As students start packing up their apartments and houses next week for summer break, they're bound to leave things behind. For some, it might be a box of tattered textbooks or an old carpet. For others, though, it's a pet who didn't factor into post-graduation plans.

Tanya Hilgendorf, the executive director of the Humane Society of Huron Valley, which serves Washtenaw County, said her office notices about a 30-percent increase in pet desertions around the time school lets out each year.

"As long as anybody can remember, the animals that are surrendered to us, abandoned and found as strays spike at the end of the school year," Hilgendorf said.

Shelter officials said the spike could be credited in part to students who neglect to make proper arrangements for their pets when their leases expire.

Hilgendorf said the animals are often left trapped inside empty homes without any food or water. Though abandoned animals taken in by the Humane Society are usually held for seven days and then put up for adoption, there are some cases, especially during the warmer months, when they have to be euthanized.

"In our shelter, animals are euthanized for serious health or aggression issues," Hilgendorf said. "But there are times when we have to euthanize animals because of lack of space. That does not happen a lot, but it certainly does happen in the spring and summer."

Around graduation time last year, Hilgendorf said, two rabbits and a cat were left alone for a week in a house on Division Street without food and water. After investigating, Hilgendorf found that although multiple students had lived in the house, no one took direct responsibility for the pets.

"Everybody else thought somebody else was going to take care of the animals," she said.

It's a trend that goes beyond Ann Arbor. Officials from the Capital Area Humane Society in Lansing, which takes in any abandoned animals from Michigan State University students, has also noticed the trend, calling it one that "happens all over the country."

"I noticed it when I worked in Pennsylvania and Florida," said Steven Heaven, the group's president. "We tend not to adopt to students because of that problem."

Ann Williams, a representative from popular campus home management firm Old Town Realty, said she'd seen animal abandonment in one of the company's properties rented to students.

Williams investigated an apartment soon after the two girls living there had moved out and found an abandoned feline.

"I think the reason that I checked it was because they didn't turn their keys in, and I went down and there was a cat just in there with no food," Williams said.

Williams said she decided to confront the residents' parents rather than contact local authorities. Though animal cruelty is a misdemeanor in the state of Michigan, Hilgendorf said it's often difficult to prove malice.

"You can be prosecuted for abandoning animals," Hilgendorf said. "It is very difficult to prove that it was intentionally done, because animals can't talk."

Many students said they were shocked that other students would leave their pets behind.

"I think it's absolutely awful that someone would leave their pet in their apartment after they leave," said LSA freshman Mike Dunleavy. "I have two pets at home, and I can't imagine treating either of them like that because that's just absolutely inhumane, and not something a responsible person should do."

Many Ann Arbor landlords and leasing agencies interviewed said they haven't encountered any pet abandonments. Of the more than 10 companies asked, only Old Time Realty reported finding neglected animals.

Hilgendorf said the shelter also encounters abandoned pets that were released into the wild.

"Some are dumped out on the street," Hilgendorf said. "Sometimes they do that mistakenly thinking the animal is better off than being brought to a shelter.

Hilgendorf said a reason that students might leave pets behind is because they underestimate the costs associated with owning a pet, even if they get the animal for free.

"A free kitten is actually quite costly, because they're not vaccinated, they're not spayed or neutered," she said. "There's a lot of costs associated with that free kitten from the farmer's market."

Hilgendorf said students with unstable housing situations should wait to take on the commitment of owning a pet until long-term residence can be established.

"I think for the most part they're not understanding the long term ramifications of their decisions," she said. "I think they're getting a pet on impulse. They don't understand the responsibilities that are attached, and they're not thinking in the long run."

Students who can't keep their pets should call the Humane Society of Huron Valley at (734) 662-5585.