House Crow

To: Birding-Aus Mailing List <>
Subject: House Crow
From: Dean Portelli <>
Date: Wed, 19 Mar 2008 05:24:44 +1100
 Hi All,

 The appearance of this single house crow and the discussion it has incited has 
scratched the surface of an entire field of study that has become known as 
‘invasive species biology’. Below is my rather lengthy contribution to this 
discussion – see my concluding remarks for an explanation!

Australia, as we are all well aware of, has a large number of introduced 
(invasive) species. The majority were deliberately introduced for biological 
control (e.g. cane toads, common mynah), acclimitisation purposes (e.g. house 
and tree sparrow, common starling, lantana, camphor laurel), or for recreation 
(e.g. rabbit, fox). Typically, many individuals were released and in some cases 
at multiple sites over a prolonged period of time. For example, the common 
starling was introduced over a period of ~30 years in the late 1800’s in 
Sydney, Melbourne, Hobart, Brisbane and Adelaide. It is noteworthy that 
numerous introduction attempts in Australia failed to establish persistent 
populations despite attempts with some of the same species being successful 
elsewhere in the world. Examples include waterbirds, gamebirds and various 
passerines from the northern hemisphere.

Invasive species that were not the result of a deliberate introduction can be 
broadly considered as either ‘accidental introductions’ or 
‘self-introductions’. Importantly, introductions in both categories are 
facilitated through human activity. This includes promoting persistence of a 
species in human-modified habitats and by providing a means of transport (e.g. 
ship assisted such as we assume to be the case with the House Crow). Among 
birds the only clear example of a successful introduction of this form is the 
cattle egret, and even in this case deliberate introductions were also 
undertaken. It is also worthy of noting that native species have been 
introduced to parts of Australia outside their normal range and have 
established persistent (and often growing) populations (e.g. laughing 
kookaburra, rainbow lorikeet, long-billed corella). While introduced species 
can spread rapidly, they can only do so once they have established a 
self-sustaining population capable of growth. This initial period of 
establishment can take much longer than the subsequent rapid spread might 

The economical and ecological impacts of introduced species that have 
established viable populations in Australia have been well documented and are 
undisputed. Consequently, it is not surprising that many people have developed 
(and expressed in the present discussion) an alarmist attitude towards 
non-native species. However, I believe adopting this attitude only hinders 
discussion about the house crow at Dee Why. For example, comparing an 
individual fox to an individual house crow is hardly valid given the greatly 
disparate ecology of the two species and their potential individual impact(s) 
on native biota (i.e. large voracious predator vs small predominantly 
scavenging bird). While I appreciate that this example was being used to make a 
point it highlights the lack of objective consideration in the present 

 So far two key questions have been raised by several contributors.
1)      How likely is it that this individual house crow at Dee Why will lead 
to the successful establishment of a viable population in Australia?

2)      How does the house crow pose a threat economically (i.e. as an 
agricultural pest) and ecologically (i.e. direct and indirect effects on native 
biota) to Australia? That is, should we be concerned that house crows may 
become established in Australia?

It appears that the first question has been overshadowed by the alarmist 
response to the second question as the answer is clearly in the affirmative 
given house crows have become a serious problem where they have been introduced 
elsewhere in the world. While this is somewhat understandable given the public 
perception towards invasive species in Australia (e.g. foxes, rabbits, cane 
toads), I believe that this question only becomes relevant after considering 
this first question and some follow-on questions, including:

3)      Given the establishment of a population of house crows in Australia how 
effectively can it be controlled or eradicated? That is, can we ‘afford’ to 
wait and see if this one house crow becomes many?

It is here that we really need more information about house crows in Australia, 
which has been neglected in the discussion thus far. In my cursory research on 
the subject I discovered that in addition to house crows appearing occasionally 
as singles or pairs at several locations in Australia (something I think many 
birders would already be aware of) at least one small population of at least 15 
individuals persisted in Perth for over two decades before finally being 
destroyed (along with all others that have been detected). A signifiant feature 
of the ecoplogy of the species is that it usually lives in close-association 
with people, both in its original range and where it has been introduced. So, 
it would appear that house crows can be readily managed if a population becomes 
established in Australia because they are large birds, prefer human settlement 
and thus easily detected, an effective control method is available (i.e. 
shooting), and the rate of population growth appears to be slow providing ample 
time to implement control actions. This leads to a fourth question:

4)      If there is no threat posed or the likelihood of any threat being 
realised is low, and/or the ability to control or eliminate the threat is high, 
what action (if any) should be taken with the Dee Why house crow? That is, we 
now have a fourth question:What are the pros and cons of the options for 
management of the house crow in Australia?

This is where this issue starts to get much more complicated as we delve into 
issues beyond the biology of the invasive species including public perception 
about wildlife management, animal ethics and resources available for monitoring 
and control of invasive species. Here are just some things to consider:

-         Is it acceptable/ethical to destroy the Dee Why house crow given it 
may not pose a threat? It is unlikely this single individual, given the ecology 
of the species, will have direct or indirect effects on native biota above and 
beyond that of ecologically similar native species. (Note that disease 
transmission has been suggested as a threat but in reality this is a low risk. 
There are numerous native species that regularly travel between Australia and 
other parts of the world that are just as likely to be carrying the same 
pathogens that the Dee Why house crow may be carrying. So the chances are if 
the Dee Why bird has it, native birds have previously been exposed).

-         Are resources available to monitor this individual house crow in the 
event that it becomes the founder of a population? This is really where the 
second question of house crow impacts in Australia (i.e. the potential threat 
they pose) becomes an important consideration. Is there a high likelihood that 
a house crow population will become established and start to spread before 
being detected and/or control/eradication measures can be implemented? Here we 
must consider the balance between the risk of the Dee Why bird becoming a 
threat (i.e. population established) and the risk of being unable to manage 
that threat should it arise. This is what underlies the precautionary principle 
advocated by the late Graham Pizzey and others.

-         Should the Dee Why bird be destroyed because future costs of 
eradicating a population may be considerably higher, even if it is unlikely 
that a population will become established?

-         Is it important that the public perceives government taking action to 
control an invasive species? That is, be seen to be doing something. The 
disastrous results of previous decisions to introduce organisms for biological 
control and the prominent negative profile of feral animals in Australia are 
important considerations when considering this question.

It was not my intention to write such a detailed reply, but the deeper I dug 
the more complex things became. In any case it was a stimulating exercise and 
helped fill the time that insomnia was otherwise wasting.

 I was originally prompted to write this reply because I take a great dislike 
to discussions on birding-aus becoming personal in nature. I feel this is both 
unnecessary and detracts from the discussion. It can also discourage people 
from contributing. This present topic is a case in point. For example, one 
person criticised another contributor saying "you seem to have failed to grasp 
the fundamentals of the argument...". I don’t think the comment was justified 
(or at least the tone of the comment wasn't) given the context of the previous 
comments by the other contributor. Furthermore, I am of the opinion that it was 
the critic that has demonstrated a poorer understanding of what was fundamental 
to the issue being discussed.

 I think birding–aus has great potential to facilitate very stimulating 
discussions about anything ‘birds’ but this, in my experience, is too often 
hindered by intolerance of the opinions of others, inflammatory comments, and a 
tendency for people to jump to conclusions. Indeed, it is for these reasons I 
now only rarely contribute to birding-aus.
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