Some winter evenings at the breakwater, you can stand at the viewing area past nine and still be waiting. Other nights, especially in late spring, the first birds are porpoising in through the shallows not long after dusk. That difference isn’t random. It’s mostly down to what’s happening under the surface of Port Phillip Bay, and it’s one of the more interesting things volunteers track season to season.
Little penguins are central-place foragers, which is a slightly clunky way of saying they have to keep returning to the same fixed point, their nest cavity in the rock wall, no matter how far they’ve had to travel to find food. That constraint shapes everything about how they hunt. A change in sea temperature of even a degree or two can shift where the small schooling fish they rely on are sitting in the water column, and that in turn changes penguin foraging trip length by hours, not minutes.
Why penguin foraging trip length isn’t fixed
A penguin heading out from the breakwater at first light isn’t working to a schedule. It’s working to a budget: how much energy it can afford to spend swimming and diving before it has to bank a return on that investment in the form of caught fish. In good conditions, with prey schooled up close and shallow, a bird might complete a full day’s foraging in under twelve hours. In poor conditions it can take considerably longer, sometimes pushing into the following evening.
Researchers who’ve followed this colony for decades, going back to Professor Mike Cullen’s fortnightly night trips from Monash University that started in 1986, have long noted that trip duration is one of the most sensitive behavioural signals available. It responds faster than breeding success or body condition, because a penguin adjusts its foraging effort night by night, well before any of those slower effects show up.
The role of sea surface temperature
Port Phillip Bay is a shallow, semi-enclosed body of water, and it warms and cools faster than the open ocean. That matters because the small pelagic fish and krill-like prey little penguins target, anchovy and pilchard among them, are themselves responding to temperature gradients, moving to stay within their preferred thermal range and following plankton blooms that shift with the seasons.
When the bay is at its cooler end through winter, prey tends to be more dispersed, and penguins have to swim further and dive more often to build a full catch. Warmer, more stable periods, particularly as spring moves into the productive months, often coincide with tighter prey aggregations closer to the colony, and correspondingly shorter trips. It isn’t a clean linear relationship. Bay temperature is only one variable, and it interacts with wind, tide and the state of the broader Bass Strait system that Port Phillip connects into. But it’s a genuine and measurable driver, and it’s consistent with what CSIRO’s marine research on southern Australian coastal systems has documented more broadly about temperature-driven shifts in small pelagic fish distribution.
Wind, swell and why some nights are just harder work
Weather adds another layer on top of temperature. Strong onshore winds churn the shallow water and reduce visibility, making it harder for penguins to spot prey visually, which is largely how they hunt in the upper water column during daylight hours. Choppy conditions also cost more energy simply to swim through, quite apart from any effect on prey behaviour.
After a few days of southerly blow through the bay, we tend to see later returns at the breakwater, and volunteers doing the evening counts will often mention it before checking anything else. It’s the kind of pattern-recognition that comes from actually standing there most nights of the week rather than reading it off a chart. I’d argue the wind effect is honestly underrated compared to temperature in a lot of the public-facing material on this, probably because it’s harder to put a clean number on it. But anyone who has done a few dozen dusk shifts here will tell you a bad blow changes the whole feel of the evening, not just the timing.
The Bureau of Meteorology’s marine wind and swell forecasts for Port Phillip Bay are a reasonable proxy for what the birds are dealing with out past the heads, even though the penguins themselves are mostly foraging within the bay rather than out in open water.
Breeding season adds its own pressure
Foraging trip length doesn’t just track ocean conditions, it also tracks what’s happening back at the nest. During incubation, which runs roughly five weeks and is shared by both parents, the bird sitting on the eggs is fasting, so its partner is under pressure to forage efficiently and get back. Once chicks hatch, typically September through December, both parents need to bring back whole fish regularly enough to keep growing chicks fed, which tends to shorten trips compared to the pre-breeding months even if sea conditions haven’t changed much.
That’s part of why late winter, when birds are re-forming pair bonds and rebuilding condition after a lean season but aren’t yet feeding chicks, can produce some of the longest and latest foraging trips of the year. There’s less urgency to get home fast, and the water is often at its coolest and prey most scattered, a combination that stacks the deck toward longer nights out.
The moult period breaks this pattern altogether. For about two weeks each summer an adult penguin can’t go to sea at all, and instead sits ashore fasting on stored fat reserves while it grows a new waterproof coat. It’s a completely different physiological state to the rest of the year, not really comparable to a “long trip”, more like a forced pause built entirely around fat stores banked in the weeks beforehand.
What this means for an evening at the breakwater
If you’re planning a visit, the practical upshot is that “dusk” is a starting point, not a guarantee. On a calm, mild evening in the warmer months you might see the first arrivals not long after the light drops. On a cold, blustery night, especially in winter, you could be waiting considerably longer, and some birds may not come in until well after most visitors have gone home.
This is one of the reasons volunteer-guided viewing exists in the first place, not just to manage torches and dogs, but because trained volunteers who are there most nights have a genuine feel for how conditions on a given evening are likely to play out. If you want the best chance of a good sighting without disturbing the birds, joining one of the organised sessions described in our guide to how volunteer-guided viewing works at the St Kilda breakwater is a far better bet than turning up cold with a torch and hoping.
It’s also worth reading the return itself as a behaviour worth respecting rather than a spectacle to force along. A penguin that has just finished an unusually long, hard foraging trip is running on close to empty by the time it hits the rocks, and needs a clear, undisturbed path back to its nest cavity. That’s part of why flash photography and handheld lights are such a serious problem here, something we’ve covered in more detail in why flash photography and torches genuinely harm St Kilda’s penguins. A bird that’s already spent extra energy fighting currents and hunting scattered prey doesn’t have much margin left for being startled or disoriented right at the doorstep.
Reading the pattern across a season
None of this is something you can fully appreciate from a single visit. The value is in the pattern across a season, watching how return times shift as the bay warms into summer, how they blow out again after a cold snap, how everything tightens up once chicks are being fed. It’s part of why the long-run monitoring that’s continued since Cullen’s original study, and the count, weigh and band work volunteers still do today, matters well beyond the raw numbers. Trip length is a live readout of how the bay is doing, translated through a one-kilogram bird that happens to nest in a rock wall seven kilometres from the middle of Melbourne.
If you want to get a feel for that seasonal rhythm yourself, our penguin calendar and seasonal life cycle page lays out what to expect month by month, and it’s worth cross-referencing against a run of visits rather than one. And if you’re curious about how the birds actually locate their food out there in the first place, the mechanics are covered in more depth in our piece on foraging behaviour of little penguins.
Anyone getting more seriously interested in what drives these patterns, rather than just watching for a sighting, might also want to look at how the broader monitoring effort works and where the data goes, covered in how to get involved protecting the St Kilda penguin colony. It’s not glamorous work, mostly cold hands and clipboards on a rock wall after dark, but it’s how we know any of this at all.
