Abstract
BackgroundBlack sparrowhawks (Accipiter melanoleucus) recently colonised the Cape Peninsula, South Africa, where the species faces competition for their nest sites from Egyptian geese (Alopochen aegyptiaca) which frequently usurp black sparrowhawk nests. In this paper, we test the hypothesis that multiple nest building by black sparrowhawks is a strategy to cope with this competitor, based on a 14-year long term data set.ResultsTwo main results support the hypothesis: first, the numbers of intact nests per breeding season in black sparrowhawk territories increased as levels of geese interactions increased, specifically when usurpation occurred. Usurpation occurred significantly more often at nests later in the season, and may provide a further explanation for the advancement of the black sparrowhawk breeding season towards earlier breeding attempts which results in an overall extension of the breeding period (over 9 months) that has been found in our study population. Second, nest usurpation had a negative impact on black sparrowhawks’ reproductive performance at the ‘nest’ level, but not at the ‘territory’ level when multiple nests were available within the same breeding season, suggesting that this strategy was effective for dealing with this competitor. However, our results do not rule out long term negative consequences of these interactions, for example, reduced adult survival rates or reduced lifetime reproductive success, due to the higher energy demand required to build several nests each breeding season.ConclusionsOur results suggest that black sparrowhawks avoid direct conflict with this large and aggressive competitor and instead choose the passive strategy in allocating more resources to multiple nest building. Our research further highlights the importance of behavioural plasticity, which might be especially important for city-dwelling species in the face of global urbanisation.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0671-7) contains supplementary material, which is available to authorized users.
Highlights
Black sparrowhawks (Accipiter melanoleucus) recently colonised the Cape Peninsula, South Africa, where the species faces competition for their nest sites from Egyptian geese (Alopochen aegyptiaca) which frequently usurp black sparrowhawk nests
Conflict with Egyptian geese For each intact black sparrowhawk nest we provided a set of binary scores (1 or 0), as follows: (i) firstly, if Egyptian geese were present during any monitoring visit over the entire season or not (1 = geese present, 0 = no geese ever present), (ii) secondly, if geese harassment was ever seen during the breeding season or not and (iii) thirdly, if geese usurpation occurred or not
We found that higher levels of geese interactions, in terms of our territory g-score, correlated with a higher number of intact nests within black sparrowhawk territories (GLMM, Fig. 1 Number of intact black sparrowhawk nests per active territory (71 territories in 14 years; N = 396 records) in each breeding season in relation to (a) the territory g-score (0 = no geese present, 1 = geese present, 2 = nest harassment and 3 = nest usurpation), (b) the morph of the breeding male and (c) the morph of the breeding female
Summary
Black sparrowhawks (Accipiter melanoleucus) recently colonised the Cape Peninsula, South Africa, where the species faces competition for their nest sites from Egyptian geese (Alopochen aegyptiaca) which frequently usurp black sparrowhawk nests. Nest structures are essential for reproduction in many bird species, which typically build one nest per breeding attempt. Build more than one nest (or nest-like structures), sometimes prior to incubation Nest usurpation is relatively common among birds (see [11] reviewing usurpers in avian families worldwide and species being usurped in families; this includes 10 African usurping species described by Martin, Broekhuysen [15]). Nest usurpation can be limited to the use of nonactive or abandoned nests, or may involve violent takeovers. The latter is the focus of our current study
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