Aarhus Universitets segl

No. 666: Biology and population dynamics in the Grey Partridge (Perdix perdix) - a review of the present knowledge

Kahlert, J., Asferg, T. & Odderskær, P. 2008: Agerhønens biologi og bestandsregulering. En gennemgang af den nuværende viden. Danmarks Miljøundersøgelser, Aarhus Universitet. 62 s. – Faglig rapport fra DMU nr. 666.

 

Summary

This report presents the results of a literature study on grey partridge based upon peer-reviewed papers, reports and popular articles from Denmark and abroad.

 

The literature study should be considered as a supplement to on-going studies on grey partridge under the research programme "Wildlife and Landscape"

 

The report considers four main issues: 1) changes in European partridge populations during the last 50-60 years, 2) general biology, 3) causes of the observed population trends and, 4) management of grey partridge in different European countries. Although the report considers the different issues from an overall European perspective, from the outset emphasis is placed on aspects that relate specifically to Danish partridges and their particular circumstances. New analyses of existing data are presented derived from the Danish hunting bag statistics and point counts organised by the Danish Ornithological Society.

 

The collation of bird count survey data and the results of analysis of bag statistics showed that grey partridge has suffered from a significant decline in numbers during the last 50-60 years, and in Denmark the number of shot partridge currently equates to less than 10% of those taken around 1940. In many countries, falling numbers seem to have shown a stepwise decline and to certain extent these patterns appear synchronized between countries. In Denmark five periods of significant decline can be identified: 1) early 1950s, 2) early 1960s, 3) around 1980, 4) late 1990s and finally from 2003 to 2006. Since 1994, the number of bagged partridges (a proxy for changes in population size) has fallen in 11 out of 13 years. Declines were reported from all regions of Denmark, which were also confirmed by the national point count surveys. In recent years, the decline in the number of bagged partridges may overestimate the actual decline in numbers due to a reduced hunting effort on grey partridge.

 

A review of the biology of the grey partridge revealed certain characteristics:

  • The grey partridge is relatively short-lived (on the average less than one year), but produces a large clutch (10-20 eggs). Accordingly, the species is relatively susceptible to changes in reproductive output (egg and chick production). However, these characteristics of partridge population dynamics give the potential for relatively rapid restoration to abundance after a decline, as long as the factors responsible for the declines are reduced in magnitude or removed.
  • The grey partridge is consistently associated with farmland throughout the annual life cycle; showing particular preferences for areas that are not directly associated with production: small uncultivated areas, hedgerows, field margins and uncropped wildlife strips. These habitats contribute to provide various basics needs, including adequate available food resources; cover for nesting and predation avoidance for adults and chicks.
  • Adults grey partridges feed almost exclusively on plant material, especially seeds from weed and green leaves from grasses and small cereal plants. During the first weeks after hatching, chicks primarily feed on insects.
  • Grey partridge shows little dispersal. Twelve studies within Europe have shown that the average dispersal remained at maximum 3.5 km.

 

The decline amongst European partridges has mainly been linked to changes in agriculture since 1950. During this period, the size and distribution of small uncultivated areas has reduced significantly. Hence, many potential nest and feeding habitats have disappeared. The use of herbicides and insecticides has also limited food resources considerably for both adults and chicks.

 

New analyses of data from Denmark confirmed that severe winters together with low temperatures and heavy rainfall during the chick period explain a substantial proportion of the year to year change in the numbers of bagged partridges since 1941. Bag statistics on red fox (as an index of population change in this important predator) suggest its abundance has some impact on grey partridge abundance, but to a lesser extent than weather conditions. In addition to red fox, grey partridge are predated by other species, including birds of prey and mustelids.

 

Studies in Denmark and elsewhere indicate cases where locally grey partridge hunting is clearly not sustainable, along side other examples presented in the report where the hunt is considered sustainable in different European countries. Guidance on how to attain a sustainable partridge hunt varied considerably between case studies, and is not necessarily based on scientific evidence. Advice ranges from official recommendations from authorities to what is considered as common sense amongst hunters.

 

There are no indications that diseases and parasites amongst grey partridge should have played a major role in the changes in numbers. The issue is, however, not supported by in-depth studies to the present time.

 

It may be that grey partridge densities in some places (including areas in Denmark) have become so low that this in itself may increase the risk of further declines. The low dispersal rate of the species contributes to this fact. In many European countries, hand-raised grey partridges are released to supplement “wild” populations. Released partridges typical suffer from lower survival rate after release, the main reason why release projects have not restored partridge populations to previous levels on a larger geographical scale.

 

It is concluded that the grey partridge still has the potential to maintain a dramatic increase in numbers, mainly because of their potential for very high annual reproductive output. If the present predictions for climate change are manifest, future weather conditions are likely to be beneficial to Danish partridges. However, the interaction of compounding factors that presently limit the grey partridge in contemporary Danish farmland, means that more than just one of the underlying factors important for population changes must prevail, if the grey partridge is to be successfully restored to previous population levels.

 

At a number of sites throughout Europe, positive effects on grey partridge population densities have been demonstrated at the local level after the provision of targeted measures, which improve the habitat in agricultural areas. This report reviews these measures and presents a synthesis of the generally good match across different European countries of what is considered as habitat improvement and enhancement for grey partridge. These recommendations include increasing the availability of nest sites and enhancing chick survival as well as winter and spring survival amongst adults. So far, these improvements have not had a significant impact on partridge numbers on the national scale, most likely because they have been carried out on a relatively small part of the total farmland area.

 

Full report  in pdf-format (987 kB)