Morepork diet

The morepork is New Zealand’s only extant native owl, and is common throughout much of the country (Higgins, 1999). The boobook owl of Australia is classified under the same species name (Higgins, 1999), although there are differences in biology between the two (Olsen & Moon, 1990). For more information on the biology of moreporks and boobooks see Chapter One.

Although a number of studies have included information on the diet of moreporks or boobooks (see Higgins, 1999 for summary, as well as Haw & Clout, 1999, Haw et al., 2001 and Penck & Queale, 2002), many were based on a small sample size of birds and/or pellets (Anderson, 1992; Baker-Gabb, 1984; Calaby, 1951; Campbell & Rose, 1996; Clark, 1992; Cunningham, 1948; Daniel, 1972; Imboden, 1975; Moon, 2005; Ramsay & Watt, 1971; Rose, 1996), and only a few of the studies were sufficient for assessment of seasonal differences in the diet (Haw & Clout, 1999; Haw et al., 2001; Lindsay & Ordish, 1964; Penck & Queale, 2002; Stephenson, 1998).

These studies indicate that moreporks are primarily insectivorous, although small amounts of vertebrate prey are also consumed (Clark, 1992; Haw & Clout, 1999; Haw et al., 2001; Lindsay & Ordish, 1964; Moon, 2005; Stephenson, 1998). Studies on the Australian boobook have shown a similar dietary composition (Campbell & Rose, 1996; Green et al., 1986; Penck & Queale, 2002; Rose, 1996). Moreporks generally survey an area from high perches, and locate prey using visual and auditory cues. Prey may be taken in flight, from trees, or on the ground (Higgins, 1999).

Beetles (Coleoptera), moths (Lepidoptera) and orthopterans including crickets (Gryllidae) and weta (Anostostomatidae and Raphidophoridae) have been reported as common prey of the morepork (Clark, 1992; Cunningham, 1948; Haw & Clout, 1999; Haw et al., 2001; Imboden, 1975; Lindsay & Ordish, 1964; Saint Girons et al., 1986) and the boobook (Campbell & Rose, 1996; Penck & Queale, 2002; Rose, 1996). However, many other invertebrate taxa are also taken including spiders (Araneida), stick insects (Phasmatidae), millipedes (Diplopoda) centipedes (Chilopoda), cicadas (Cicadidae), and mantids (Mantodea), and there is a considerable amount of variance in the composition of the diet between studies (see Higgins, 1999 for summary, as well as Haw & Clout, 1999, Haw et al., 2001 and Penck & Queale, 2002).

In New Zealand, vertebrate prey includes rodents such as house mice (Mus musculus) (Clark, 1992; Lindsay & Ordish, 1964; Stephenson, 1998), ship rats (Rattus rattus) (Haw et al., 2001) and kiore (Rattus exulans) (Saint Girons et al., 1986), as well as small birds (Aves) (Brown, 1997; Cunningham, 1948; Haw & Clout, 1999; Haw et al., 2001; Hogg & Skegg, 1961), bats (Chiroptera) (Dwyer, 1962; Worthy & Holdaway, 2002) and lizards (Reptilia) (Moon, 2005; Ramsay & Watt, 1971). It appears that vertebrate prey may only be taken by moreporks in large numbers when they are particularly common (Haw et al., 2001; Saint Girons et al., 1986). The Australian boobook also consumes birds, rodents, reptiles and bats, but, in addition may take small marsupials (Marsupiala) (Baker-Gabb, 1984; Campbell & Rose, 1996; Green et al., 1986; Penck & Queale, 2002; Rose, 1996).

Moreporks and boobooks have been observed flushing cicadas from foliage (Stephenson, 1998), attacking birds caught in mist-nets (Imboden, 1975) and catching insects attracted to lights (Ramsay, 1988; Rose, 1973). Thus, they have been described as generalist, opportunistic predators that exploit seasonally abundant prey species (Cunningham, 1948; Green et al., 1986; Haw & Clout, 1999; Haw et al., 2001; Lindsay & Ordish, 1964; Penck & Queale, 2002; Stephenson, 1998).

Haw and Clout (1999) and Haw et al. (2001) looked specifically at seasonal variation in the morepork diet based on the study of 53 stomach samples from throughout New Zealand, and 189 pellet samples from Pureora Forest Park respectively. Both studies found beetles, orthopterans and spiders to be consumed throughout the study period whilst Haw and Clout (1999) found there was a peak in consumption of moths and cicadas in summer, and Haw et al. (2001) noted a peak in stick insect consumption in winter. Lindsay and Ordish (1964) also noted that weta appeared to be a staple of the morepork diet, whilst there was more variation in seasonally abundant taxa such as cicadas (based on analysis of 25 stomach samples). Similarly, Penck and Queale (2002) analysed the gizzard contents of 117 boobooks and found that beetles were frequent prey throughout the year, with increased prevalence of moths during spring, and of mice during autumn. Based on their findings, the authors of these studies, along with several others on morepork diet (Cunningham, 1948; Saint Girons et al., 1986; Stephenson, 1998), have suggested a relationship between the diet and fluctuations in the availability of prey.

However, no study of morepork (or boobook) diet has concurrently measured the relative abundance of its prey. Rather, the suggestion of a functional relationship has been based on circumstantial evidence or general trends in seasonal abundance revealed by spatially or temporally distinct studies. This has the potential to be misleading, as comparisons may have been made between quite different habitats, or the seasonal abundance of prey species may differ substantially between years.

As a common, generalist predator (Higgins, 1999) the morepork is a component of ecosystems throughout the country, and has the potential to interact with many other species, as a predator and a competitor. As such, simultaneous monitoring of morepork diet and prey abundance would help to improve understanding of the bird’s feeding behaviour, and its role in communities.

Research of this kind would also help to elucidate the importance of vertebrate prey to the species, and the potential for secondary poisoning. Dead moreporks have been found following rodent poisoning operations using brodifacoum (Eason & Spurr, 1995; Ogilvie et al., 1997; Stephenson et al., 1999; Walker & Elliott, 1997) and 1080 (sodium monofluoroacetate) (Spurr & Powlesland, 1997), presumably as a result of secondary poisoning. Of the birds that were not too decayed to be assayed by Ogilvie et al. (1997) (n=1) and Stephenson et al. (1999) (n=2), all contained brodifacoum residues in their tissues. With this in mind, recent studies on morepork diet in New Zealand have focused on the amount of vertebrate prey included, and the associated risk of secondary poisoning during pest control operations. Stephenson et al. (1999) found that there was a moderate risk to morepork populations on Mokoia Island based on their relatively high consumption rates of mice (which were probably abundant due to the absence of rats), and mortality rates following a poisoning operation. On the other hand, Haw et al. (2001) found that consumption of rodents in Pureora Forest Park was low, despite the fact that they were considered to be fairly abundant throughout the study. This lead to the suggestion that moreporks only eat rodents when they are very abundant (as they may be on some islands), such that the risk of secondary poisoning in mainland forest is probably low. However, they pointed out that their study did not include the period from January to April when rodents may have been more abundant (Haw et al., 2001).

Another point for consideration is that no studies on morepork diet, and only one on the boobook (Baker-Gabb, 1984) have estimated the biomass or calorific value of the vertebrate prey consumed. Instead, prey species in the diet have been measured by either their number or frequency of occurrence in pellets, stomachs or gizzards. For this reason, the importance of vertebrate prey, which could contribute a large amount of biomass to the diet, may have been under-emphasised. There have also been variable suggestions about the role of vertebrate prey in the diet; either that inclusion is purely opportunistic (Haw et al., 2001; Penck & Queale, 2002), or that it is selected for during the breeding season (Imboden, 1985; Schodde & Mason, 1980).