Spotlight: Alarm Flights in Australia | The beach
by Deborah Buehler originally published in Wader Study 130(1)
For many humans, it means relaxation and recreation. It’s a place to fish, or walk the dog, or sunbathe, or swim. For many shorebird species, the beach is necessary for survival and reproduction. Unfortunately, beaches are not always tranquil. Shorebirds can be attacked by birds of prey or flushed by human beachgoers and their pets. How much does this happen? What causes the disturbance? Can anything be done to reduce the harm?
In this issue of Wader Study, Khwaja and colleagues harness the power of citizen scientist volunteers to address these questions. Using a standardised monitoring protocol, they collected data on the disturbances that shorebirds face when resting on the beaches of Roebuck Bay.1
Roebuck Bay is a large, tropical embayment in north Western Australia, south of the town of Broome. It is an important non-breeding site on the migration flyway extending from Arctic Russia and North America, along the east coast of Asia, to the southern limits of Australia and New Zealand (the East Asian-Australasian Flyway). Researchers at the Broome Bird Observatory regularly record 29 shorebird species supported by the bay. Because most of these species are migratory and breed in the northern hemisphere, the number of shorebirds in the bay varies greatly between roughly 20,000 individuals through the austral winter (dry season) to roughly 100,000 in the austral summer (wet season). Recently, populations of the migratory species in particular have declined alarmingly within the flyway. Although these declines have been linked to habitat loss in other parts of the flyway (specifically the disappearing mudflats of the Yellow Sea), understanding what happens in Australia improves the chances that the birds will be helped rather than further hindered when they reach the beaches of Roebuck Bay.
The tidal mudflats in Roebuck Bay are extensive, and low spring tides can expose around 175 km2 of mudflats where shorebirds feed. However, such a high tidal range also means that there is a four-hour period around high tide when the mudflats are totally submerged. During these times, the birds need safe places—called high tide roosts— to rest.
Finding a good high tide roost is not easy. Optimally such sites should be close to the feeding areas and have clear lines of sight to spot approaching danger. In tropical areas like Roebuck Bay, these roosts must also be close to the water during daytime high tides to prevent heat stress. When a good site is found, it makes sense to rest and conserve energy for the duration of high tide, but undisturbed roosting isn’t always possible.
Disturbance is usually defined as the disruption of normal activities caused by an animal’s response to an encounter with an external stimulus. Khwaja and colleagues chose alarm flights—when shorebirds take off steeply and rapidly turn to avoid perceived danger—to quantify disturbance. Models suggest that alarm flights are around three times more energetically costly than ‘commuting’ flights when birds move between areas undisturbed.2,3 The researchers then enlisted Broome Bird Observatory (BBO) staff and trained volunteers to conduct systematic watches on five beaches in Roebuck Bay. These study sites represented a range of disturbance levels and had been regular roost sites for shorebirds since at least the late 1990s.
Observers worked in pairs when possible, but when volunteer availability was low, experienced bird watchers could work alone. Teams performed watches at each of the five study beaches, simultaneously, sitting close enough to see any shorebirds present, but distant enough not to disturb them. There were two watch days per month from May 2005 until April 2006, and from August 2019 until July 2020, with one watch-day falling on a weekday and the other on a weekend. To gauge how often the birds were disturbed, observers counted all alarm flights made in the four-hour high tide period. They also estimated the average amount of time the birds spent in flight, by performing a short scan every 10 minutes throughout the watch and noting the proportion of birds in flight. The researchers then calculated the average proportion flying per minute and multiplied by the 240 minutes in a four-hour watch. Finally, they tried to determine what caused the alarm flights and recorded details of any visits made by people to the beaches.
The observers completed 214 watches over the two years: 96 in 2005–2006, and 118 in 2019–2020. Birds were present on the beach for 196 of the 214 watches and when birds were present, 2020 alarm flights were recorded: 918 in 2005–2006, and 1,102 in 2019–2020. On average the birds were disturbed 2.44 times per hour and the alarm flights usually involved all, or nearly all, of the birds present on the beach. Across all watches, the researchers estimated that each bird spent about 2.86 minutes per hour in flight.
Several factors affected both the number of alarm flights and the estimated time in flight. First, more alarm flights occurred, and birds spent significantly longer inflight, in the dry season (winter) than the wet season (summer). This makes sense because, in the dry season, birds of prey are more abundant in Roebuck Bay, Broome’s population swells with tourists and seasonal workers, and the access road to the northern beaches (study sites) is less likely to flood. Location of the beach was also associated with the number of alarm flights and time spent in flight, but the effect depended on the year of study. This interaction between location and year means that efforts to mitigate disturbance will need to be tailored to the circumstances at individual beaches over time. Finally, more alarm flights were noted when more birds were present on the beach, perhaps because the presence of more individuals increased the likelihood that one of them would detect an actual or perceived threat.
The researchers were able to identify the apparent cause of disturbance for 60% of alarm flights in 2005–2006, and 71% in 2019–2020. Most alarm flights were triggered by birds of prey (raptors) in both years and it is possible that they may have caused even more alarm flights than recorded. In both years, 29 to 40% of alarm flights could not be traced to a stimulus and birds of prey are more likely than other stimuli to have been missed by observers. For example, a brief, silent appearance by a raptor behind a dune is likely to be missed by an observer and can be enough to provoke an alarm flight. Aircraft are noisier and people disturbing shorebirds usually more obvious.
Human visitors were a less frequently identified cause of disturbance than birds of prey, but still accounted for about 20% of the alarm flights with an identified stimulus. Humans were most often seen walking or fishing on the beach, and walkers were more likely to alarm shorebirds (37% or 33 of 90 occasions) than people fishing (21% or 35 of 167 occasions). This is likely because walkers are mobile while fishers are stationary. However, observations from this study also suggest that fishing might have an indirect effect on disturbance because scraps left by human visitors attract birds of prey to beaches.
The researchers suggest various ways to mitigate human-caused disturbance in Roebuck Bay. Examples include: public awareness campaigns about the importance of not disturbing shorebirds on the beach, information for fishers on the importance of not leaving scraps or bycatch on beaches, and even installing floating roost platforms on beaches where human use is already high, to increase available habitat for birds without reducing accessibility to people.
The biggest challenge in this study, and a challenge for anyone attempting to measure the impact of disturbance on wild animals, is that is that researchers can only assume the consequences of an observed behaviour but cannot directly measure those consequences. Khwaja and colleagues were interested in the energetic costs of disturbance on shorebirds but acknowledge that they made no estimate of the true energetic cost of alarm flights, nor was doing so possible with the data they collected. They estimated a response to disturbance and used that as an index of the actual energetic costs. The use of indices is common but requires assumptions and raises the question of whether the exhibited behavior really means what we think it does.
In the case of alarm flights in Roebuck Bay, the assumption is that alarm flights are costly2 and that when the cost of multiple alarm flights exceeds the cost of a “commuting flight” to a “less disturbed” roost site, the birds will go elsewhere.3 Khwaja and colleagues estimated that shorebirds exceeded this threshold during the winter at all beaches studied in 2005–2006, and at two of the five beaches in 2019–2020. Yet, some birds clearly continue to roost on highly disturbed beaches in winter. What does it mean when a bird stays put? In many studies, staying is interpreted as low disturbance, but is that always the case? Perhaps the birds also experience disturbance at the nearest alternative roost. Perhaps they stay put and endure the disturbance—which also has costs—because they have nowhere better to go.4
This study by Khwaja and colleagues is impressive because the energy of volunteers was successfully harnessed in a standardised citizen science monitoring protocol. In this way, the researchers were able collect robust data on shorebird alarm flights and their possible causes, for two full years fourteen years apart. As we move towards summer and beach season in the north, in Roebuck Bay, the austral winter beach season is also beginning. This study is a nice reminder that though we can’t yet directly measure energetic or fitness costs of disturbing shorebirds, we can do our best to minimize our part in it.
1 Khwaja, N., C.J. Hassell, M.J. Taylor, J.A. Taylor, J. Lewis & D.I. Rogers. 2023. Repeated monitoring suggests shorebirds are disturbed consistently during winter at a globally important roost in tropical Australia. Wader Study 130(1): 38–51.
2 Nudds, R.L. & D.M. Bryant. 2000. The energetic costs of short flights in birds. Journal of Experimental Biology 203: 1561–1572.
3 Rogers, D.I., T. Piersma & C.J. Hassell. 2006. Roost availability may constrain shorebird distribution: exploring the energetic costs of roosting and disturbance around a tropical bay. Biological Conservation 133: 225–235.
4 Gill, J.A., K. Norris & W.J. Sutherland. 2001. Why behavioural responses may not reflect the population consequences of disturbance. Biological Conservation 97: 265–268.
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Featured image: Bar-tailed Godwit Limosa lapponica, Orielton Lagoon, Tasmania, Australia. © J. J. Harrison.