SEPG 1316 - Date Awarded 1996

K Bowley

A Comparative Study of the Song Delivery Method of Sedge Warblers Acrocephalus schoenobaenas Utilising Rape Crops and Traditional Habitats, in North East England

Introduction

For most song birds song behaviour is crucial in attracting a mate and establishing a territory (Cambell & Lack 1985). Hence, the study of a species' song, the vocabulary of which is aimed at attracting a female and repelling rival males, may be key in revealing aspects of the species' habitat utilisation; as well as its mate acquisition (Carthy 1979). The sedge warbler's song is considered unusual in its extreme elaboration and its function, its purpose being almost solely for attracting a female (Catchpole 1973a).

Study Sites

All of the study sites fell within the area delineated by the borough boundaries of the Metropolitan Borough of Gateshead, Tyne & Wear. Gateshead is situated at approximately 54 degrees, 54 minutes N, the mid‑latitude of the sedge warbler's world range (Murton & Westwood 1977).

Oilseed rape is grown widely across the borough, all of the crops studied during the study period, 1996 to 1997, being located at altitudes of between 15 m and 195 m above Ordnance Datum.

Methods

Transect surveys were conducted around the crops and traditional habitat study site during the peak song‑period for sedge warblers, visits being made on a regular basis between late April and early June (Catchpole 1973a) in 1996 and 1997. The presence of sedge warblers was indicated by song activity or the visual observation of birds.

These were usually conducted in the morning, between the hours of four and eight a.m. (B.ST), the most productive period to register singing males (Catchpole 1973a) although a considerable number of transects had to be undertaken at other times of the day, mainly prior to noon but with some afternoon and evening visits.

The singing position of 'advertising song' in the sedge warbler is quite characteristic. Males usually choose a prominent song perch, e.g. a willow or hawthorn, and, in combination with their song and pale underparts, this makes them visually and acoustically obvious in open habitats (Catchpole 1972). Visual advertising is further enhanced in the species by regular song‑flights, in which the song appears co‑ordinated with the elaborate flight (Catchpole 1973a).

In order to determine whether males diffiered in their advertising song behaviour a study of the way in which song was delivered in each of the habitats was undertaken. Each observed 'song incident= was classified into one of three categories:

i) 'concealed', bird in full active song but concealed, or part concealed, within the vegetation or foliage around its song post;

ii) 'perched', bird was in an exposed position on its chosen song post; and,

ii)'song‑flight', bird launched into a display song‑flight (Cramp 1992).

One might consider that singing from a concealed position would minimise the risk from avian predators, such as the sparrowhawk Accipiter nisus, and that perched and song‑flighting incidents would increase such risks (Armstrong 1947). The potential increase in risk of predation, as a result of increased song‑flighting, might be offset by a lack of cover for stealth orientated hunters, in the more open country usually associated with arable crops (pers. obs). By contrast, traditional habitats would provide such predators more cover from which to hunt. If this were the case, reduced song‑flighting by males in traditional habitats may have an adaptive value in reducing predation risk in these more structurally diverse habitats. However, the reverse might be considered true in respect of the potential efficacy of each the song delivery methods with regards attracting a mate; which is (Catchpole 1973a) the principal purpose of sedge warbler's song. Therefore, one might assume, the more obvious a singing sedge warbler is to a passing female, the greater the chance of his pairing with her.

These differences in song behaviour will have differing ecological and energetic costs and benefits attached to them as far as the males are concerned. For instance, the greater the number of song‑flights the greater the expenditure of energy ‑ this might be accommodated if the habitat compensates by easier prey availability, higher prey density or greater energy content per foraged unit. This assumes that the reproductive success attained by birds nesting in rape crops more than offsets any such potential risks/costs to the individual.

It is clear that song flights have significant ecological importance to male sedge warblers. Therefore, one might assume, the observed habitat‑related differences in song delivery behaviour, are likely to be manifested by the differing ecological demands placed upon the birds by the different habitats. Male song birds exhibiting larger song repertoires are more likely to elicit pairing with females (Kroodsma 1976, Catchpole 1980, Catchpole et al. 1984) and, through this, may induce earlier breeding in females, thereby increasing the potential for reproductive success. Perhaps males sedge warblers in rape exhibiting higher frequencies of song‑flighting, are similarly benefited in their relations with female sedge warblers?

Previously, it has been demonstrated that the structure of bird song is correlated with habitat (Morton 1975, Hunter & Krebs 1979), it would appear that, for sedge warblers in rape at least, the manner of the delivery of song is also modified by the habitat, the outcome, more song‑flights.

References