Birds can positively impact soil fertilization in a variety of ways. Their droppings contain nitrogen and phosphorus which can enrich soils. Additionally, some species disturb and aerate soil while foraging, allowing better water and air penetration. Birds can also aid in seed dispersal, helping plants propagate across a landscape. However, the extent to which birds fertilize soils depends on the species, habitat, and density of birds present. Overall, avian contributions to soil fertility may be minor compared to other factors, but can still provide benefits in certain ecosystems.
Do bird droppings fertilize soil?
Yes, bird droppings can fertilize soil by providing nitrogen and phosphorus. Droppings from wild birds tend to be higher in these nutrients compared to livestock manure. For example, analysis of Canada goose droppings shows they contain 2.6% nitrogen and 1.4% phosphorus.[1] In comparison, cow manure averages 1.1% nitrogen and 0.5% phosphorus.[2] The more concentrated nitrogen and phosphorus in wild bird feces means their droppings can enrich soils, supporting plant growth.
This fertilizing effect is most notable below bird roosting and nesting sites, where droppings accumulate. Studies of seabird colonies show soils often have higher nitrogen levels compared to surrounding areas.[3] Penguins in Antarctica also create ‘hotspots’ of enriched soil beneath their rookeries.[4] Similar localized enrichment can occur under starling, pigeon, and other bird roosts and nests. Therefore, areas concentrated with bird droppings do see fertilizing effects.
Key factors determining fertilizing impact
However, some key factors influence whether bird droppings significantly fertilize soils across a landscape:
- Bird species – Dropping nutrient content varies. Seabirds and waterfowl produce more concentrated, fertilizing droppings. Passerines have less impact.
- Bird density – Areas with more birds deposit more droppings, resulting in greater nutrient inputs.
- Habitat – Droppings accumulate more in roosting and nesting areas.
- Diet – Birds eating protein-rich foods produce more nutrient-rich droppings.
- Weathering – Rain can wash away unabsorbed nutrients before fertilizing plants.
So while bird droppings do fertilize soils, the extent depends on these factors. The localized enrichment beneath bird colonies is more notable than any landscape-scale fertilizing effect for most regions.
Do bird droppings acidify soil?
Bird droppings are acidic, with a pH of around 5-6.[5] When fresh droppings fall, they can lower the pH of directly impacted soils. However, the acidification is transient rather than long-term. Acidic components in the feces quickly wash out with rainfall, allowing soil pH to recover.[6]
On landscapes with highly buffered soils containing calcium carbonate, any acidification from bird droppings has negligible effects. The pH change is localized and brief, returning to normal levels within weeks after deposition.[7]
In ecosystems with sensitive, unbuffered soils low in calcium, temporary acid spikes from bird feces deposition could impact pH-dependent processes like plant nutrient availability.[8] However, research shows bird fecal deposition causes no lasting acidification even in sensitive environments like Arctic tundra.[9]
Overall, while bird droppings are acidic, their fertilizing effect dominates over any transient pH change. Except perhaps in extremely sensitive soils, bird feces do not acidify soils in a substantial or lasting way.
Do birds disturb and aerate soil?
Some bird behaviors, especially foraging, can beneficially disturb soils. Niches created by birds overturning litter and soil improve aeration, water infiltration, and seedling germination.
For example, Malleefowl in Australia dig pits up to 5 feet deep to incubate eggs.[10] This extensive excavation mixes and aerates surface soil layers. Studies in deserts show desert-dwelling birds aid seedling establishment by raking soil during foraging, creating niches for improved germination.[11] Even smaller birds like Bushtits in California benefit shrub germination by digging circular foraging pits that aerate soil.[12]
Some seabird colonies also illustrate how extensive soil disturbances help vegetation thrive. Burrowing seabirds like puffins create burrows that mix and aerate soils. Plants growing in and around the burrows have better structure, moisture, and nitrogen levels compared to undisturbed soils only a few meters away.[13]
Therefore, while the fertilizing effect is their droppings is notable, some birds also enhance soil conditions and plant growth by physically disturbing soils while foraging and nesting.
Key factors influencing soil disturbance
As with fertilization, the degree of positive soil disturbance depends on:
- Bird species – Larger birds with robust foraging behaviors make biggest impact.
- Bird density – More birds means more niches and soil turnover.
- Habitat – Arid, low-productivity areas benefit most from added soil disturbances.
- Plant community – Early successional stage vegetation gains most advantage from soil niches.
The positive soil enriching and disturbing effects are most pronounced in threatened desert ecosystems and around seabird colonies. Overall landscape level impacts are small in most regions.
Do birds aid in seed dispersal?
Birds can contribute to seed dispersal through:
- Endozoochory – Passing seeds through the digestive system
- Ectozoochory – Carrying seeds externally on feathers and feet
Over 70% of plant species in some habitats rely on birds for dispersal by one or both of these mechanisms.[14]
Endozoochory aids dispersal distance since birds can move seeds far from the parent plant. Passage through the digestive systems can also scarify seeds, improving germination rates. Ectozoochory allows local dispersal and colonization of new habitats.
By dispersing seeds away from parent plants, birds reduce intraspecific competition and help plants colonize new areas. This facilitates plant population spread and contributes to vegetation community diversity.[15]
Key factors for seed dispersal:
- Bird species – Some birds disperse more seeds than others based on diet and habitat.
- Plant species – Plants adapted for epizoochory or endozoochory benefit most.
- Vegetative phase – Early successional communities gain most advantage from bird dispersal.
So avian seed dispersal provides another mechanism for birds to benefit plant communities, enriching biodiversity and fostering successional development.
Relative importance of birds for soil fertility
The enriching and disturbing influences described can provide localized benefits to soils and plant growth. However, at a landscape scale across most ecosystems, birds play a minor role compared to other factors in maintaining soil fertility, such as:
- Plant litter decomposition and biomass turnover
- Atmospheric nitrogen deposition
- Nitrogen fixation by bacteria and legumes
- Animal wastes from livestock, native fauna
- Use of composts and fertilizers
- Geologic weathering of parent material
In regional case studies quantifying nitrogen inputs, avian contributions from droppings were dwarfed by other sources like nitrogen fixation, atmospheric deposition, and leguminous plants.[16] Similar small contributions are found for phosphorus inputs from birds.[17]
However, birds can make meaningful localized differences as described. In fragile habitats like Arctic tundra or deserts, their disturbances and dispersals take on greater importance for plant communities and ecosystem function.[18] Seabird colonies also illustrate how large concentrations of birds can substantially enrich soils, stimulating plant growth.
Fertilizing Mechanism | Relative Importance for Soil Fertility |
---|---|
Droppings containing N and P | Minor overall, but locally substantial beneath bird colonies |
Soil disturbance from foraging | Greatest benefits in threatened desert, tundra, and island ecosystems |
Seed dispersal | Aids plant community diversity and succession, but minor effect on nutrient availability |
So while birds contribute to nutrient inputs and soil conditions, their influence on landscape-level soil fertility is small compared to other factors for most regions. However, they still provide measurable benefits to habitats where their behaviors concentrate and accumulate.
Conclusion
Birds can fertilize and enrich soils through their droppings, disturbances, and seed dispersals. However, their relative contributions are minor for overall soil fertility across most ecosystems. Locally around roosting and nesting sites, seabird colonies, and in threatened deserts and tundras, avian influences on soils become more pronounced and beneficial. But landscape-scale impacts on soil nutrient availability are dominated by other natural mechanisms like nitrogen fixation and rock weathering. With their mobility and behaviors, though, birds provide another way biological processes interact to sustain plant communities and ecosystem function. Their varied roles in different environments illustrate nature’s interconnectedness. While their fertilizing function may be minor overall, it remains an important part of the habitat puzzle across many regions.