The Insect Deficit: why game bird chicks hunt what is no longer there

Galliform chicks are effectively obligate insectivores for the first fortnight of life. Over the same half century in which that requirement has stayed constant, the invertebrate supply on British arable land has fallen by more than a third.

1. The requirement

Grey partridge, red-legged partridge and pheasant chicks all hatch precocial and begin foraging within hours. For approximately the first two weeks the diet is composed almost entirely of invertebrates. This is not a preference that can be substituted at will; it reflects a digestive system that has not yet developed the capacity to handle a cereal-based ration, combined with a growth rate that demands protein at a concentration grain cannot supply.

The nutritional demand in that window is specific rather than general. Rapid feather development requires sulphur-containing amino acids — feather keratin is unusually rich in cystine — and skeletal growth in a bird that will double its mass repeatedly over a short period requires calcium and phosphorus in a bioavailable form. Insects supply all three simultaneously, which is why the association is evolutionary rather than incidental.

2. The supply

The Game & Wildlife Conservation Trust's Sussex Study is the longest running scientific monitoring programme of the cereal ecosystem anywhere in the world. Running continuously since 1970, it has by 2019 accumulated 4,757 samples containing 2.98 million invertebrates from the same arable landscape.

Across that fifty-year record, overall insect abundance in cereal fields fell by 37%. Several groups of direct relevance to chick diet fell considerably further:

Group

Change, 1970–2019

Overall insect abundance −37%

Aphids −90%

Ground beetles (Carabidae) −80%

Ladybirds (Coccinellidae) −78%

The mechanism is well described and largely uncontroversial. Herbicides removed the broad-leaved arable weeds; the phytophagous invertebrates that depended on those weeds declined with them; and the predatory groups that fed on those in turn followed. The chick-food resource is the third link in a chain that was broken at the first.

3. The consequence

The clearest quantification of the effect comes from grey partridge, because the species has been monitored longest and because its chicks are the most narrowly dependent.

Potts and Aebischer (1995) recorded mean chick survival rate on the Sussex study area at 48.6% before the decline, against 32.3% in the conventionally farmed area afterwards. On the managed area between 2009 and 2018, chick survival averaged 46% against 33% on conventional ground, although that difference did not reach statistical significance.

Potts and Aebischer (1991) also derived a Chick Food Index, a measure of the abundance of the invertebrate taxa that actually appear in chick crops. A CFI of 0.7 is sufficient to stabilise grey partridge numbers. Below it, populations decline regardless of what else is done for them.

4. Why the requirement does not disappear at fourteen days

The acute dependency ends as the digestive tract matures and the bird moves onto a mixed diet. The behavioural drive does not end with it. Adult galliforms continue to take invertebrates opportunistically throughout the year, and the search behaviour — working leaf litter, turning surface material, moving through vegetation with the head down — persists as a default foraging mode.

That persistence has a practical consequence, which is the subject of the second article in this series. A bird that retains a strong appetitive response to invertebrate prey can be influenced by where that prey is available, long after it has ceased to depend on it nutritionally.

5. Supplementary insect protein

Where habitat management has been done and the invertebrate resource is still below threshold — a common position on recovering ground, and near-universal in a cold, wet June — supplementary insect material is one of the few remaining levers.

Dried black soldier fly larvae (Hermetia illucens) are the most widely available option at scale.

Two components merit specific comment. Lauric acid (C12:0) has documented antimicrobial activity against Gram-positive organisms and has been reviewed in the feed context by the EFSA FEEDAP Panel (2021) and characterised in black soldier fly specifically by Spranghers et al. (2017). Chitin, present in the cuticle, is poorly digestible in the conventional sense but has been associated with immune modulation in poultry species, and is a normal component of any wild insectivorous diet.

Direct work on the species of interest remains limited, but is beginning to appear. A study published in the Journal of Animal and Feed Sciences fed full-fat Hermetia illucens larvae meal to laying pheasants at 100 and 200 g/kg. The higher inclusion group produced the greatest number of eggs and the lowest proportion of culled eggs, recorded the highest albumen height and Haugh index, and showed increased yolk dry matter and crude protein. The authors also observed a tendency toward improved chick survival to days 7 and 14 post-hatch, increasing with dose.

That study used larvae meal within a formulated ration, not whole larvae scattered on the ground. It is therefore indicative of the nutritional contribution rather than directly transferable to field supplementation, and should be read as such.

6. What it does not do

Supplementary insect protein does not restore an invertebrate population. It supplies the nutritional component that a depleted population no longer provides; it does not rebuild the food chain that produced it. Conservation headlands, beetle banks, unsprayed margins and reduced summer insecticide use remain the only interventions that address the cause rather than the symptom, and the Sussex Study data are as much an argument for those as for anything else.

The realistic position is that supplementation is a tool for the gap: the season in which the weather fails, the ground still in recovery, the brood that would otherwise not make it through a fortnight of rain. Treating it as a substitute for habitat would be a misreading of every figure above.

References

EFSA Panel on Additives and Products or Substances used in Animal Feed (FEEDAP), 2021. Scientific opinions on feed additives and undesirable substances.

Game & Wildlife Conservation Trust. The Sussex Study, 1970–2019. Fifty years of monitoring the cereal ecosystem.

Kierończyk, B. et al. Insects as a natural component of pheasant diets: effects of full-fat Hermetia illucens meal on egg production and quality, hatchability and selected physicochemical egg indices. Journal of Animal and Feed Sciences.

Potts, G.R. and Aebischer, N.J., 1991. Modelling the population dynamics of the grey partridge: conservation and management.

Potts, G.R. and Aebischer, N.J., 1995. Population dynamics of the grey partridge Perdix perdix 1793–1993.

Spranghers, T. et al., 2017. Nutritional composition of black soldier fly (Hermetia illucens) prepupae reared on different organic waste substrates. Journal of the Science of Food and Agriculture.