All habitats seem to vary from place to place in quality and attractiveness to the birds they support. One known mechanism through which density dependence in mortality or reproduction could occur during a period of population change involves the 'buffer effect'. This occurs when birds occupy the best habitat areas first and, as they fill these areas to capacity, they spread increasingly to poorer areas as their numbers continue to rise. As survival or reproduction is poorer in the less good areas, the mean per capita performance in the population as a whole declines as overall numbers grow, in a density-dependent manner. Sequential habitat fill of this type is seen: (1) as birds arrive in their breeding areas in spring, or their wintering areas in autumn, when they occupy the best places first, so that later arrivals are relegated to poorer places (Chapter 14); (2) in the annual fluctuations of populations, where numbers remain more stable from year to year in the preferred habitats (or territories) than in the secondary ones (Kluijver & Tinbergen 1953, Zimmerman 1982, Rodenhouse et al. 2003); and (3) in the progressive occupation of habitat areas (or territories) of different quality, as a population grows over a period of years (e.g. Mearns & Newton 1988, Ferrer & Donazur 1996, Lohmus 2001). All these processes, which result from habitat variation, can help to regulate bird populations (for further examples of each type, see Newton 1998b). Food and other resources are involved in the regulation because they influence the quality and carrying capacity of habitats. In the reverse situation, as populations decline, birds usually withdraw from the poorer habitats first, leaving an increasing proportion of individuals in the good habitats, where survival or reproduction is highest.
The Icelandic population of the Black-tailed Godwit Limosa l. islandica wintering in Britain has risen four-fold since the 1970s, but rates of increase within individual estuaries have varied from zero to six-fold (Gill et al. 2001). In accordance with the buffer effect, rates of increase were greatest on estuaries with low initial numbers, and godwits on these sites were found to have lower prey intake rates and lower survival rates than godwits on longer-occupied sites with stable populations. In this species, therefore, population growth could have resulted in a progressively larger proportion of the population wintering in poorer estuaries, with measured consequences on feeding rates and survival. The buffer effect, acting on a large spatial scale, could thus have been a major density-dependent process acting to constrain population growth in this migratory species.
A similar spread to poor sites during a period of population growth was earlier noted in Grey Plovers Pluvialis squatarola wintering in different parts of Britain (Moser 1988), in Brent Geese Branta bernicla wintering in the Netherlands (Ebbinge 1992), and in Great Cormorants Phalacrocorax carbo wintering in Switzerland (Suter 1995). In the latter, the process was stepwise, and each category of habitat experienced a rapid build-up in numbers, followed by stabilisation, before the next type of habitat was occupied. In each of these studies, the first-filled habitat was assumed to be better, in which case survival would have been poorer in the secondary habitats, leading to progressive decline in mean per capita survival as the population grew, although this was not confirmed (but for effects on reproduction of Brent Geese see Ebbinge 1992). Hence, although the presence of secondary habitat would permit a population to attain a higher level than possible in the primary habitat alone, the poorer performance of individuals in secondary habitat would put ever increasing constraints on further population growth.
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