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Wetter springs in Mauritius are making the island's kestrels breed later in the season than they did 30 years ago, researchers have discovered.

The knock-on effect is that the Mauritius kestrel – classed as vulnerable on the IUCN Red List – can't raise as many chicks each season as it used to, which could eventually lead to a drop in their numbers.

The researchers found that springs in Mauritius are now 60 per cent wetter than they were in the early 60s, which is almost certainly a result of climate change. Where there were on average 14 days of rainfall in the 1960s, now there are around 23 days. So instead of laying eggs early in September, female kestrels now lay later in the month.

The scientists think this is because the male kestrel, which feeds the female early in the breeding season, struggles to find the birds' favourite food – the day gecko. The gecko isn't fond of getting wet, so hides when it rains. This means the females aren't getting the food they need early enough in the breeding season.

"When there's lots of spring rainfall, the hunting efficiency of the males goes down. But the female's physiological condition needs to be optimum for her to lay eggs, so if she's not getting enough food, she won't be in good condition and will lay later," explains Dr. Deepa Senapathi from the University of Reading, lead author of the study.

"The birds aren't deciding when to breed, but are instead waiting for the best time possible to breed and raise chicks," she adds.

Senapathi and her colleagues found no change in the number of Mauritius kestrels during the study period, from 1990 onwards – at least for now. But with just 42 to 45 breeding pairs left on the island, Senapathi says they need to keep a close eye on the situation in case the population does drop. 'If the trend for wetter springs continues, the population could go down,' she says.

But right now, Senapathi says there's no cause for alarm. "Our research gives people the heads-up for what to look for." By monitoring the population, she says conservationists could make sure they take whatever steps are needed to keep it stable.

The Mauritius kestrel, as its name suggests, is only found on the tiny 200 km2 island of Mauritius. Back in the 70s, the population dropped to two breeding pairs as a result of overuse of the pesticide DDT (which was eventually banned in the 80s) and widespread deforestation.

"The island was originally sprayed with DDT in an attempt to eradicate malaria. But this had a two-pronged effect: the toxin accumulated up the food chain resulting in adult mortality, and also thinned the shells of the kestrel's eggs, so they broke easily and didn't survive," says Senapathi.

As part of a recovery programme in the 80s, scientists re-introduced a new population to the island to bolster numbers. Since then, the birds have been monitored intensively.

Climate change has led to the declines of a number of plants and animals in recent years: populations of the checkerspot butterfly have disappeared, because of habitat loss and a change in the local climate. Populations of emperor penguins in the Antarctic have dropped because it's got warmer, and flowers in Norway are coming out earlier for the same reason.

But studies on how changes in the climate affect plants and animals have mainly been confined to regions where temperature has changed. Far fewer tropical regions have been studied.

"In the tropics, temperature won't vary as much as in temperate regions, so we thought let's look at rainfall," says Senapathi.

So she and colleagues from the University of Reading, France, Jersey and Mauritius analysed data which has come out of the detailed monitoring programme, alongside weather records.

The data included a huge range of information, including how many eggs each female laid, how many of these hatched, how many birds fledged, which partners the female paired up with, and whether or not she stayed with her partner.

"We were most surprised that the frequency of rainfall had a greater effect on these birds than how much it rained," Senapathi says.

"This study demonstrates that changes in the climate affect the tropics through different mechanisms. While temperature affects seasonality in temperate regions, in the tropics, rainfall patterns are more important," says Professor Ken Norris from the University of Reading, co-author of the study. "We shouldn't just focus on temperature changes."