Small habitat islands are inferior breeding habitats but are used by some great tits – competition or ignorance?

Abstract

Great tit breeding performance in small habitat islands in an agricultural landscape was compared to that in large deciduous woods. Clutch size was similar in both habitats, but more nestlings starved and fledgling weight was lower in the small habitat islands. The area of wooded habitat in the territories of tits breeding in the small habitat islands was less than what is common to encounter in 'optimum' habitat, deciduous woods. I suggest that this may be responsible, ultimately or proximately, for the poor performance in the small habitat islands. Two possible mechanisms for this effect are discussed. It is possible that the small habitat islands were perceived as inferior by the tits and used by competitively inferior individuals that also were less successful in raising a brood. However, the results were obtained even when female age and weight were controlled for. Therefore, the competitive ability (and probably also young raising competence) may be the same for tits breeding in small habitat islands as for those breeding in optimum habitat. Such a pattern is conceivable if tits have been selected for maintaining a large territory as a response to the close presence of other tits and not as a means of securing a large feeding area. Historically, territories large enough to reduce predation or mate competition (or any cause of territoriality) may always have contained ample food. The presence of small islands, without close neighbours but with insufficient food, may be evolutionarily new. It is probably typical of modern agricultural landscape.