioural mechanism that could not only limit densities in a given year, but also regulate densities over a number of years, with increasing proportions of potential settlers excluded as their overall numbers rise. However, the level at which density stabilises can vary from year to year in line with prevailing food supply and other features of the habitat itself (Newton 1998b).

Although this model was developed for territorial birds (Brown 1969), in which competition for space is most apparent, it could apply equally to colonial or flocking birds, summer or winter, given the competitive interactions that occur over food or other resources (Newton 1998b). In fact, any competitive interaction can regulate density, providing it results in an uneven sharing of resources among competing individuals, leading some to remain and others to leave or die. Local densities are thereby brought in line with local resource levels, and the proportion of birds excluded could increase as total numbers rise.

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