In Praise of Crows

Note: This is not a researched scholarly article based on accepted statistical measurement or scientific data. It examines possibilities and presents unverified outcomes based on my personal interpretation of available data. The purpose is to encourage thinking and raise questions, and to look at the potential contribution made by Corvus splendens or the House Crow to urban India.

Introduction

It is an understatement to say that they are not much loved. The House Crow, or Corvus splendens is a bird almost everyone loves to hate. 'Splendens' in Latin translates to brilliant - a difficult adjective to use when talking about crows. That it is dark, big, and black is not enough – it is singularly noisy and also a source of urban defilement. It is, therefore, not a surprise that Corvus splendens is regarded as a public nuisance in many countries!

The crow is regarded as a nuisance for a reason. It actually causes much visible damage. The list of its crimes are long, and reads like a charge-sheet. The house crow inflicts economic and ecological damage by predating chicks and eggs, destroying crops, causing severe damage to fruit in orchards, and presents a threat to tourist amenities and industry in some places. It is also an intestinal carrier of at least eight human enteric diseases. 
The charges against it are especially vitriolic in the many countries where this intelligent and resourceful bird has established itself as an unwanted alien.

But we, in India, are not dealing with an intruding pest. This is very much an Indian (or subcontinental) bird, and we have all walked the path together for ages. Almost everyone in India is familiar with the House Crow. No city dawn breaks without its raucous call preparing the citizens for another day of its shenanigans. It defiles, its robs, it attacks – we know that, and have learned to live with it. For what? Is there any service that this bird provides that diminishes or balances the misdeeds? Indeed there is, and it is a multi-crore contribution, one potentially big enough to invite researchers to look deeper into the subject and perhaps help restore dignity to this much-maligned commensal of man. 


The role of the crow

House Crows are known to be useful in removing pests. They were, in fact, introduced to Malaysia and Oman to control caterpillars and livestock ticks. But, perhaps, the reason they were introduced in the island of Zanzibar holds the real key to this discussion – the birds were introduced to clean up refuse or garbage!

Having raised the issue of garbage and crows, let us plunge ahead to the heart of this discussion which is not about 'crows are garbage', but how crows contribute to the management of garbage. House Crows are omnivorous scavengers with a wide-ranging and opportunistic diet. They eat absolutely everything that is edible, and some that may not even be so. The crow feeds largely on refuse around human habitations, carrion, small reptiles, insects, small invertebrates, eggs, nestlings, plants, grain and fruits etc.. Being closely associated with people (no populations are known to live independently of man), the crow takes advantage of scavenging opportunities provided by discarded food items and refuse dumps. This is particularly true in urban environments where other food sources like fruits, grain, insects, eggs etc. are limited.

Garbage or refuse is waste matter, something that is unwanted or useless. In urban areas, garbage is mostly due to poor decisions and is a by-product of our activities. Although one man's trash can be another man's treasure (particularly so in India) much of this waste is household waste which goes under the name of 'Municipal Waste'. Studies indicate that Indians are not as wasteful as people in more developed countries, but even then an average Indian generates between 200 - 600 grams (g) of waste a day. Given our population, that is a significant amount that we don't want, and unless we dispose or reuse it efficiently we may get drowned in waste in a very short time because accumulated waste leads to degradation of urban environment, stresses natural resources and leads to health problems. According to published reports, Mumbai city generated 6256 tons of Municipal Solid Waste (MSW) per day in 2001. Kolkata city was not far behind with 4,000 tons generated a day. A large part (30-55%) of the MSW is non-recyclable organic waste, stuff that crows eat. Huge amounts of money are spent by civic bodies to dispose of waste. In 2001, the Municipal Council of Greater Mumbai spent Rs. 61,435 million towards MSW management. The cost for the country as a whole must be staggering considering that we generated 42 million tons of MSW in 2004 -2005.

But our story is that we actually generated more garbage. How much more and who disposed it? Of course the crows removed garbage before we collected it! Just how much is what we need to find out.

How much does a crow eat?

On then to crows and how much garbage they remove. The first thing to determine is just how much a crow eats. As with many subjects in India, there seems to be no data on this subject. There is considerable work on how much crows destroy, but no one thought it important to find out how much it actually consumes! The crow is a omnivore and its diet consists of food that has mixed calorific value. Studies indicate that a free ranging wild bird usually consumes one half its body weight every day as a rule of thumb. But this differs between bird types with hummingbirds consuming as much as twice their body weight each day, and raptors requiring a quarter of body weight. The House Crow belongs to the corvid family. Studies in the west with similar members of the corvid family suggest that a 'well-grown” American Crow (Corvus brachyrhynchos) needed 600grams of food every day to maintain their body weight of around 600g (Forbush, 1907). Some other studies with Carrion Crows (Corvus corone) also suggest a daily food requirement close to the birds body weight. The House Crow weighs between 245 to 370 grams, or about 300g on average. If the studies on corvids are used as a guide, then the House Crow needs about 300g of food daily.

Much of what the average House Crow eats is garbage or human waste – but not all. Crows predate, consume insects, drink nectar, eat fruits, grain etc. and occasionally rob human food. Urban crows have less ability to obtain many of these goodies and are more reliant on our wasteful habits for survival. For the sake of discussion, let us assume that urban crows consume at least 225g of waste/day while their rural brethren consume, say, 50% or 110g of waste/day. 

Having determined an individual crows appetite for trash, we need to move on to figuring out how much food is consumed by all House Crows in India. Child's math now that we know how much one bird eats. All we need is the number of crows in India. Easy!

How many crows?

But this, as you probably guessed, is where we come across the biggest hurdle. There is no scientific study on the crow population in India. The best we have is a citizen science project by the members of birdsofbombay at Yahoo groups, who (led by Sunjoy Monga) estimated that there were roughly around 5,00,000 - 6,00,000 House Crows in the Mumbai city area in 2001, based primarily on sample nest counts. My presumption is there were more. House Crows have two distinct populations – one that is territorial and sedentary and another that disperses everyday to forage. The territorial lot is easy to count as they have nests within their area and do not stray far from home. A very large percentage of the crow population are non-nesters (juvenile, old?) and disperse widely only coming back to a fixed communal roost. Any nest count data will ignore these birds, and they can be half the actual population. [This behaviour difference makes even roost counts suspect as it ignores territorial birds]. To give you an idea, I counted crows flying in to a nearby roost every day for a period. My line of sight only covered 5-6 percent of the arrival path to the roosting area. My average daily count of non-territorial crows was c.2,000. If we extrapolate this number to cover all access points to the roost area, we would end up with about 60,000 crows by a simplistic calculation! - crows that roost in an area but forage elsewhere.

The birdsofbombay teams exercise is laudable as an excellent initiative and provides preliminary information on crow populations. There are more rigorous studies on House Crow populations – but they are in foreign countries. The island of Singapore counted crows in 2001 and found 133,000 of them. Tiny (35 sq.km.) Kharg Island in Iran woke up one day to the call of 5,500 House Crows. There have been other counts in Zanizibar and elsewhere, but mostly in alien areas by people who were preparing to cull or eradicate these birds. There is no real count of the species in the subcontinent, and none seem to be likely in the future. 

The hypothesis

What then is the way out? How does one even roughly estimate the crow population in India? One way is to go by figures of population density from areas where they have been scientifically counted. But that throws up greatly divergent figures and does not factor in habitat differences. For example, Singapore had a population density of 192 birds per sq. km in 2001 despite culling. Kharg Island had a density of 157 birds/sq. Km and Pemba Island in Zanzibar boasted of 122 crows in every sq. km. on average. Closer home, in the sparsely populated Uttara area of Dhaka, Bangladesh a study found 40 House Crows in every sq. km, and a study in the Delhi area in 2007 by Neeraj Khera and others found 40 crows in each hectare in urban areas. The area density indicator, therefore, is extremely variable and unsuitable for generalization. The alternative is to look for a different correlation which can give us a rough and ready estimate of the crow population. And here one characteristic of the House Crow stands out and makes it an exception from all other birds – Corvus splendens is a obligate associate of man. It only survives where humans are present and depends on them for survival. Is it possible, therefore, to draw a correlation between human population and House Crow numbers in the subcontinent where the birds are well-established natives, especially in urban areas where the bird almost entirely depends on human waste? I propose as much. It is a logical hypothesis which could be tested in the field. 


If we test this hypothesis with data from Singapore we arrive at a index of one House Crow for every 38 humans, and Singapore is known as one of the cleanest places in Asia. Data from Pemba Island suggests that at some time there was one crow to every 5 humans. Closer home, if we take Mumbai's crow estimate and compare it with Mumbai's population in 2001, we arrive at 16 humans/crows. My own observations based on the small study area around my locality offers a ratio of 11 humans/crows based on data collected over 3 years. These are numbers which are much closer to each other and may offer us a tool, at least in urban India. While actual data for more areas will take effort and time to generate, it seems plausible that there is 1 crow to every 10-15 human is city areas of India.

We need to pick a number here as a yardstick to move on with our exercise. I have chosen 15humans/crows as being the representative ratio for urban India, based largely on the results from the Mumbai exercise. At this stage there is no point in refining this number further as it is a very crude number and does not take into account variables like habitat, food habits and hygiene of citizens, efficiency of garbage handling, alternate food sources etc. etc. The idea here is to try and look at the big picture and get a sense of scale.

Population estimate

Now that we have a preliminary index, we can move forward to the next steps. The first of which is to determine the urban crow population in India. India's urban population is estimated at 287 million as at 2010. With one crow needing 15 humans to support it, the estimated crow population is then 1,91,00,000 or 19 million in urban areas. In rural areas, the population density of crows is lower and there is less crow-human dependency from waste creation. This is partly because organic solid waste is much less, and much of the stuff that we can't consume has other takers like farm animals. The Dhaka study suggests a large fall in density in semi-urban areas and the Delhi study found crow densities dropping to 1/4^th urban densities in agricultural areas. Not the entire non-urban area is agricultural, and at the same time there are large tracts which do not have crow populations despite human presence. By factoring these differences in, we may assume that the crow to humans ratio in the rural and semi-urban areas of India is around 50humans : 1crow (around 1/3rd). Again this is a unverified assumption and is based largely on density differences observed in one study. India's rural population was 742 mn in 2010. Based on that number the rural crow population across the country should be in the region of 1,50,00,000 crows or 15 million rural crows, giving us a grand total of 34 million crows in the country.

Analysis

Now that we have a derived estimate of the strength of the House Crow in India, the rest is easy. Let us first establish how much solid waste our crows consume on a daily basis. The calculations are different for rural and urban crows. The urban crows almost entirely live off refuse, the rural ones possibly have a much more variable diet.

Based on our earlier estimate of daily consumption, an adult crow will consume 300g daily. The calculations are as under:

So total garbage cleared by crows is:

Urban crows [1,91,00,000 x 165g] = 3,150 tons/day = 11,50,000tons/year

Rural crows  [1,50,00,000 x 110g] = 1,650 tons/day = 6,00,000tons/year

Here is where we drop the rural crows from our exercise. Much of rural India does not have Municipal services and there is no significant cost-saving due to the crow's presence. In fact it is very likely that the damage to agriculture and farming is as significant as any environmental costs the crow may save.

Contribution of urban crows

Urban solid waste management costs anywhere between Rs.3,000 – 4,000+ per ton on average across India. Applying a rate of Rs 3,000, the total amount of money that is saved by the House Crow annually in urban areas of India in Municipal Solid Waste management cost is estimated to be [11,50,000x3000] Rs 340,00,00,000 or Rs. 340 crores. Add environmental costs associated with garbage (Rs 790/ton in Mumbai in 2003 according to a study) and the final contribution by House Crows can easily cross Rs 400crores (about US$ 90mn) annually – and this is citizen tax savings!

Conclusion

As I said at the beginning, this is not a scholarly article based on accepted statistical measurement or scientific data. It examines possibilities and plausible estimates based on available data from some areas The purpose is to encourage thinking and raise questions, and to look at the House Crow with less than hate-filled eyes. After all, higher Municipal taxes are not exactly what you may want to trade off in exchange for a mischievous native garbage destroyer!

Acknowledgements and references

Many thanks to Arka Sarkar and K. S. Gopi Sundar for checking the draft for logical inaccuracies and suggesting changes. They are in no way responsible for the contents of this blog entry.

1. House Crow Monitor

2. NNSS - Indian house crow, Corvus splendens 

3. Global Invasive Species Database - Corvus splendens (bird)

4. Crows.net - Description of the American Crow, Corvus brachyrhynchos 

5. Familiar Birds - American Crow 

6. Behavioral Ecology - False feeding;... Daniela Canestrari et al (2009) 

7. Crows Eradication Programme - Department of Commercial Crops, Fruits and Forestry, Zanzibar, Tanzania (pdf)

8. Diversity and Population of Avifauna of Two Urban Sites in Dhaka, Bangladesh by Noor Jahan Sarkar and others (2010) (pdf)

9. Habitat-wise distribution of the House Sparrow (Passer domesticus) in Delhi, India by Neeraj Khera & others (2009)

10. Optimization model for integrated municipal solid waste management in Mumbai, India by Sarika Rathi (2007) (pdf)

11. Assimilation efficiency in birds: A function of taxon or food type? - Gonzalo Castro et.al. (1989)

12. Animal weights and their food and water requirements - Ministry of Environment British Coloumbia

13. Results of crow count at birdsofbombay yahoo group 

14. Mortality rates and survival of birds - D B Botkin et. al. in The Ameriacn Naturlist (1974)

15. Wikipedia

Sumit Sen
August, 2011

> Comment received by mail from Rose DeNeve: I am a sincere believer in the philosophy that every species and all creatures have a role to play in the grand scheme of life. Thank you for such a clear and logical presentation in making a case for the humble house crow. There is much to be learned from and about this intelligent, if cantankerous, bird!