Whenever territories vary in quality in a consistent way, individual birds may develop behaviour that allows them to maximize territory quality. Individuals may settle preferentially in good territories or shift to better sites whenever possible. The benefit of this selective behaviour will be especially high if individuals limit all of their activities to the territory, if they remain for a long time in the territory, and if territory quality does not vary over time. Territory quality can be constant also if territories are defended for future purposes instead of, or in addition to, immediate needs. The Eurasian Nuthatch (Sitta europaea) is a small hole-nesting passerine that lives in territorial pairs throughout the year and spends nearly all of its time in the territory. First-year birds establish territories in vacant areas within weeks after fledging but may move to another territory at any time of year. Territory quality may change seasonally because of dietary changes, but territories are probably defended for future purposes as well. These include winter food reserves but may include nest sites or mates.
Studies have shown that birds that shifted territories generally moved to territories with a higher breeding score. Breeding score is therefore considered an indicator of territory quality. Owners of high-quality territories lost less body mass in winter and survived better than owners of low-quality territories. High-quality territories tended to be larger and to contain gardens and oaks but not conifers. Low-quality territories functioned as population reserves by allowing juveniles to settle in summer and wait for a breeding vacancy in a high-quality territory (Matthysen 1990).
Fragmentation of natural habitats is a potential threat to the persistence of animal and plant populations in many different kinds of landscapes and habitat types. Many studies have demonstrated a lower diversity and/or abundance of various organisms in habitat fragments compared with larger habitat tracts. Two major groups of hypotheses to explain these patterns involve changes in population structure and habitat quality, respectively. Changes in population structure include a reduction in population size, diminishing dispersal between patches or between local populations within a metapopulation, and loss of genetic variation. Changes in habitat quality may result from the increasing influence of abiotic and biotic elements from the surrounding landscape (the matrix) on the habitat remnant, or from changes in community structure within the habitat.
Studies on the Nuthatch (Sitta europaea) found no differences in reproductive parameters between fragments and two large forests, nor any relationship with the size or degree of isolation of individual fragments. Failed nests more often were taken over by nest competitors (particularly starlings) in fragments, but this does not seem to affect overall success rates. Pairs nesting in parks have a lower chance to produce recruits than pairs in similar-sized oak fragments, and early broods recruited more offspring than late broods. It is concluded that fragmentation does not affect the suitability of mature oak stands for reproduction of nuthatches within the size range of oak stands frequented by this species (Matthysen and Adriaensen 1998).
- Matthysen.E., (1990) Behavioural and Ecological Correlates of Territory Quality in the Eurasian Nuthatch (Sitta europaea). The Auk, 107, 85-95.
- Matthysen.E., & Adriaensen.F., (1998) Forest Size and Isolation Have No Effect on the Reproductive Success of Eurasian Nuthatches (Sitta europaea). The Auk, 115, 955-963.