Habitat use of redstarts breeding in orchards

Although the redstart still is a widespread species in Switzerland, its abundance is low in most regions. Since 1990, the breeding population has considerably declined.

Aims

The study aims to find out whether low and patchy vegetation is an important ecological component in the breeding and foraging habitat of redstarts.

Approach

The study was carried out at three levels:
1. We investigated whether patchy vegetation was more prominent in redstart territories than in control plots within the same orchard, but outside territories. Within 50 meters of the nest (territory) and around a potential nest site (control plot), respectively, the coverage of different vegetation types was recorded.
2. In an area where several redstart pairs had been breeding in recent years (Bruderholz-Süd, canton Basel-Landschaft), we experimentally created 10 stripes of patchy vegetation and bare ground using rotary cultivators. At boundaries between the managed stripes and the surrounding densely growing grassland, we set poles as perches for foraging redstarts. In some plots, supplementary food (mealworms) was supplied. Subsequently, we determined the habitat preferences of redstarts when foraging in the different vegetation types.
3. Experiments with captive redstarts were conducted in aviaries with different types of ground vegetation (dense grassland, low-density grassland, one-year old fallow stripe) and different densities of food (mealworms). Again, foraging habitat preferences were determined.

Significance

Orchards and managed grasslands with fruit trees are important structures in the traditional agricultural landscapes of Switzerland. Since the middle of the last century, habitat quality for the birds typically breeding in orchards has dramatically decreased due to the intensification of grassland management below the fruit trees. Eurasian hoopoe, little owl, wryneck and woodchat shrike have all disappeared from many parts of Switzerland. We argue that, in addition to the decreasing number of orchards, low food availability in the remaining orchards is another key factor accounting for the population declines of birds breeding in orchards. For the redstart in particular, the critical factor is probably the accessibility of prey items rather than their biomass in the vegetation.

Results

The three approaches yielded consistent results on feeding habitat preferences of the redstart. We compared the density of ground vegetation in 24 redstart territories and 24 control plots. In the field experiments investigating the foraging preferences of mechanically managed stripes, we collected data on seven individuals, and in the aviaries, on nine captive birds. The proportion of area covered by dense grassland vegetation was nearly three times lower in occupied redstart territories compared to control plots. Accordingly, redstarts foraged significantly more often in the managed stripes (95 % of all foraging flights) than in the adjacent tall and densely growing grassland. However, after the grass in the orchards had been mown, this difference disappeared. Captive birds confirmed the results from the wild: they avoided the dense vegetation, even when food abundance in the densely growing plots was experimentally increased to be four times higher than in the grassland of low vegetation density.

Project management

This study was conducted as a master thesis by Nicolas Martinez at the University of Basel and supervised by Niklaus Zbinden and Lukas Jenni from the Swiss Ornithological Institute.